Ranges of Normal Values in Human Whole Blood (B), Plasma (P), or Serum (S)a Normal Value (Varies with Procedure Used) Determination Traditional Units SI Units Normal Value (Varies with Procedure Used) Determination
Traditional Units
SI Units
Acetoacetate plus acetone (S)
0.3–2.0 mg/dL
3–20 mg/L
Aldosterone (supine) (P)
3.0–10 ng/dL
83–227 pmol/L
Alpha-amino nitrogen (P)
3.0–5.5 mg/dL
2.1–3.9 mmol/L
Aminotransferases Alanine aminotransferase
3–48 units/L
Aspartate aminotransferase
0–55 units/L
Ammonia (B)
12–55 μmol/L
12–55 μmol/L
Amylase (S)
53–123 units/L
884–2050 nmol s–1/L
Ascorbic acid (B)
0.4–1.5 mg/dL (fasting)
23–85 μmol/L
Bilirubin (S)
Conjugated (direct): up to 0.4 mg/dL
Up to 7 μmol/L
Total (conjugated plus free): up to 1.0 mg/dL
Up to 17 μmol/L
Calcium (S)
8.5–10.5 mg/dL; 4.3–5.3 meq/L
2.1–2.6 mmol/L
Carbon dioxide content (S)
24–30 meq/L
24–30 mmol/L
Carotenoids (S)
0.8–4.0 μg/mL
1.5–7.4 μmol/L
Ceruloplasmin (S)
23–43 mg/dL
240–430 mg/L
Chloride (S)
100–108 meq/L
100–108 mmol/L
Cholesterol (S)
< 200 mg/dL
< 5.17 mmol/L
Cholesteryl esters (S)
60–70% of total cholesterol
Copper (total) (S)
70–155 μg/dL
11.0–24.4 μmol/L
Cortisol (P) (AM, fasting)
5–25 μg/dL
0.14–0.69 μmol/L
Creatinine (P)
0.6–1.5 mg/dL
53–133 μmol/L
Glucose, fasting (P)
70–110 mg/dL
3.9–6.1 mmol/L
Iron (S)
50–150 μg/dL
9.0–26.9 μmol/L
Lactic acid (B)
0.5–2.2 meq/L
0.5–2.2 mmol/L
Lipase (S)
3–19 units/L
Lipids, total (S)
450–1000 mg/dL
4.5–10 g/L
Magnesium (S)
1.4–2.0 meq/L
0.7–1.0 mmol/L
Osmolality (S)
280–296 mosm/kg H2O
280–296 mmol/kg H2O
PCO2 (arterial) (B)
35–45 mm Hg
4.7–6.0 kPa
Pepsinogen (P)
200–425 units/mL
pH (B) 7.35–7.45
Phenylalanine (S)
0–2 mg/dL
0–120 μmol/L
Phosphatase, acid (S)
Males: 0–0.8 sigma unit/mL Females: 0.01–0.56 sigma unit/mL 13–39 units/L (adults)
0.22–0.65 μmol s–1/L
Phospholipids (S)
9–16 mg/dL as lipid phosphorus
2.9–5.2 mmol/L
Phosphorus, inorganic (S)
2.6–4.5 mg/dL (infants in first year: up to 6.0 mg/dL)
0.84–1.45 mmol/L
PO2 (arterial) (B)
75–100 mm Hg
10.0–13.3 kPa
Potassium (S)
3.5–5.0 meq/L
3.5–5.0 mmol/L
Total (S)
6.0–8.0 g/dL
60–80 g/L
Albumin (S)
3.1–4.3 g/dL
31–43 g/L
Globulin (S)
2.6–4.1 g/dL
26–41 g/L
Pyruvic acid (P)
0–0.11 meq/L
0–110 μmol/L
Sodium (S)
135–145 meq/L
135–145 mmol/L
Urea nitrogen (S)
8–25 mg/dL
2.9–8.9 mmol/L
Women
2.3–6.6 mg/dL
137–393 μmol/L
Men
3.6–8.5 mg/dL
214–506 μmol/L
Phosphatase, alkaline (S)
Protein
Uric acid (S)
a
Based in part on Kratz A, et al. Laboratory reference values. N Engl J Med 2004;351:1548. Ranges vary somewhat from one laboratory to another depending on the details of the methods used, and specific values should be considered in the context of the range of values for the laboratory that made the determination.
a LANGE medical book
Ganong’s Review of
Medical Physiology Twenty-Third Edition
Kim E. Barrett, PhD
Scott Boitano, PhD
Professor Department of Medicine Dean of Graduate Studies University of California, San Diego La Jolla, California
Associate Professor, Physiology Arizona Respiratory Center Bio5 Collaborative Research Institute University of Arizona Tucson, Arizona
Susan M. Barman, PhD
Heddwen L. Brooks, PhD
Professor Department of Pharmacology/Toxicology Michigan State University East Lansing, Michigan
Associate Professor Department of Physiology College of Medicine University of Arizona Tucson, Arizona
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Copyright © 2010 by The McGraw-Hill Companies, Inc. All rights reserved. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. ISBN: 978-0-07-160568-7 MHID: 0-07-160568-1 The material in this eBook also appears in the print version of this title: ISBN: 978-0-07-160567-0, MHID: 0-07-160567-3. All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. Where such designations appear in this book, they have been printed with initial caps. McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. To contact a representative please e-mail us at
[email protected]. Medicine is an ever-changing science. As new research and clinical experience broaden our knowledge, changes in treatment and drug therapy are required. The authors and the publisher of this work have checked with sources believed to be reliable in their efforts to provide information that is complete and generally in accord with the standards accepted at the time of publication. However, in view of the possibility of human error or changes in medical sciences, neither the authors nor the publisher nor any other party who has been involved in the preparation or publication of this work warrants that the information contained herein is in every respect accurate or complete, and they disclaim all responsibility for any errors or omissions or for the results obtained from use of the information contained in this work. Readers are encouraged to confirm the information contained herein with other sources. For example and in particular, readers are advised to check the product information sheet included in the package of each drug they plan to administer to be certain that the information contained in this work is accurate and that changes have not been made in the recommended dose or in the contraindications for administration. This recommendation is of particular importance in connection with new or infrequently used drugs. TERMS OF USE This is a copyrighted work and The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and its licensors reserve all rights in and to the work. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw-Hill’s prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms. THE WORK IS PROVIDED “AS IS.” McGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THE WORK, INCLUDING ANY INFORMATION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill and its licensors do not warrant or guarantee that the functions contained in the work will meet your requirements or that its operation will be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccuracy, error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill has no responsibility for the content of any information accessed through the work. Under no circumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise.
Dedication to
WILLIAM FRANCIS GANONG William Francis (“Fran”) Ganong was an outstanding scientist, educator, and writer. He was completely dedicated to the field of physiology and medical education in general. Chairman of the Department of Physiology at the University of California, San Francisco, for many years, he received numerous teaching awards and loved working with medical students. Over the course of 40 years and some 22 editions, he was the sole author of the best selling Review of Medical Physiology, and a co-author of 5 editions of Pathophysiology of Disease: An Introduction to Clinical Medicine. He was one of the “deans” of the Lange group of authors who produced concise medical text and review books that to this day remain extraordinarily popular in print and now in digital formats. Dr. Ganong made a gigantic impact on the education of countless medical students and clinicians. A general physiologist par excellence and a neuroendocrine physiologist by subspecialty, Fran developed and maintained a rare understanding of the entire field of physiology. This allowed him to write each new edition (every 2 years!) of the Review of Medical Physiology as a sole author, a feat remarked on and admired whenever the book came up for discussion
among physiologists. He was an excellent writer and far ahead of his time with his objective of distilling a complex subject into a concise presentation. Like his good friend, Dr. Jack Lange, founder of the Lange series of books, Fran took great pride in the many different translations of the Review of Medical Physiology and was always delighted to receive a copy of the new edition in any language. He was a model author, organized, dedicated, and enthusiastic. His book was his pride and joy and like other best-selling authors, he would work on the next edition seemingly every day, updating references, rewriting as needed, and always ready and on time when the next edition was due to the publisher. He did the same with his other book, Pathophysiology of Disease: An Introduction to Clinical Medicine, a book that he worked on meticulously in the years following his formal retirement and appointment as an emeritus professor at UCSF. Fran Ganong will always have a seat at the head table of the greats of the art of medical science education and communication. He died on December 23, 2007. All of us who knew him and worked with him miss him greatly.
iii
Key Features of the 23rd Edition of
Ganong’s Review of Medical Physiology • Thoroughly updated to reflect the latest research and developments in important areas such as the cellular basis of neurophysiology • Incorporates examples from clinical medicine throughout the chapters to illustrate important physiologic concepts • Delivers more detailed, clinically-relevant, high-yield information per page than any similar text or review • NEW full-color illustrations—the authors have worked with an outstanding team of medical illustrators, photographers, educators, and students to provide an unmatched collection of 600 illustrations and tables • NEW boxed clinical cases—featuring examples of diseases that illustrate important physiologic principles • NEW high-yield board review questions at the end of each chapter • NEW larger 8½ X 11” trim-size enhances the rich visual content • NEW companion online learning center (LangeTextbooks.com) offers a wealth of innovative learning tools and illustrations
NEW iPod-compatible review—Medical PodClass
offers audio and text for study on the go
Full-color illustrations enrich the text
iv
KEY FEATURES
Clinical Cases illustrate essential physiologic principles
Summary tables and charts encapsulate important information
Chapters conclude with Chapter Summaries and review questions
v
About the Authors KIM E. BARRETT Kim Barrett received her PhD in biological chemistry from University College London in 1982. Following postdoctoral training at the National Institutes of Health, she joined the faculty at the University of California, San Diego, School of Medicine in 1985, rising to her current rank of Professor of Medicine in 1996. Since 2006, she has also served the University as Dean of Graduate Studies. Her research interests focus on the physiology and pathophysiology of the intestinal epithelium, and how its function is altered by commensal, probiotics, and pathogenic bacteria as well as in specific disease states, such as inflammatory bowel diseases. She has published almost 200 articles, chapters, and reviews, and has received several honors for her research accomplishments including the Bowditch and Davenport Lectureships from the American Physiological Society and the degree of Doctor of Medical Sciences, honoris causa, from Queens University, Belfast. She is also a dedicated and award-winning instructor of medical, pharmacy, and graduate students, and has taught various topics in medical and systems physiology to these groups for more than 20 years. Her teaching experiences led her to author a prior volume (Gastrointestinal Physiology, McGraw-Hill, 2005) and she is honored to have been invited to take over the helm of Ganong.
SUSAN M. BARMAN Susan Barman received her PhD in physiology from Loyola University School of Medicine in Maywood, Illinois. Afterward she went to Michigan State University (MSU) where she is currently a Professor in the Department of Pharmacology/ Toxicology and the Neuroscience Program. Dr Barman has had a career-long interest in neural control of cardiorespiratory function with an emphasis on the characterization and origin of the naturally occurring discharges of sympathetic and phrenic nerves. She was a recipient of a prestigious National Institutes of Health MERIT (Method to Extend Research in Time) Award. She is also a recipient of an Outstanding University Woman Faculty Award from the MSU Faculty Professional Women's Association and an MSU College of Human Medicine Distinguished Faculty Award. She has been very active in the vi
American Physiological Society (APS) and recently served on its council. She has also served as Chair of the Central Nervous System Section of APS as well as Chair of both the Women in Physiology and Section Advisory Committees of APS. In her spare time, she enjoys daily walks, aerobic exercising, and mind-challenging activities like puzzles of various sorts.
SCOTT BOITANO Scott Boitano received his PhD in genetics and cell biology from Washington State University in Pullman, Washington, where he acquired an interest in cellular signaling. He fostered this interest at University of California, Los Angeles, where he focused his research on second messengers and cellular physiology of the lung epithelium. He continued to foster these research interests at the University of Wyoming and at his current positions with the Department of Physiology and the Arizona Respiratory Center, both at the University of Arizona.
HEDDWEN L. BROOKS Heddwen Brooks received her PhD from Imperial College, University of London and is an Associate Professor in the Department of Physiology at the University of Arizona (UA). Dr Brooks is a renal physiologist and is best known for her development of microarray technology to address in vivo signaling pathways involved in the hormonal regulation of renal function. Dr Brooks’ many awards include the American Physiological Society (APS) Lazaro J. Mandel Young Investigator Award, which is for an individual demonstrating outstanding promise in epithelial or renal physiology. She will receive the APS Renal Young Investigator Award at the 2009 annual meeting of the Federation of American Societies for Experimental Biology. Dr Brooks is a member of the APS Renal Steering Section and the APS Committee of Committees. She is on the Editorial Board of the American Journal of Physiology-Renal Physiology (since 2001), and she has also served on study sections of the National Institutes of Health and the American Heart Association.
Contents Preface
ix
S E C T I O N
I
CELLULAR & MOLECULAR BASIS FOR MEDICAL PHYSIOLOGY 1 1. General Principles & Energy Production in Medical Physiology 1 2. Overview of Cellular Physiology in Medical Physiology 31 3. Immunity, Infection, & Inflammation 63 S E C T I O N
II
PHYSIOLOGY OF NERVE & MUSCLE CELLS 79
15. Electrical Activity of the Brain, Sleep–Wake States, & Circadian Rhythms 229 16. Control of Posture & Movement 241 17. The Autonomic Nervous System 261 18. Hypothalamic Regulation of Hormonal Functions 273 19. Learning, Memory, Language, & Speech 289 S E C T I O N
IV
ENDOCRINE & REPRODUCTIVE PHYSIOLOGY 301 20. The Thyroid Gland 301
4. Excitable Tissue: Nerve 79 5. Excitable Tissue: Muscle 93 6. Synaptic & Junctional Transmission 115 7. Neurotransmitters & Neuromodulators 129 8. Properties of Sensory Receptors 149 9. Reflexes 157 S E C T I O N
III
CENTRAL & PERIPHERAL NEUROPHYSIOLOGY 167 10. Pain & Temperature 167 11. Somatosensory Pathways 173 12. Vision 181 13. Hearing & Equilibrium 203
21. Endocrine Functions of the Pancreas & Regulation of Carbohydrate Metabolism 315 22. The Adrenal Medulla & Adrenal Cortex 337 23. Hormonal Control of Calcium and Phosphate Metabolism & the Physiology of Bone 363 24. The Pituitary Gland 377 25. The Gonads: Development & Function of the Reproductive System 391 S E C T I O N
V
GASTROINTESTINAL PHYSIOLOGY 429 26. Overview of Gastrointestinal Function & Regulation 429
14. Smell & Taste 219 vii
viii
CONTENTS
27. Digestion, Absorption, & Nutritional Principles 451
S E C T I O N
VII
RESPIRATORY PHYSIOLOGY 587
28. Gastrointestinal Motility 469
35. Pulmonary Function 587
29. Transport & Metabolic Functions of the Liver 479
36. Gas Transport & pH in the Lung 609
S E C T I O N
VI
CARDIOVASCULAR PHYSIOLOGY 489 30. Origin of the Heartbeat & the Electrical Activity of the Heart 489
37. Regulation of Respiration 625 S E C T I O N
VIII
RENAL PHYSIOLOGY
639
38. Renal Function & Micturition 639
31. The Heart as a Pump 507
39. Regulation of Extracellular Fluid Composition & Volume 665
32. Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow 521
40. Acidification of the Urine & Bicarbonate Excretion 679
33. Cardiovascular Regulatory Mechanisms 555 34. Circulation Through Special Regions 569
Answers to Multiple Choice Questions Index
689
687
Preface
From the Authors
New 81/2 x 11 Format
We are very pleased to launch the 23rd edition of Ganong's Review of Medical Physiology. The current authors have attempted to maintain the highest standards of excellence, accuracy, and pedagogy developed by Fran Ganong over the 46 years in which he educated countless students worldwide with this textbook. At the same time, we have been attuned to the evolving needs of both students and professors in medical physiology. Thus, in addition to usual updates on the latest research and developments in areas such as the cellular basis of physiology and neurophysiology, this edition has added both outstanding pedagogy and learning aids for students. We are truly grateful for the many helpful insights, suggestions, and reviews from around the world that we received from colleagues and students. We hope you enjoy the new features and the 23rd edition! This edition is a revision of the original works of Dr. Francis Ganong.
• Based on student and instructor focus groups, we have increased the trim size, which will provide additional white space and allow our new art program to really show!
New 4 Color Illustrations • We have worked with a large team of medical illustrators, photographers, educators, and students to build an accurate, up-to-date, and visually appealing new illustration program. Full-color illustrations and tables are provided throughout, which also include detailed figure legends that tell a short story or describes the key point of the illustration.
New Boxed Clinical Cases • Highlighted in a shaded background, so students can recognize the boxed clinical cases, examples of diseases illustrating important physiological principles are provided.
New End of Chapter Board Review Questions • New to this edition, chapters now conclude with board review questions.
New Media • This new edition has focused on creating new student content that is built upon learning outcomes and assessing student performance. Free with every student copy is an iPod Review Tutorial Product. Questions and art based from each chapter tests students comprehension and is easy to navigate with a simple click of the scroll bar! • Online Learning Center will provide students and faculty with cases and art and board review questions on a dedicated website.
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SECTION I CELLULAR & MOLECULAR BASIS OF MEDICAL PHYSIOLOGY
C
General Principles & Energy Production in Medical Physiology
1
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■ ■
Name the different fluid compartments in the human body. Define moles, equivalents, and osmoles. Define pH and buffering. Understand electrolytes and define diffusion, osmosis, and tonicity. Define and explain the resting membrane potential. Understand in general terms the basic building blocks of the cell: nucleotides, amino acids, carbohydrates, and fatty acids. Understand higher-order structures of the basic building blocks: DNA, RNA, proteins, and lipids. Understand the basic contributions of these building blocks to cell structure, function, and energy balance.
INTRODUCTION In unicellular organisms, all vital processes occur in a single cell. As the evolution of multicellular organisms has progressed, various cell groups organized into tissues and organs have taken over particular functions. In humans and other vertebrate animals, the specialized cell groups include a gastrointestinal system to digest and absorb food; a respiratory system to take up O2 and eliminate CO2; a urinary system to remove wastes; a cardiovascular system to distribute nutrients, O2, and the products of metabolism; a reproductive system to perpetuate the species; and nervous and endocrine systems to coordinate and integrate the functions of the other systems. This book
is concerned with the way these systems function and the way each contributes to the functions of the body as a whole. In this section, general concepts and biophysical and biochemical principles that are basic to the function of all the systems are presented. In the first chapter, the focus is on review of basic biophysical and biochemical principles and the introduction of the molecular building blocks that contribute to cellular physiology. In the second chapter, a review of basic cellular morphology and physiology is presented. In the third chapter, the process of immunity and inflammation, and their link to physiology, are considered. 1
2
SECTION I Cellular & Molecular Basis of Medical Physiology
GENERAL PRINCIPLES THE BODY AS AN ORGANIZED “SOLUTION” The cells that make up the bodies of all but the simplest multicellular animals, both aquatic and terrestrial, exist in an “internal sea” of extracellular fluid (ECF) enclosed within the integument of the animal. From this fluid, the cells take up O2 and nutrients; into it, they discharge metabolic waste products. The ECF is more dilute than present-day seawater, but its composition closely resembles that of the primordial oceans in which, presumably, all life originated. In animals with a closed vascular system, the ECF is divided into two components: the interstitial fluid and the circulating blood plasma. The plasma and the cellular elements of the blood, principally red blood cells, fill the vascular system, and together they constitute the total blood volume. The interstitial fluid is that part of the ECF that is outside the vascular system, bathing the cells. The special fluids considered together as transcellular fluids are discussed in the following text. About a third of the total body water is extracellular; the remaining two thirds is intracellular (intracellular fluid). In the average young adult male, 18% of the body weight is protein and related substances, 7% is mineral, and 15% is fat. The remaining 60% is water. The distribution of this water is shown in Figure 1–1A. The intracellular component of the body water accounts for about 40% of body weight and the extracellular component for about 20%. Approximately 25% of the extracellular component is in the vascular system (plasma = 5% of body weight) and 75% outside the blood vessels (interstitial fluid = 15% of body weight). The total blood volume is about 8% of body weight. Flow between these compartments is tightly regulated.
UNITS FOR MEASURING CONCENTRATION OF SOLUTES In considering the effects of various physiologically important substances and the interactions between them, the number of molecules, electric charges, or particles of a substance per unit volume of a particular body fluid are often more meaningful than simply the weight of the substance per unit volume. For this reason, physiological concentrations are frequently expressed in moles, equivalents, or osmoles.
Moles A mole is the gram-molecular weight of a substance, ie, the molecular weight of the substance in grams. Each mole (mol) consists of 6 × 1023 molecules. The millimole (mmol) is 1/1000 of a mole, and the micromole (μmol) is 1/1,000,000 of a mole. Thus, 1 mol of NaCl = 23 g + 35.5 g = 58.5 g, and 1 mmol = 58.5 mg. The mole is the standard unit for expressing the amount of substances in the SI unit system.
The molecular weight of a substance is the ratio of the mass of one molecule of the substance to the mass of one twelfth the mass of an atom of carbon-12. Because molecular weight is a ratio, it is dimensionless. The dalton (Da) is a unit of mass equal to one twelfth the mass of an atom of carbon-12. The kilodalton (kDa = 1000 Da) is a useful unit for expressing the molecular mass of proteins. Thus, for example, one can speak of a 64-kDa protein or state that the molecular mass of the protein is 64,000 Da. However, because molecular weight is a dimensionless ratio, it is incorrect to say that the molecular weight of the protein is 64 kDa.
Equivalents The concept of electrical equivalence is important in physiology because many of the solutes in the body are in the form of charged particles. One equivalent (eq) is 1 mol of an ionized substance divided by its valence. One mole of NaCl dissociates into 1 eq of Na+ and 1 eq of Cl–. One equivalent of Na+ = 23 g, but 1 eq of Ca2+ = 40 g/2 = 20 g. The milliequivalent (meq) is 1/1000 of 1 eq. Electrical equivalence is not necessarily the same as chemical equivalence. A gram equivalent is the weight of a substance that is chemically equivalent to 8.000 g of oxygen. The normality (N) of a solution is the number of gram equivalents in 1 liter. A 1 N solution of hydrochloric acid contains both H+ (1 g) and Cl– (35.5 g) equivalents, = (1 g + 35.5 g)/L = 36.5 g/L.
WATER, ELECTROLYTES, & ACID/BASE The water molecule (H2O) is an ideal solvent for physiological reactions. H2O has a dipole moment where oxygen slightly pulls away electrons from the hydrogen atoms and creates a charge separation that makes the molecule polar. This allows water to dissolve a variety of charged atoms and molecules. It also allows the H2O molecule to interact with other H2O molecules via hydrogen bonding. The resultant hydrogen bond network in water allows for several key properties in physiology: (1) water has a high surface tension, (2) water has a high heat of vaporization and heat capacity, and (3) water has a high dielectric constant. In layman’s terms, H2O is an excellent biological fluid that serves as a solute; it provides optimal heat transfer and conduction of current. Electrolytes (eg, NaCl) are molecules that dissociate in water to their cation (Na+) and anion (Cl–) equivalents. Because of the net charge on water molecules, these electrolytes tend not to reassociate in water. There are many important electrolytes in physiology, notably Na+, K+, Ca2+, Mg2+, Cl–, and HCO3–. It is important to note that electrolytes and other charged compounds (eg, proteins) are unevenly distributed in the body fluids (Figure 1–1B). These separations play an important role in physiology.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
Intestines
Stomach
Skin Kidneys
Blood plasma: 5% body weight
Lungs Extracellular fluid: 20% body weight
3
Interstitial fluid: 15% body weight
Intracellular fluid: 40% body weight
A
Extracellular fluid
200 Plasma
Intracellular fluid
Cl−
Na+
Cl−
Cell membrane
Na+
Capillaries
meq/L H2O
100
Misc. phosphates
Interstitial fluid
150
K+
Na+ Prot−
HCO3−
50 K+
Prot−
K+
HCO3−
HCO3−
0 B
Cl−
FIGURE 1–1 Organization of body fluids and electrolytes into compartments. A) Body fluids are divided into Intracellular and extracellular fluid compartments (ICF and ECF, respectively). Their contribution to percentage body weight (based on a healthy young adult male; slight variations exist with age and gender) emphasizes the dominance of fluid makeup of the body. Transcellular fluids, which constitute a very small percentage of total body fluids, are not shown. Arrows represent fluid movement between compartments. B) Electrolytes and proteins are unequally distributed among the body fluids. This uneven distribution is crucial to physiology. Prot –, protein, which tends to have a negative charge at physiologic pH.
4
SECTION I Cellular & Molecular Basis of Medical Physiology
pH AND BUFFERING The maintenance of a stable hydrogen ion concentration ([H+]) in body fluids is essential to life. The pH of a solution is defined as the logarithm to the base 10 of the reciprocal of the H+ concentration ([H+]), ie, the negative logarithm of the [H+]. The pH of water at 25 °C, in which H+ and OH– ions are present in equal numbers, is 7.0 (Figure 1–2). For each pH unit less than 7.0, the [H+] is increased tenfold; for each pH unit above 7.0, it is decreased tenfold. In the plasma of healthy individuals, pH is slightly alkaline, maintained in the narrow range of 7.35 to 7.45. Conversely, gastric fluid pH can be quite acidic (on the order of 2.0) and pancreatic secretions can be quite alkaline (on the order of 8.0). Enzymatic activity and protein structure are frequently sensitive to pH; in any given body or cellular compartment, pH is maintained to allow for maximal enzyme/protein efficiency. Molecules that act as H+ donors in solution are considered acids, while those that tend to remove H+ from solutions are considered bases. Strong acids (eg, HCl) or bases (eg, NaOH) dissociate completely in water and thus can most change the [H+] in solution. In physiological compounds, most acids or bases are considered “weak,” that is, they contribute relatively few H+ or take away relatively few H+ from solution. Body pH is stabilized by the buffering capacity of the body fluids. A buffer is a substance that has the ability to bind or release H+ in solution, thus keeping the pH of the solution relatively constant despite the addition of considerable quantities of acid or base. Of course there are a number of buffers at work in biological fluids at any given time. All buffer pairs in a homogenous solution are in equilibrium with the same [H+]; this is known as the isohydric principle. One outcome of this principle is that by assaying a single buffer system, we can understand a great deal about all of the biological buffers in that system.
When acids are placed into solution, there is a dissociation of some of the component acid (HA) into its proton (H+) and free acid (A–). This is frequently written as an equation: + – →H +A . HA ←
According to the laws of mass action, a relationship for the dissociation can be defined mathematically as: Ka = [H+] [A–] / [HA] where Ka is a constant, and the brackets represent concentrations of the individual species. In layman’s terms, the product of the proton concentration ([H+]) times the free acid concentration ([A–]) divided by the bound acid concentration ([HA]) is a defined constant (K). This can be rearranged to read: [H+] = Ka [HA]/[A–] If the logarithm of each side is taken: log [H+] = logKa + log[HA]/[A–] Both sides can be multiplied by –1 to yield: –log [H+] = –logKa + log[A–]/[HA] This can be written in a more conventional form known as the Henderson Hasselbach equation: pH = pKa + log [A–]/[HA] This relatively simple equation is quite powerful. One thing that we can discern right away is that the buffering capacity of a particular weak acid is best when the pKa of that acid is equal to the pH of the solution, or when: [A–] = [HA], pH = pKa Similar equations can be set up for weak bases. An important buffer in the body is carbonic acid. Carbonic acid is a weak acid, and thus is only partly dissociated into H+ and bicarbonate: + – → H + HCO3 H2CO3 ←
pH
10−1 10−2 10−3 10−4 10−5 10−6 10−7 10−8 10−9 10−10 10−11 10−12 10−13 10−14
1 2 3 4 5 6 7 8 9 10 11 12 13 14
ACIDIC
H+ concentration (mol/L)
ALKALINE
For pure water, [H+] = 10−7 mol/L
FIGURE 1–2
Proton concentration and pH. Relative proton (H+) concentrations for solutions on a pH scale are shown. (Redrawn from Alberts B et al: Molecular Biology of the Cell, 4th ed. Garland Science, 2002.)
If H+ is added to a solution of carbonic acid, the equilibrium shifts to the left and most of the added H+ is removed from solution. If OH– is added, H+ and OH– combine, taking H+ out of solution. However, the decrease is countered by more dissociation of H2CO3, and the decline in H+ concentration is minimized. A unique feature of bicarbonate is the linkage between its buffering ability and the ability for the lungs to remove carbon dioxide from the body. Other important biological buffers include phosphates and proteins.
DIFFUSION Diffusion is the process by which a gas or a substance in a solution expands, because of the motion of its particles, to fill all the available volume. The particles (molecules or atoms) of a substance dissolved in a solvent are in continuous random movement. A given particle is equally likely to move into or
CHAPTER 1 General Principles & Energy Production in Medical Physiology out of an area in which it is present in high concentration. However, because there are more particles in the area of high concentration, the total number of particles moving to areas of lower concentration is greater; that is, there is a net flux of solute particles from areas of high to areas of low concentration. The time required for equilibrium by diffusion is proportionate to the square of the diffusion distance. The magnitude of the diffusing tendency from one region to another is directly proportionate to the cross-sectional area across which diffusion is taking place and the concentration gradient, or chemical gradient, which is the difference in concentration of the diffusing substance divided by the thickness of the boundary (Fick’s law of diffusion). Thus, J = –DA Δc Δx where J is the net rate of diffusion, D is the diffusion coefficient, A is the area, and Δc/Δx is the concentration gradient. The minus sign indicates the direction of diffusion. When considering movement of molecules from a higher to a lower concentration, Δc/Δx is negative, so multiplying by –DA gives a positive value. The permeabilities of the boundaries across which diffusion occurs in the body vary, but diffusion is still a major force affecting the distribution of water and solutes.
OSMOSIS When a substance is dissolved in water, the concentration of water molecules in the solution is less than that in pure water, because the addition of solute to water results in a solution that occupies a greater volume than does the water alone. If the solution is placed on one side of a membrane that is permeable to water but not to the solute, and an equal volume of water is placed on the other, water molecules diffuse down their concentration (chemical) gradient into the solution (Figure 1–3). This process—the diffusion of solvent molecules into a region in which there is a higher concentration of a solute to which the membrane is impermeable—is called osmosis. It is an important factor in physiologic processes. The tendency for movement of solvent molecules to a region of greater solute concentration can be prevented by applying pressure to the more concentrated solution. The pressure necessary to prevent solvent migration is the osmotic pressure of the solution. Osmotic pressure—like vapor pressure lowering, freezingpoint depression, and boiling-point elevation—depends on the number rather than the type of particles in a solution; that is, it is a fundamental colligative property of solutions. In an ideal solution, osmotic pressure (P) is related to temperature and volume in the same way as the pressure of a gas: nRT P = ---------V
where n is the number of particles, R is the gas constant, T is the absolute temperature, and V is the volume. If T is held constant, it is clear that the osmotic pressure is proportional to the number of particles in solution per unit volume of solution.
Semipermeable membrane
5
Pressure
FIGURE 1–3 Diagrammatic representation of osmosis. Water molecules are represented by small open circles, solute molecules by large solid circles. In the diagram on the left, water is placed on one side of a membrane permeable to water but not to solute, and an equal volume of a solution of the solute is placed on the other. Water molecules move down their concentration (chemical) gradient into the solution, and, as shown in the diagram on the right, the volume of the solution increases. As indicated by the arrow on the right, the osmotic pressure is the pressure that would have to be applied to prevent the movement of the water molecules. For this reason, the concentration of osmotically active particles is usually expressed in osmoles. One osmole (Osm) equals the gram-molecular weight of a substance divided by the number of freely moving particles that each molecule liberates in solution. For biological solutions, the milliosmole (mOsm; 1/1000 of 1 Osm) is more commonly used. If a solute is a nonionizing compound such as glucose, the osmotic pressure is a function of the number of glucose molecules present. If the solute ionizes and forms an ideal solution, each ion is an osmotically active particle. For example, NaCl would dissociate into Na+ and Cl– ions, so that each mole in solution would supply 2 Osm. One mole of Na2SO4 would dissociate into Na+, Na+, and SO42– supplying 3 Osm. However, the body fluids are not ideal solutions, and although the dissociation of strong electrolytes is complete, the number of particles free to exert an osmotic effect is reduced owing to interactions between the ions. Thus, it is actually the effective concentration (activity) in the body fluids rather than the number of equivalents of an electrolyte in solution that determines its osmotic capacity. This is why, for example, 1 mmol of NaCl per liter in the body fluids contributes somewhat less than 2 mOsm of osmotically active particles per liter. The more concentrated the solution, the greater the deviation from an ideal solution. The osmolal concentration of a substance in a fluid is measured by the degree to which it depresses the freezing point, with 1 mol of an ideal solution depressing the freezing point 1.86 °C. The number of milliosmoles per liter in a solution equals the freezing point depression divided by 0.00186. The osmolarity is the number of osmoles per liter of solution (eg, plasma), whereas the osmolality is the number of osmoles per kilogram of solvent. Therefore, osmolarity is affected by the volume of the various solutes in the solution and the temperature, while the osmolality is not. Osmotically active substances in the body are dissolved in water, and the density of water is 1, so osmolal concentrations can be expressed as osmoles per
6
SECTION I Cellular & Molecular Basis of Medical Physiology
liter (Osm/L) of water. In this book, osmolal (rather than osmolar) concentrations are considered, and osmolality is expressed in milliosmoles per liter (of water). Note that although a homogeneous solution contains osmotically active particles and can be said to have an osmotic pressure, it can exert an osmotic pressure only when it is in contact with another solution across a membrane permeable to the solvent but not to the solute.
OSMOLAL CONCENTRATION OF PLASMA: TONICITY The freezing point of normal human plasma averages –0.54 °C, which corresponds to an osmolal concentration in plasma of 290 mOsm/L. This is equivalent to an osmotic pressure against pure water of 7.3 atm. The osmolality might be expected to be higher than this, because the sum of all the cation and anion equivalents in plasma is over 300. It is not this high because plasma is not an ideal solution and ionic interactions reduce the number of particles free to exert an osmotic effect. Except when there has been insufficient time after a sudden change in composition for equilibrium to occur, all fluid compartments of the body are in (or nearly in) osmotic equilibrium. The term tonicity is used to describe the osmolality of a solution relative to plasma. Solutions that have the same osmolality as plasma are said to be isotonic; those with greater osmolality are hypertonic; and those with lesser osmolality are hypotonic. All solutions that are initially isosmotic with plasma (ie, that have the same actual osmotic pressure or freezing-point depression as plasma) would remain isotonic if it were not for the fact that some solutes diffuse into cells and others are metabolized. Thus, a 0.9% saline solution remains isotonic because there is no net movement of the osmotically active particles in the solution into cells and the particles are not metabolized. On the other hand, a 5% glucose solution is isotonic when initially infused intravenously, but glucose is metabolized, so the net effect is that of infusing a hypotonic solution. It is important to note the relative contributions of the various plasma components to the total osmolal concentration of plasma. All but about 20 of the 290 mOsm in each liter of normal plasma are contributed by Na+ and its accompanying anions, principally Cl– and HCO3–. Other cations and anions make a relatively small contribution. Although the concentration of the plasma proteins is large when expressed in grams per liter, they normally contribute less than 2 mOsm/L because of their very high molecular weights. The major nonelectrolytes of plasma are glucose and urea, which in the steady state are in equilibrium with cells. Their contributions to osmolality are normally about 5 mOsm/L each but can become quite large in hyperglycemia or uremia. The total plasma osmolality is important in assessing dehydration, overhydration, and other fluid and electrolyte abnormalities (Clinical Box 1–1).
CLINICAL BOX 1–1 Plasma Osmolality & Disease Unlike plant cells, which have rigid walls, animal cell membranes are flexible. Therefore, animal cells swell when exposed to extracellular hypotonicity and shrink when exposed to extracellular hypertonicity. Cells contain ion channels and pumps that can be activated to offset moderate changes in osmolality; however, these can be overwhelmed under certain pathologies. Hyperosmolality can cause coma (hyperosmolar coma). Because of the predominant role of the major solutes and the deviation of plasma from an ideal solution, one can ordinarily approximate the plasma osmolality within a few mosm/liter by using the following formula, in which the constants convert the clinical units to millimoles of solute per liter: Osmolality (mOsm/L) = 2[Na+] (mEq/L) + 0.055[Glucose] (mg/dL) + 0.36[BUN] (mg/dL) BUN is the blood urea nitrogen. The formula is also useful in calling attention to abnormally high concentrations of other solutes. An observed plasma osmolality (measured by freezing-point depression) that greatly exceeds the value predicted by this formula probably indicates the presence of a foreign substance such as ethanol, mannitol (sometimes injected to shrink swollen cells osmotically), or poisons such as ethylene glycol or methanol (components of antifreeze).
NONIONIC DIFFUSION Some weak acids and bases are quite soluble in cell membranes in the undissociated form, whereas they cannot cross membranes in the charged (ie, dissociated) form. Consequently, if molecules of the undissociated substance diffuse from one side of the membrane to the other and then dissociate, there is appreciable net movement of the undissociated substance from one side of the membrane to the other. This phenomenon is called nonionic diffusion.
DONNAN EFFECT When an ion on one side of a membrane cannot diffuse through the membrane, the distribution of other ions to which the membrane is permeable is affected in a predictable way. For example, the negative charge of a nondiffusible anion hinders diffusion of the diffusible cations and favors diffusion of the diffusible anions. Consider the following situation, X Y m K+
K+
Cl–
Cl–
Prot–
CHAPTER 1 General Principles & Energy Production in Medical Physiology in which the membrane (m) between compartments X and Y is impermeable to charged proteins (Prot–) but freely permeable to K+ and Cl–. Assume that the concentrations of the anions and of the cations on the two sides are initially equal. Cl– diffuses down its concentration gradient from Y to X, and some K+ moves with the negatively charged Cl– because of its opposite charge. Therefore [K+x] > [K+y] Furthermore, [K+x] + [Cl–x] + [Prot–x] > [K+y] + [Cl–y] that is, more osmotically active particles are on side X than on side Y. Donnan and Gibbs showed that in the presence of a nondiffusible ion, the diffusible ions distribute themselves so that at equilibrium their concentration ratios are equal: +
[K x] [K+y]
=
[Cl–
y] [Cl–x]
is the equilibrium potential. Its magnitude can be calculated from the Nernst equation, as follows: ECl =
This is the Gibbs–Donnan equation. It holds for any pair of cations and anions of the same valence. The Donnan effect on the distribution of ions has three effects in the body introduced here and discussed below. First, because of charged proteins (Prot–) in cells, there are more osmotically active particles in cells than in interstitial fluid, and because animal cells have flexible walls, osmosis would make them swell and eventually rupture if it were not for Na, K ATPase pumping ions back out of cells. Thus, normal cell volume and pressure depend on Na, K ATPase. Second, because at equilibrium the distribution of permeant ions across the membrane (m in the example used here) is asymmetric, an electrical difference exists across the membrane whose magnitude can be determined by the Nernst equation. In the example used here, side X will be negative relative to side Y. The charges line up along the membrane, with the concentration gradient for Cl– exactly balanced by the oppositely directed electrical gradient, and the same holds true for K+. Third, because there are more proteins in plasma than in interstitial fluid, there is a Donnan effect on ion movement across the capillary wall.
FORCES ACTING ON IONS The forces acting across the cell membrane on each ion can be analyzed mathematically. Chloride ions (Cl–) are present in higher concentration in the ECF than in the cell interior, and they tend to diffuse along this concentration gradient into the cell. The interior of the cell is negative relative to the exterior, and chloride ions are pushed out of the cell along this electrical gradient. An equilibrium is reached between Cl– influx and Cl– efflux. The membrane potential at which this equilibrium exists
RT FZCl
ln
[Clo–] [Cli–]
where ECl = equilibrium potential for Cl– R = gas constant T = absolute temperature F = the faraday (number of coulombs per mole of charge) ZCl = valence of Cl– (–1) [Clo–] = Cl– concentration outside the cell [Cli–] = Cl– concentration inside the cell Converting from the natural log to the base 10 log and replacing some of the constants with numerical values, the equation becomes: ECl = 61.5 log
Cross-multiplying, [K+x] + [Cl–x] = [K+y] + [Cl–y]
7
[Cli–] [Clo–]
at 37 °C
Note that in converting to the simplified expression the concentration ratio is reversed because the –1 valence of Cl– has been removed from the expression. The equilibrium potential for Cl– (ECl), calculated from the standard values listed in Table 1–1, is –70 mV, a value identical to the measured resting membrane potential of –70 mV. Therefore, no forces other than those represented by the chemical and electrical gradients need be invoked to explain the distribution of Cl– across the membrane. A similar equilibrium potential can be calculated for K+ (EK): EK =
RT
[Ko+]
[Ko+]
ln = 61.5log + at 37 °C FZK [Ki+] [Ki ]
where EK = equilibrium potential for K+ ZK = valence of K+ (+1) [Ko+] = K+ concentration outside the cell [Ki+] = K+ concentration inside the cell R, T, and F as above In this case, the concentration gradient is outward and the electrical gradient inward. In mammalian spinal motor neurons, EK is –90 mV (Table 1–1). Because the resting membrane potential is –70 mV, there is somewhat more K+ in the neurons than can be accounted for by the electrical and chemical gradients. The situation for Na+ is quite different from that for K+ and Cl–. The direction of the chemical gradient for Na+ is inward, to the area where it is in lesser concentration, and the electrical gradient is in the same direction. ENa is +60 mV (Table 1–1). Because neither EK nor ENa is equal to the membrane potential,
8
SECTION I Cellular & Molecular Basis of Medical Physiology
TABLE 1–1 Concentration of some ions inside and outside mammalian spinal motor neurons.
NH2
Concentration (mmol/L of H2O)
15.0
150.0
O−
+60
K+
150.0
5.5
–90
Cl–
9.0
125.0
–70
N
CH2
−O
P
O− O
P
O− O
P — —
Outside Cell
— —
Na+
Inside Cell
Adenine N
Equilibrium Potential (mV)
— —
Ion
N
N
O
O
O
O
O CH H C H H HO OH
Ribose
Adenosine 5'-monophosphate (AMP)
Resting membrane potential = –70 mV
Adenosine 5'-diphosphate (ADP)
one would expect the cell to gradually gain Na+ and lose K+ if only passive electrical and chemical forces were acting across the membrane. However, the intracellular concentration of Na+ and K+ remain constant because of the action of the Na, K ATPase that actively transports Na+ out of the cell and K+ into the cell (against their respective electrochemical gradients).
GENESIS OF THE MEMBRANE POTENTIAL The distribution of ions across the cell membrane and the nature of this membrane provide the explanation for the membrane potential. The concentration gradient for K+ facilitates its movement out of the cell via K+ channels, but its electrical gradient is in the opposite (inward) direction. Consequently, an equilibrium is reached in which the tendency of K+ to move out of the cell is balanced by its tendency to move into the cell, and at that equilibrium there is a slight excess of cations on the outside and anions on the inside. This condition is maintained by Na, K ATPase, which uses the energy of ATP to pump K+ back into the cell and keeps the intracellular concentration of Na+ low. Because the Na, K ATPase moves three Na+ out of the cell for every two K+ moved in, it also contributes to the membrane potential, and thus is termed an electrogenic pump. It should be emphasized that the number of ions responsible for the membrane potential is a minute fraction of the total number present and that the total concentrations of positive and negative ions are equal everywhere except along the membrane.
ENERGY PRODUCTION ENERGY TRANSFER Energy is stored in bonds between phosphoric acid residues and certain organic compounds. Because the energy of bond formation in some of these phosphates is particularly high, relatively large amounts of energy (10–12 kcal/mol) are released when the bond is hydrolyzed. Compounds containing such bonds are called high-energy phosphate compounds. Not all organic phosphates are of the high-energy type. Many, like glucose 6-phosphate, are low-energy phosphates that on
Adenosine 5'-triphosphate (ATP)
FIGURE 1–4
Energy-rich adenosine derivatives. Adenosine triphosphate is broken down into its backbone purine base and sugar (at right) as well as its high energy phosphate derivatives (across bottom). (Reproduced, with permission, from Murray RK et al: Harper’s Biochemistry,
26th ed. McGraw-Hill, 2003.)
hydrolysis liberate 2–3 kcal/mol. Some of the intermediates formed in carbohydrate metabolism are high-energy phosphates, but the most important high-energy phosphate compound is adenosine triphosphate (ATP). This ubiquitous molecule (Figure 1–4) is the energy storehouse of the body. On hydrolysis to adenosine diphosphate (ADP), it liberates energy directly to such processes as muscle contraction, active transport, and the synthesis of many chemical compounds. Loss of another phosphate to form adenosine monophosphate (AMP) releases more energy. Another group of high-energy compounds are the thioesters, the acyl derivatives of mercaptans. Coenzyme A (CoA) is a widely distributed mercaptan-containing adenine, ribose, pantothenic acid, and thioethanolamine (Figure 1–5). Reduced CoA (usually abbreviated HS–CoA) reacts with acyl groups (R–CO–) to form R–CO–S–CoA derivatives. A prime example is the reaction of HS-CoA with acetic acid to form acetylcoenzyme A (acetyl-CoA), a compound of pivotal importance in intermediary metabolism. Because acetyl-CoA has a much higher energy content than acetic acid, it combines readily with substances in reactions that would otherwise require outside energy. Acetyl-CoA is therefore often called “active acetate.” From the point of view of energetics, formation of 1 mol of any acyl-CoA compound is equivalent to the formation of 1 mol of ATP.
BIOLOGIC OXIDATIONS Oxidation is the combination of a substance with O2, or loss of hydrogen, or loss of electrons. The corresponding reverse processes are called reduction. Biologic oxidations are catalyzed by specific enzymes. Cofactors (simple ions) or coenzymes (organic, nonprotein substances) are accessory substances that
CHAPTER 1 General Principles & Energy Production in Medical Physiology
β-Alanine
Pantothenic acid H3C
OH
C
CH
CH2 O O
O
H N
C
Thioethanolamine
CH2
CH2
H N
C
CH2
CH2
SH
H3C
NH2 O− N
P
O
9
N Adenine
Pyrophosphate
O O
P
N
N CH2 O
O
O−
Coenzyme A
H H H
H OH
O −O
P
Ribose 3-phosphate
O
O
O
O−
R
C
OH + HS
CoA
C
R
S
CoA + HOH
FIGURE 1–5
Coenzyme A (CoA) and its derivatives. Left: Formula of reduced coenzyme A (HS-CoA) with its components highlighted. Right: Formula for reaction of CoA with biologically important compounds to form thioesters. R, remainder of molecule.
transferred to the flavoprotein–cytochrome system, reoxidizing the NAD+ and NADP+. Flavin adenine dinucleotide (FAD) is formed when riboflavin is phosphorylated, forming flavin mononucleotide (FMN). FMN then combines with AMP, forming the dinucleotide. FAD can accept hydrogens in a similar fashion, forming its hydro (FADH) and dihydro (FADH2) derivatives. The flavoprotein–cytochrome system is a chain of enzymes that transfers hydrogen to oxygen, forming water. This process occurs in the mitochondria. Each enzyme in the chain is reduced
usually act as carriers for products of the reaction. Unlike the enzymes, the coenzymes may catalyze a variety of reactions. A number of coenzymes serve as hydrogen acceptors. One common form of biologic oxidation is removal of hydrogen from an R–OH group, forming R=O. In such dehydrogenation reactions, nicotinamide adenine dinucleotide (NAD+) and dihydronicotinamide adenine dinucleotide phosphate (NADP+) pick up hydrogen, forming dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) (Figure 1–6). The hydrogen is then
NH2 N
N
H
OH* OH H
O
CH2O H
P
O
P — —
H
N
— —
N
CONH2
O−
OH
O
O
O H H
Adenine
Ribose H
Diphosphate H
FIGURE 1–6
OH OH Ribose
H Nicotinamide
CONH2
+ R'H2
R Oxidized coenzyme
H
H
CONH2 N+
+N
OCH2
+ H+ + R' N R Reduced coenzyme
Structures of molecules important in oxidation reduction reactions to produce energy. Top: Formula of the oxidized form of nicotinamide adenine dinucleotide (NAD +). Nicotinamide adenine dinucleotide phosphate (NADP +) has an additional phosphate group at the location marked by the asterisk. Bottom: Reaction by which NAD+ and NADP+ become reduced to form NADH and NADPH. R, remainder of molecule; R’, hydrogen donor.
10
SECTION I Cellular & Molecular Basis of Medical Physiology
H+
Outer lamella
Inner lamella
ATP
ADP
FIGURE 1–7
Simplified diagram of transport of protons across the inner and outer lamellas of the inner mitochondrial membrane. The electron transport system (flavoprotein-cytochrome system) helps create H+ movement from the inner to the outer lamella. Return movement of protons down the proton gradient generates ATP.
and then reoxidized as the hydrogen is passed down the line. Each of the enzymes is a protein with an attached nonprotein prosthetic group. The final enzyme in the chain is cytochrome c oxidase, which transfers hydrogens to O2, forming H2O. It contains two atoms of Fe and three of Cu and has 13 subunits. The principal process by which ATP is formed in the body is oxidative phosphorylation. This process harnesses the energy from a proton gradient across the mitochondrial membrane to produce the high-energy bond of ATP and is briefly outlined in Figure 1–7. Ninety percent of the O2 consumption in the basal state is mitochondrial, and 80% of this is coupled to ATP synthesis. About 27% of the ATP is used for protein synthesis, and about 24% is used by Na, K ATPase, 9% by gluconeogenesis, 6% by Ca2+ ATPase, 5% by myosin ATPase, and 3% by ureagenesis.
MOLECULAR BUILDING BLOCKS NUCLEOSIDES, NUCLEOTIDES, & NUCLEIC ACIDS Nucleosides contain a sugar linked to a nitrogen-containing base. The physiologically important bases, purines and pyrimidines, have ring structures (Figure 1–8). These structures are
N1 H C2
C 6
N 7
5C
Adenine:
6-Aminopurine
Guanine:
1-Amino6-oxypurine
8 CH 3 N
Hypoxanthine: 6-Oxypurine
Type of Compound
Components
H
Xanthine:
Nucleoside
Purine or pyrimidine plus ribose or 2-deoxyribose
Nucleotide (mononucleotide)
Nucleoside plus phosphoric acid residue
Nucleic acid
Many nucleotides forming double-helical structures of two polynucleotide chains
Nucleoprotein
Nucleic acid plus one or more simple basic proteins
Contain ribose
Ribonucleic acids (RNA)
Contain 2-deoxyribose
Deoxyribonucleic acids (DNA)
H
H
C2
C 4
1 N
2,6-Dioxypurine
Cytosine: 4-Amino2-oxypyrimidine 5C
H
6C
H
Uracil:
2,4-Dioxypyrimidine
Thymine: 5-Methyl2,4-dioxypyrimidine
Pyrimidine nucleus
FIGURE 1–8
TABLE 1–2 Purine- and pyrimidinecontaining compounds.
9 N
4C
Purine nucleus
N3
bound to ribose or 2-deoxyribose to complete the nucleoside. When inorganic phosphate is added to the nucleoside, a nucleotide is formed. Nucleosides and nucleotides form the backbone for RNA and DNA, as well as a variety of coenzymes and regulatory molecules (eg, NAD+, NADP+, and ATP) of physiological importance (Table 1–2). Nucleic acids in the diet are digested and their constituent purines and pyrimidines absorbed, but most of the purines and pyrimidines are synthesized from amino acids, principally in the liver. The nucleotides and RNA and DNA are then synthesized. RNA is in dynamic equilibrium with the amino acid pool, but DNA, once formed, is metabolically stable throughout life. The purines and pyrimidines released by the breakdown of nucleotides may be reused or catabolized. Minor amounts are excreted unchanged in the urine. The pyrimidines are catabolized to the β-amino acids, βalanine and β-aminoisobutyrate. These amino acids have their amino group on β-carbon, rather than the α-carbon typical to physiologically active amino acids. Because β-aminoisobutyrate is a product of thymine degradation, it can serve as a measure of DNA turnover. The β-amino acids are further degraded to CO2 and NH3. Uric acid is formed by the breakdown of purines and by direct synthesis from 5-phosphoribosyl pyrophosphate (5PRPP) and glutamine (Figure 1–9). In humans, uric acid is excreted in the urine, but in other mammals, uric acid is further oxidized to allantoin before excretion. The normal blood uric acid level in humans is approximately 4 mg/dL (0.24 mmol/L). In the kidney, uric acid is filtered, reabsorbed, and secreted. Normally, 98% of the filtered uric acid is reabsorbed and the remaining 2% makes up approximately 20% of the amount excreted. The remaining 80% comes from the tubular secretion. The uric acid excretion on a purine-free diet is about 0.5 g/24 h and on a regular diet about 1 g/24 h. Excess uric acid in the blood or urine is a characteristic of gout (Clinical Box 1–2).
Principal physiologically important purines and pyrimidines. Purine and pyrimidine structures are shown next to representative molecules from each group. Oxypurines and oxypyrimidines may form enol derivatives (hydroxypurines and hydroxypyrimidines) by migration of hydrogen to the oxygen substituents.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
Adenosine
Guanosine
CLINICAL BOX 1–2
Hypoxanthine 5-PRPP + Glutamine
Xanthine oxidase Xanthine O Xanthine oxidase C HN
NH C C
O
C
O
C N H
NH
Uric acid (excreted in humans)
O NH H2N
C
C
C H
C O
N H
11
O
NH
Allantoin (excreted in other mammals)
FIGURE 1–9
Synthesis and breakdown of uric acid. Adenosine is converted to hypoxanthine, which is then converted to xanthine, and xanthine is converted to uric acid. The latter two reactions are both catalyzed by xanthine oxidase. Guanosine is converted directly to xanthine, while 5-PRPP and glutamine can be converted to uric acid. An additional oxidation of uric acid to allantoin occurs in some mammals.
Gout Gout is a disease characterized by recurrent attacks of arthritis; urate deposits in the joints, kidneys, and other tissues; and elevated blood and urine uric acid levels. The joint most commonly affected initially is the metatarsophalangeal joint of the great toe. There are two forms of “primary” gout. In one, uric acid production is increased because of various enzyme abnormalities. In the other, there is a selective deficit in renal tubular transport of uric acid. In “secondary” gout, the uric acid levels in the body fluids are elevated as a result of decreased excretion or increased production secondary to some other disease process. For example, excretion is decreased in patients treated with thiazide diuretics and those with renal disease. Production is increased in leukemia and pneumonia because of increased breakdown of uric acid-rich white blood cells. The treatment of gout is aimed at relieving the acute arthritis with drugs such as colchicine or nonsteroidal anti-inflammatory agents and decreasing the uric acid level in the blood. Colchicine does not affect uric acid metabolism, and it apparently relieves gouty attacks by inhibiting the phagocytosis of uric acid crystals by leukocytes, a process that in some way produces the joint symptoms. Phenylbutazone and probenecid inhibit uric acid reabsorption in the renal tubules. Allopurinol, which directly inhibits xanthine oxidase in the purine degradation pathway, is one of the drugs used to decrease uric acid production.
DNA Deoxyribonucleic acid (DNA) is found in bacteria, in the nuclei of eukaryotic cells, and in mitochondria. It is made up of two extremely long nucleotide chains containing the bases adenine (A), guanine (G), thymine (T), and cytosine (C) (Figure 1–10). The chains are bound together by hydrogen bonding between the bases, with adenine bonding to thymine and guanine to cytosine. This stable association forms a double-helical structure (Figure 1–11). The double helical structure of DNA is compacted in the cell by association with histones, and further compacted into chromosomes. A diploid human cell contains 46 chromosomes. A fundamental unit of DNA, or a gene, can be defined as the sequence of DNA nucleotides that contain the information for the production of an ordered amino acid sequence for a single polypeptide chain. Interestingly, the protein encoded by a single gene may be subsequently divided into several different physiologically active proteins. Information is accumulating at an accelerating rate about the structure of genes and their regulation. The basic structure of a typical eukaryotic gene is shown in diagrammatic form in Figure 1–12. It is made up of a strand of DNA that includes coding and noncoding regions. In eukaryotes, unlike prokaryotes, the portions of the genes that
dictate the formation of proteins are usually broken into several segments (exons) separated by segments that are not translated (introns). Near the transcription start site of the gene is a promoter, which is the site at which RNA polymerase and its cofactors bind. It often includes a thymidine–adenine–thymidine–adenine (TATA) sequence (TATA box), which ensures that transcription starts at the proper point. Farther out in the 5' region are regulatory elements, which include enhancer and silencer sequences. It has been estimated that each gene has an average of five regulatory sites. Regulatory sequences are sometimes found in the 3'-flanking region as well. Gene mutations occur when the base sequence in the DNA is altered from its original sequence. Such alterations can affect protein structure and be passed on to daughter cells after cell division. Point mutations are single base substitutions. A variety of chemical modifications (eg, alkylating or intercalating agents, or ionizing radiation) can lead to changes in DNA sequences and mutations. The collection of genes within the full expression of DNA from an organism is termed its genome. An indication of the complexity of DNA in the human haploid genome (the total genetic message) is its size; it is made up of 3 × 109 base pairs that can code for approximately 30,000 genes. This genetic message is the blueprint for
12
SECTION I Cellular & Molecular Basis of Medical Physiology
NH2 Phosphate
NH2 Phosphate
N Base (cytosine)
O –
O
P
O
CH2
N
C H
A
O
–O
O
O– H
N Base (cytosine)
O P
O
CH2 O
O– C
H
C
C
OH
H
O
N
C
Sugar (deoxyribose) H
H
Typical deoxyribonucleotide
H
H
C
C
OH
OH
C Sugar (ribose) H
Typical ribonucleotide
Phosphate
NH2 N
O O P O CH2 O–
N
N
Adenine (DNA and RNA)
N
O
O Sugar
N
O Nucleotide
HN Guanine (DNA and RNA)
O P O CH2 O–
N
NH2
N
O
NH2 N
O
Cytosine (DNA and RNA)
O P O CH2 O–
O
N O
O CH3
NH
O O P O CH2 O–
N
Thymine (DNA only)
O
O O NH
O O P O CH2 O–
Uracil (RNA only)
N
O
O
B
FIGURE 1–10 Basic structure of nucleotides and nucleic acids. A) At left, the nucleotide cytosine is shown with deoxyribose and at right with ribose as the principal sugar. B) Purine bases adenine and guanine are bound to each other or to pyrimidine bases, cytosine, thymine, or uracil via a phosphodiester backbone between 2'-deoxyribosyl moieties attached to the nucleobases by an N-glycosidic bond. Note that the backbone has a polarity (ie, a 5' and a 3' direction). Thymine is only found in DNA, while the uracil is only found in RNA.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
13
REPLICATION: MITOSIS & MEIOSIS G
C T
A T G
C A
A
T
Minor groove G C
3.4 nm G
C A
T
Major groove
T
A
2.0 nm
FIGURE 1–11
Double-helical structure of DNA. The compact structure has an approximately 2.0 nm thickness and 3.4 nm between full turns of the helix that contains both major and minor grooves. The structure is maintained in the double helix by hydrogen bonding between purines and pyrimidines across individual strands of DNA. Adenine (A) is bound to thymine (T) and cytosine (C) to guanine (G).
(Reproduced with permission from Murray RK et al: Harper’s Biochemistry, 26th ed. McGraw-Hill, 2003.)
the heritable characteristics of the cell and its descendants. The proteins formed from the DNA blueprint include all the enzymes, and these in turn control the metabolism of the cell. Each nucleated somatic cell in the body contains the full genetic message, yet there is great differentiation and specialization in the functions of the various types of adult cells. Only small parts of the message are normally transcribed. Thus, the genetic message is normally maintained in a repressed state. However, genes are controlled both spatially and temporally. First, under physiological conditions, the double helix requires highly regulated interaction by proteins to unravel for replication, transcription, or both.
Regulatory region
Basal promoter region
At the time of each somatic cell division (mitosis), the two DNA chains separate, each serving as a template for the synthesis of a new complementary chain. DNA polymerase catalyzes this reaction. One of the double helices thus formed goes to one daughter cell and one goes to the other, so the amount of DNA in each daughter cell is the same as that in the parent cell. The life cycle of the cell that begins after mitosis is highly regulated and is termed the cell cycle (Figure 1–13). The G1 (or Gap 1) phase represents a period of cell growth and divides the end of mitosis from the DNA synthesis (or S) phase. Following DNA synthesis, the cell enters another period of cell growth, the G2 (Gap 2) phase. The ending of this stage is marked by chromosome condensation and the beginning of mitosis (M stage). In germ cells, reduction division (meiosis) takes place during maturation. The net result is that one of each pair of chromosomes ends up in each mature germ cell; consequently, each mature germ cell contains half the amount of chromosomal material found in somatic cells. Therefore, when a sperm unites with an ovum, the resulting zygote has the full complement of DNA, half of which came from the father and half from the mother. The term “ploidy” is sometimes used to refer to the number of chromosomes in cells. Normal resting diploid cells are euploid and become tetraploid just before division. Aneuploidy is the condition in which a cell contains other than the haploid number of chromosomes or an exact multiple of it, and this condition is common in cancerous cells.
RNA The strands of the DNA double helix not only replicate themselves, but also serve as templates by lining up complementary bases for the formation in the nucleus of ribonucleic acids (RNA). RNA differs from DNA in that it is single-stranded, has uracil in place of thymine, and its sugar moiety is ribose rather than 2'-deoxyribose (Figure 1–13). The production of RNA from DNA is called transcription. Transcription can lead to several types of RNA including: messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), and other RNAs. Transcription is catalyzed by various forms of RNA polymerase. Poly(A) addition site
Transcription start site Exon
DNA
5'
CAAT
AATAAA 5' Noncoding region
FIGURE 1–12
Exon
TATA Intron
3'
3' Noncoding region
Diagram of the components of a typical eukaryotic gene. The region that produces introns and exons is flanked by noncoding regions. The 5'-flanking region contains stretches of DNA that interact with proteins to facilitate or inhibit transcription. The 3'-flanking region contains the poly(A) addition site. (Modified from Murray RK et al: Harper’s Biochemistry, 26th ed. McGraw-Hill, 2003.)
14
SECTION I Cellular & Molecular Basis of Medical Physiology
hase
sis
Telo p Cy
tok
P
ine
p ro
ha
se
Met
aph
ase
Anaphase
Mitotic phase
Mitosis
G2 Final growth and activity before mitosis
G1 Centrioles replicate
S DNA replication
Interphase
FIGURE 1–13
Sequence of events during the cell cycle. Immediately following mitosis (M) the cell enters a gap phase (G1) before a DNA synthesis phase (S) a second gap phase (G2) and back to mitosis. Collectively G1, S, and G2 phases are referred to as interphase (I).
Typical transcription of an mRNA is shown in Figure 1–14. When suitably activated, transcription of the gene into a premRNA starts at the cap site and ends about 20 bases beyond the AATAAA sequence. The RNA transcript is capped in the nucleus by addition of 7-methylguanosine triphosphate to the 5' end; this cap is necessary for proper binding to the ribosome. A poly(A) tail of about 100 bases is added to the untranslated segment at the 3' end to help maintain the stability of the mRNA. The pre-mRNA formed by capping and addition of the poly(A) tail is then processed by elimination of the introns, and once this posttranscriptional modification is complete, the mature mRNA moves to the cytoplasm. Posttranscriptional modification of the pre-mRNA is a regulated process where
differential splicing can occur to form more than one mRNA from a single pre-mRNA. The introns of some genes are eliminated by spliceosomes, complex units that are made up of small RNAs and proteins. Other introns are eliminated by selfsplicing by the RNA they contain. Because of introns and splicing, more than one mRNA can be formed from the same gene. Most forms of RNA in the cell are involved in translation, or protein synthesis. A brief outline of the transition from transcription to translation is shown in Figure 1–15. In the cytoplasm, ribosomes provide a template for tRNA to deliver specific amino acids to a growing polypeptide chain based on specific sequences in mRNA. The mRNA molecules are smaller than the DNA molecules, and each represents a
CHAPTER 1 General Principles & Energy Production in Medical Physiology
Flanking DNA
Introns
AMINO ACIDS & PROTEINS
Exons
AMINO ACIDS
Gene
Transcription PremRNA
15
Flanking DNA
Cap
Poly(A)
RNA processing
Poly(A)
mRNA
Poly(A) Translation
FIGURE 1–14
Transcription of a typical mRNA. Steps in transcription from a typical gene to a processed mRNA are shown. Cap, cap site. (Modified from Baxter JD: Principles of endocrinology. In: Cecil Textbook of
Medicine, 16th ed. Wyngaarden JB, Smith LH Jr (editors). Saunders, 1982.)
transcript of a small segment of the DNA chain. For comparison, the molecules of tRNA contain only 70–80 nitrogenous bases, compared with hundreds in mRNA and 3 billion in DNA.
Amino acids that form the basic building blocks for proteins are identified in Table 1–3. These amino acids are often referred to by their corresponding three-letter, or single-letter abbreviations. Various other important amino acids such as ornithine, 5-hydroxytryptophan, L-dopa, taurine, and thyroxine (T4) occur in the body but are not found in proteins. In higher animals, the L isomers of the amino acids are the only naturally occurring forms in proteins. The L isomers of hormones such as thyroxine are much more active than the D isomers. The amino acids are acidic, neutral, or basic in reaction, depending on the relative proportions of free acidic (–COOH) or basic (–NH2) groups in the molecule. Some of the amino acids are nutritionally essential amino acids, that is, they must be obtained in the diet, because they cannot be made in the body. Arginine and histidine must be provided through diet during times of rapid growth or recovery from illness and are termed conditionally essential. All others are nonessential amino acids in the sense that they can be synthesized in vivo in amounts sufficient to meet metabolic needs.
DNA RNA strand formed on DNA strand (transcription) tRNA adenylate
Amino acid Posttranscriptional modification
Chain separation Activating enzyme Messenger RNA Coding triplets for A3 A4
A
2
A1
Translation
Posttranslational modification
Ribosome
tRNA-amino acid-adenylate complex
FIGURE 1–15
A4
A3 A2 A1 Peptide chain
Diagrammatic outline of transcription to translation. From the DNA molecule, a messenger RNA is produced and presented to the ribosome. It is at the ribosome where charged tRNA match up with their complementary codons of mRNA to position the amino acid for growth of the polypeptide chain. DNA and RNA are represented as lines with multiple short projections representing the individual bases . Small boxes labeled A represent individual amino acids.
16
SECTION I Cellular & Molecular Basis of Medical Physiology
TABLE 1–3 Amino acids found in proteins.* Amino acids with aliphatic side chains
Amino acids with acidic side chains, or their amides
Alanine (Ala, A)
Aspartic acid (Asp, D)
Valine (Val, V)
Asparagine (Asn, N)
Leucine (Leu, L)
Glutamine (Gln, Q)
Isoleucine (IIe, I)
Glutamic acid (Glu, E)
Hydroxyl-substituted amino acids Serine (Ser, S) Threonine (Thr, T) Sulfur-containing amino acids
γ-Carboxyglutamic acidb (Gla) Amino acids with side chains containing basic groups Argininec (Arg, R) Lysine (Lys, K)
Cysteine (Cys, C)
Hydroxylysineb (Hyl)
Methionine (Met, M)
Histidinec (His, H)
Selenocysteinea Amino acids with aromatic ring side chains
Imino acids (contain imino group but no amino group) Proline (Pro, P)
Phenylalanine (Phe, F)
4-Hydroxyprolineb (Hyp)
Tyrosine (Tyr, Y)
3-Hydroxyprolineb
Tryptophan (Trp, W) *Those in bold type are the nutritionally essential amino acids. The generally accepted three-letter and one-letter abbreviations for the amino acids are shown in parentheses. a
Selenocysteine is a rare amino acid in which the sulfur of cysteine is replaced by selenium. The codon UGA is usually a stop codon, but in certain situations it codes for selenocysteine.
b
There are no tRNAs for these four amino acids; they are formed by post-translational modification of the corresponding unmodified amino acid in peptide linkage. There are tRNAs for selenocysteine and the remaining 20 amino acids, and they are incorporated into peptides and proteins under direct genetic control.
c
Arginine and histidine are sometimes called “conditionally essential”—they are not necessary for maintenance of nitrogen balance, but are needed for normal growth.
THE AMINO ACID POOL Although small amounts of proteins are absorbed from the gastrointestinal tract and some peptides are also absorbed, most ingested proteins are digested and their constituent amino acids absorbed. The body’s own proteins are being continuously hydrolyzed to amino acids and resynthesized. The turnover rate of endogenous proteins averages 80–100 g/d, being highest in the intestinal mucosa and practically nil in the extracellular structural protein, collagen. The amino acids formed by endogenous protein breakdown are identical to those derived from ingested protein. Together they form a common amino acid pool that supplies the needs of the body (Figure 1–16).
this text, amino acid chains containing 2–10 amino acid residues are called peptides, chains containing more than 10 but fewer than 100 amino acid residues are called polypeptides, and chains containing 100 or more amino acid residues are called proteins.
Amino acid pool
Urinary excretion
Creatine
PROTEINS
Body protein
Diet
Inert protein (hair, etc)
Transamination Amination Deamination
Common metabolic pool +
NH4 Purines, Hormones, pyrimidines neurotransmitters
Urea
Proteins are made up of large numbers of amino acids linked into chains by peptide bonds joining the amino group of one amino acid to the carboxyl group of the next (Figure 1–17). In addition, some proteins contain carbohydrates (glycoproteins) and lipids (lipoproteins). Smaller chains of amino acids are called peptides or polypeptides. The boundaries between peptides, polypeptides, and proteins are not well defined. For
FIGURE 1–16
Amino acids in the body. There is an extensive network of amino acid turnover in the body. Boxes represent large pools of amino acids and some of the common interchanges are represented by arrows. Note that most amino acids come from the diet and end up in protein, however, a large portion of amino acids are interconverted and can feed into and out of a common metabolic pool through amination reactions.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
H
O
R
H H
N
C C
H C
OH
O
H
H
17
H–N
C
N C
C
O
R
H R Amino acid
Polypeptide chain
FIGURE 1–17
Amino acid structure and formation of peptide bonds. The dashed line shows where peptide bonds are formed between two amino acids. The highlighted area is released as H 2O. R, remainder of the amino acid. For example, in glycine, R = H; in glutamate, R = —(CH2)2—COO–.
The order of the amino acids in the peptide chains is called the primary structure of a protein. The chains are twisted and folded in complex ways, and the term secondary structure of a protein refers to the spatial arrangement produced by the twisting and folding. A common secondary structure is a regular coil with 3.7 amino acid residues per turn (α-helix). Another common secondary structure is a β-sheet. An antiparallel β-sheet is formed when extended polypeptide chains fold back and forth on one another and hydrogen bonding occurs between the peptide bonds on neighboring chains. Parallel β-sheets between polypeptide chains also occur. The tertiary structure of a protein is the arrangement of the twisted chains into layers, crystals, or fibers. Many protein molecules are made of several proteins, or subunits (eg, hemoglobin), and the term quaternary structure is used to refer to the arrangement of the subunits into a functional structure.
PROTEIN SYNTHESIS The process of protein synthesis, translation, is the conversion of information encoded in mRNA to a protein (Figure 1–15). As described previously, when a definitive mRNA reaches a ribosome in the cytoplasm, it dictates the formation of a polypeptide chain. Amino acids in the cytoplasm are activated by combination with an enzyme and adenosine monophosphate (adenylate), and each activated amino acid then combines with a specific molecule of tRNA. There is at least one tRNA for each of the 20 unmodified amino acids found in large quantities in the body proteins of animals, but some amino acids have more than one tRNA. The tRNA–amino acid–adenylate complex is next attached to the mRNA template, a process that occurs in the ribosomes. The tRNA “recognizes” the proper spot to attach on the mRNA template because it has on its active end a set of three bases that are complementary to a set of three bases in a particular spot on the mRNA chain. The genetic code is made up of such triplets (codons), sequences of three purine, pyrimidine, or purine and pyrimidine bases; each codon stands for a particular amino acid. Translation typically starts in the ribosomes with an AUG (transcribed from ATG in the gene), which codes for methionine. The amino terminal amino acid is then added, and the chain is lengthened one amino acid at a time. The mRNA attaches to the 40S subunit of the ribosome during protein
synthesis, the polypeptide chain being formed attaches to the 60S subunit, and the tRNA attaches to both. As the amino acids are added in the order dictated by the codon, the ribosome moves along the mRNA molecule like a bead on a string. Translation stops at one of three stop, or nonsense, codons (UGA, UAA, or UAG), and the polypeptide chain is released. The tRNA molecules are used again. The mRNA molecules are typically reused approximately 10 times before being replaced. It is common to have more than one ribosome on a given mRNA chain at a time. The mRNA chain plus its collection of ribosomes is visible under the electron microscope as an aggregation of ribosomes called a polyribosome.
POSTTRANSLATIONAL MODIFICATION After the polypeptide chain is formed, it “folds” into its biological form and can be further modified to the final protein by one or more of a combination of reactions that include hydroxylation, carboxylation, glycosylation, or phosphorylation of amino acid residues; cleavage of peptide bonds that converts a larger polypeptide to a smaller form; and the further folding, packaging, or folding and packaging of the protein into its ultimate, often complex configuration. Protein folding is a complex process that is dictated primarily by the sequence of the amino acids in the polypeptide chain. In some instances, however, nascent proteins associate with other proteins called chaperones, which prevent inappropriate contacts with other proteins and ensure that the final “proper” conformation of the nascent protein is reached. Proteins also contain information that helps to direct them to individual cell compartments. Many proteins that are going to be secreted or stored in organelles and most transmembrane proteins have at their amino terminal a signal peptide (leader sequence) that guides them into the endoplasmic reticulum. The sequence is made up of 15 to 30 predominantly hydrophobic amino acid residues. The signal peptide, once synthesized, binds to a signal recognition particle (SRP), a complex molecule made up of six polypeptides and 7S RNA, one of the small RNAs. The SRP stops translation until it binds to a translocon, a pore in the endoplasmic reticulum that is a heterotrimeric structure made up of Sec 61 proteins. The ribosome also binds, and the signal peptide leads the growing peptide chain into the cavity of the endoplasmic reticulum (Figure 1–18). The signal
18
SECTION I Cellular & Molecular Basis of Medical Physiology
CATABOLISM OF AMINO ACIDS
5' 3' N SRP
UAA N
N
N C
N C
N
C N
C N
FIGURE 1–18
Translation of protein into endoplasmic reticulum according to the signal hypothesis. The ribosomes synthesizing a protein move along the mRNA from the 5' to the 3' end. When the signal peptide of a protein destined for secretion, the cell membrane, or lysosomes emerges from the large unit of the ribosome, it binds to a signal recognition particle (SRP), and this arrests further translation until it binds to the translocon on the endoplasmic reticulum. N, amino end of protein; C, carboxyl end of protein. (Reproduced,
with permission, from Perara E, Lingappa VR: Transport of proteins into and across the endoplasmic reticulum membrane. In: Protein Transfer and Organelle Biogenesis. Das RC, Robbins PW (editors). Academic Press, 1988.)
peptide is next cleaved from the rest of the peptide by a signal peptidase while the rest of the peptide chain is still being synthesized. SRPs are not the only signals that help to direct proteins to their proper place in or out of the cell; other signal sequences, posttranslational modifications, or both (eg, glycosylation) can serve this function.
PROTEIN DEGRADATION Like protein synthesis, protein degradation is a carefully regulated, complex process. It has been estimated that overall, up to 30% of newly produced proteins are abnormal, such as can occur during improper folding. Aged normal proteins also need to be removed as they are replaced. Conjugation of proteins to the 74-amino-acid polypeptide ubiquitin marks them for degradation. This polypeptide is highly conserved and is present in species ranging from bacteria to humans. The process of binding ubiquitin is called ubiquitination, and in some instances, multiple ubiquitin molecules bind (polyubiquitination). Ubiquitination of cytoplasmic proteins, including integral proteins of the endoplasmic reticulum, marks the proteins for degradation in multisubunit proteolytic particles, or proteasomes. Ubiquitination of membrane proteins, such as the growth hormone receptors, also marks them for degradation, however these can be degraded in lysosomes as well as via the proteasomes. There is an obvious balance between the rate of production of a protein and its destruction, so ubiquitin conjugation is of major importance in cellular physiology. The rates at which individual proteins are metabolized vary, and the body has mechanisms by which abnormal proteins are recognized and degraded more rapidly than normal body constituents. For example, abnormal hemoglobins are metabolized rapidly in individuals with congenital hemoglobinopathies.
The short-chain fragments produced by amino acid, carbohydrate, and fat catabolism are very similar (see below). From this common metabolic pool of intermediates, carbohydrates, proteins, and fats can be synthesized. These fragments can enter the citric acid cycle, a final common pathway of catabolism, in which they are broken down to hydrogen atoms and CO2. Interconversion of amino acids involve transfer, removal, or formation of amino groups. Transamination reactions, conversion of one amino acid to the corresponding keto acid with simultaneous conversion of another keto acid to an amino acid, occur in many tissues: Alanine + α-Ketoglutarate ← → Pyruvate + Glutamate The transaminases involved are also present in the circulation. When damage to many active cells occurs as a result of a pathologic process, serum transaminase levels rise. An example is the rise in plasma aspartate aminotransferase (AST) following myocardial infarction. Oxidative deamination of amino acids occurs in the liver. An imino acid is formed by dehydrogenation, and this compound is hydrolyzed to the corresponding keto acid, with production of NH4+: Amino acid + NAD+ → Imino acid + NADH + H+ Imino acid + H2O → Keto acid + NH4+ Interconversions between the amino acid pool and the common metabolic pool are summarized in Figure 1–19. Leucine, isoleucine, phenylalanine, and tyrosine are said to be ketogenic because they are converted to the ketone body acetoacetate (see below). Alanine and many other amino acids are glucogenic or gluconeogenic; that is, they give rise to compounds that can readily be converted to glucose.
UREA FORMATION Most of the NH4+ formed by deamination of amino acids in the liver is converted to urea, and the urea is excreted in the urine. The NH4+ forms carbamoyl phosphate, and in the mitochondria it is transferred to ornithine, forming citrulline. The enzyme involved is ornithine carbamoyltransferase. Citrulline is converted to arginine, after which urea is split off and ornithine is regenerated (urea cycle; Figure 1–20). The overall reaction in the urea cycle consumes 3 ATP (not shown) and thus requires significant energy. Most of the urea is formed in the liver, and in severe liver disease the blood urea nitrogen (BUN) falls and blood NH3 rises (see Chapter 29). Congenital deficiency of ornithine carbamoyltransferase can also lead to NH3 intoxication, even in individuals who are heterozygous for this deficiency.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
Hydroxyproline Serine Cysteine Threonine Glycine Tryptophan
19
Lactate
Transaminase Alanine
Acetyl-CoA
Pyruvate
Phosphoenolpyruvate carboxykinase Glucose
Phosphoenolpyruvate
Tyrosine Phenylalanine
Oxaloacetate
Fumarate
Transaminase
Aspartate Citrate Isoleucine Methionine Valine
Succinyl-CoA CO2 α-Ketoglutarate
Propionate Histidine Proline Glutamine Arginine
CO2
Transaminase Glutamate
FIGURE 1–19 Involvement of the citric acid cycle in transamination and gluconeogenesis. The bold arrows indicate the main pathway of gluconeogenesis. Note the many entry positions for groups of amino acids into the citric acid cycle. (Reproduced with permission from Murray RK et al: Harper’s Biochemistry, 26th ed. McGraw-Hill, 2003.)
METABOLIC FUNCTIONS OF AMINO ACIDS In addition to providing the basic building blocks for proteins, amino acids also have metabolic functions. Thyroid hormones, catecholamines, histamine, serotonin, melatonin, and intermediates in the urea cycle are formed from specific amino acids. Methionine and cysteine provide the sulfur contained in proteins, CoA, taurine, and other biologically important compounds. Methionine is converted into S-adenosylmethionine, which is the active methylating agent in the synthesis of compounds such as epinephrine.
CARBOHYDRATES Carbohydrates are organic molecules made of equal amounts of carbon and H2O. The simple sugars, or monosaccharides, including pentoses (5 carbons; eg, ribose) and hexoses (6 carbons; eg, glucose) perform both structural (eg, as part of nucleotides discussed previously) and functional roles (eg, inositol 1,4,5 trisphosphate acts as a cellular signaling molecules) in the body. Monosaccharides can be linked together to form disaccharides (eg, sucrose), or polysaccharides (eg, glycogen). The placement of sugar moieties onto proteins (glycoproteins) aids in cellular targeting, and in the case of some
Argininosuccinate Aspartate
Fumarate Cyto
H2N
H2N + C— — NH2
C— —O HN (CH 2 ) 3 HC
HN
Citrulline + NO
Arginine (CH 2 ) 3
+
NH3
HC Ornithine
COO − Pi
FIGURE 1–20
(CH 2 ) 3 HC
Carbamoyl phosphate NH 4+
COO −
H3N + Mito
Urea +
NH3
COO − NH3
NH3+
NH 2 —O C—
NH 2
Urea cycle. The processing of NH3 to urea for excretion contains several coordinative steps in both the cytoplasm (Cyto) and the mitochondria (Mito). The production of carbamoyl phosphate and its conversion to citrulline occurs in the mitochondria, whereas other processes are in the cytoplasm.
SECTION I Cellular & Molecular Basis of Medical Physiology
HO C
O
H C
OH
H C
O
CH2OH
OH
C
— —
H C
— —
H C
— —
20
O
H
HO C
H
HO C
H C
OH
HO C
H
H C
OH
H C
OH
H C
OH
H C
OH
CH2OH D-Glucose
CH2OH D-Galactose
H
CH2OH D-Fructose
FIGURE 1–21
Structures of principal dietary hexoses. Glucose, galactose, and fructose are shown in their naturally occurring D isomers.
receptors, recognition of signaling molecules. In this section we will discuss a major role for carbohydrates in physiology, the production and storage of energy. Dietary carbohydrates are for the most part polymers of hexoses, of which the most important are glucose, galactose, and fructose (Figure 1–21). Most of the monosaccharides occurring in the body are the D isomers. The principal product of carbohydrate digestion and the principal circulating sugar is glucose. The normal fasting level of plasma glucose in peripheral venous blood is 70 to 110 mg/dL (3.9–6.1 mmol/ L). In arterial blood, the plasma glucose level is 15 to 30 mg/ dL higher than in venous blood. Once it enters the cells, glucose is normally phosphorylated to form glucose 6-phosphate. The enzyme that catalyzes this reaction is hexokinase. In the liver, there is an additional enzyme called glucokinase, which has greater specificity for glucose and which, unlike hexokinase, is increased by insulin and decreased in starvation and diabetes. The glucose 6-phosphate is either polymerized into glycogen or catabolized. The process of glycogen formation is called glycogenesis, and glycogen breakdown is called glycogenolysis. Glycogen, the storage form of glucose, is present in most body tissues, but the major supplies are in the liver and skeletal muscle. The breakdown of glucose to pyruvate or lactate (or both) is called glycolysis. Glucose catabolism proceeds via cleavage through fructose to trioses or via oxidation and decarboxylation to pentoses. The pathway to pyruvate through the trioses is the Embden–Meyerhof pathway, and that through 6-phosphogluconate and the pentoses is the direct oxidative pathway (hexose monophosphate shunt). Pyruvate is converted to acetyl-CoA. Interconversions between carbohydrate, fat, and protein include conversion of the glycerol from fats to dihydroxyacetone phosphate and conversion of a number of amino acids with carbon skeletons resembling intermediates in the Embden–Meyerhof pathway and citric acid cycle to these intermediates by deamination. In this way, and by conversion of lactate to glucose, nonglucose molecules can be converted to glucose (gluconeogenesis). Glucose can be converted to fats through acetyl-CoA, but because the conversion of pyruvate to acetyl-CoA, unlike most reactions in glycolysis, is irreversible, fats are not converted to glucose via this pathway. There is therefore very little net conversion of fats to carbohydrates in
the body because, except for the quantitatively unimportant production from glycerol, there is no pathway for conversion.
CITRIC ACID CYCLE The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a sequence of reactions in which acetyl-CoA is metabolized to CO2 and H atoms. Acetyl-CoA is first condensed with the anion of a four-carbon acid, oxaloacetate, to form citrate and HS-CoA. In a series of seven subsequent reactions, 2CO2 molecules are split off, regenerating oxaloacetate (Figure 1–22). Four pairs of H atoms are transferred to the flavoprotein– cytochrome chain, producing 12ATP and 4H2O, of which 2H2O is used in the cycle. The citric acid cycle is the common pathway for oxidation to CO2 and H2O of carbohydrate, fat, and some amino acids. The major entry into it is through acetylCoA, but a number of amino acids can be converted to citric acid cycle intermediates by deamination. The citric acid cycle requires O2 and does not function under anaerobic conditions.
ENERGY PRODUCTION The net production of energy-rich phosphate compounds during the metabolism of glucose and glycogen to pyruvate depends on whether metabolism occurs via the Embden– Meyerhof pathway or the hexose monophosphate shunt. By oxidation at the substrate level, the conversion of 1 mol of phosphoglyceraldehyde to phosphoglycerate generates 1 mol of ATP, and the conversion of 1 mol of phosphoenolpyruvate to pyruvate generates another. Because 1 mol of glucose 6phosphate produces, via the Embden–Meyerhof pathway, 2 mol of phosphoglyceraldehyde, 4 mol of ATP is generated per mole of glucose metabolized to pyruvate. All these reactions occur in the absence of O2 and consequently represent anaerobic production of energy. However, 1 mol of ATP is used in forming fructose 1,6-diphosphate from fructose 6-phosphate and 1 mol in phosphorylating glucose when it enters the cell. Consequently, when pyruvate is formed anaerobically from glycogen, there is a net production of 3 mol of ATP per mole of glucose 6-phosphate; however, when pyruvate is formed from 1 mol of blood glucose, the net gain is only 2 mol of ATP. A supply of NAD+ is necessary for the conversion of phosphoglyceraldehyde to phosphoglycerate. Under anaerobic conditions (anaerobic glycolysis), a block of glycolysis at the phosphoglyceraldehyde conversion step might be expected to develop as soon as the available NAD+ is converted to NADH. However, pyruvate can accept hydrogen from NADH, forming NAD+ and lactate: + Pyruvate + NADH ← → Lactate + NAD
In this way, glucose metabolism and energy production can continue for a while without O2. The lactate that accumulates is converted back to pyruvate when the O2 supply is restored, with NADH transferring its hydrogen to the flavoprotein– cytochrome chain.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
21
Pyruvate 3C NAD+ CO2
NADH + H+ Acetyl-CoA 2C
Oxaloacetate 4C NADH +
H+
Citrate 6C
NAD+ Malate 4C
Isocitrate 6C Fumarate 4C
NAD+
FADH2
NADH + H+
CO2
FAD
α-Ketoglutarate 5C
Succinate 4C P CO2
NAD+
GTP GDP
Succinyl-CoA 4C
NADH + H+
FIGURE 1–22
Citric acid cycle. The numbers (6C, 5C, etc) indicate the number of carbon atoms in each of the intermediates. The conversion of pyruvate to acetyl-CoA and each turn of the cycle provide four NADH and one FADH 2 for oxidation via the flavoprotein-cytochrome chain plus formation of one GTP that is readily converted to ATP.
During aerobic glycolysis, the net production of ATP is 19 times greater than the two ATPs formed under anaerobic conditions. Six ATPs are formed by oxidation via the flavoprotein–cytochrome chain of the two NADHs produced when 2 mol of phosphoglyceraldehyde is converted to phosphoglycerate (Figure 1–22), six ATPs are formed from the two NADHs produced when 2 mol of pyruvate is converted to acetyl-CoA, and 24 ATPs are formed during the subsequent two turns of the citric acid cycle. Of these, 18 are formed by oxidation of six NADHs, 4 by oxidation of two FADH2s, and 2 by oxidation at the substrate level when succinyl-CoA is converted to succinate. This reaction actually produces GTP, but the GTP is converted to ATP. Thus, the net production of ATP per mol of blood glucose metabolized aerobically via the Embden–Meyerhof pathway and citric acid cycle is 2 + [2 × 3] + [2 × 3] + [2 × 12] = 38. Glucose oxidation via the hexose monophosphate shunt generates large amounts of NADPH. A supply of this reduced coenzyme is essential for many metabolic processes. The pentoses formed in the process are building blocks for nucleotides (see below). The amount of ATP generated depends on the amount of NADPH converted to NADH and then oxidized.
“DIRECTIONAL-FLOW VALVES” Metabolism is regulated by a variety of hormones and other factors. To bring about any net change in a particular metabolic process, regulatory factors obviously must drive a chemical reaction in one direction. Most of the reactions in intermediary metabolism are freely reversible, but there are a number of “directional-flow valves,” ie, reactions that proceed in one direction under the influence of one enzyme or transport mechanism and in the opposite direction under the influence of another. Five examples in the intermediary metabolism of carbohydrate are shown in Figure 1–23. The different pathways for fatty acid synthesis and catabolism (see below) are another example. Regulatory factors exert their influence on metabolism by acting directly or indirectly at these directional-flow valves.
GLYCOGEN SYNTHESIS & BREAKDOWN Glycogen is a branched glucose polymer with two types of glycoside linkages: 1:4α and 1:6α (Figure 1–24). It is synthesized on glycogenin, a protein primer, from glucose 1-phosphate via uridine diphosphoglucose (UDPG). The enzyme glycogen synthase catalyses the final synthetic step. The availability of
22
SECTION I Cellular & Molecular Basis of Medical Physiology glycogenin is one of the factors determining the amount of glycogen synthesized. The breakdown of glycogen in 1:4α linkage is catalyzed by phosphorylase, whereas another enzyme catalyzes the breakdown of glycogen in 1:6α linkage.
1. Glucose entry into cells and glucose exit from cells Hexokinase 2. Glucose
Glucose 6-phosphate
Glucose 6-phosphatase
FACTORS DETERMINING THE PLASMA GLUCOSE LEVEL
Glycogen synthase 3. Glucose 1-phosphate
Glycogen Phosphorylase
The plasma glucose level at any given time is determined by the balance between the amount of glucose entering the bloodstream and the amount leaving it. The principal determinants are therefore the dietary intake; the rate of entry into the cells of muscle, adipose tissue, and other organs; and the glucostatic activity of the liver (Figure 1–25). Five percent of ingested glucose is promptly converted into glycogen in the liver, and 30–40% is converted into fat. The remainder is metabolized in muscle and other tissues. During fasting, liver glycogen is broken down and the liver adds glucose to the bloodstream. With more prolonged fasting, glycogen is depleted and there is increased gluconeogenesis from amino acids and glycerol in the liver. Plasma glucose declines modestly to about 60 mg/dL during prolonged starvation in normal individuals, but symptoms of hypoglycemia do not occur because gluconeogenesis prevents any further fall.
Phosphofructokinase 4. Fructose 6-phosphate
Fructose 1,6biphosphate
Fructose 1,6biphosphatase
5. Phosphoenolpyruvate
ADP ATP Pyruvate kinase
Pyruvate
Phosphoenolpyruvate carboxykinase Pyruvate
Oxaloacetate
Oxaloacetate Malate
Malate
FIGURE 1–23
Directional flow valves in energy production reactions. In carbohydrate metabolism there are several reactions that proceed in one direction by one mechanism and in the other direction by a different mechanism, termed “directional-flow valves.” Five examples of these reactions are illustrated (numbered at left). The double line in example 5 represents the mitochondrial membrane. Pyruvate is converted to malate in mitochondria, and the malate diffuses out of the mitochondria to the cytosol, where it is converted to phosphoenolpyruvate.
CH2OH
CH2OH
O
O O
O
1:6α linkage
Glycogen
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
O
O
O
O
O
O
O
O
O
CH2
O
Glycogen synthase 1:4α linkage Uridine diphosphoglucose
Phosphorylase a O− CH2OH O
CH2O O
O− O
P
P
O −
O
O
O−
Glucose 1-phosphate
FIGURE 1–24
Glucose 6-phosphate
Glycogen formation and breakdown. Glycogen is the main storage for glucose in the cell. It is cycled: built up from glucose 6-phosphate when energy is stored and broken down to glucose 6-phosphate when energy is required. Note the intermediate glucose 1-phosphate and enzymatic control by phosphorylase a and glycogen kinase.
CHAPTER 1 General Principles & Energy Production in Medical Physiology
Amino Glycerol acids
Diet
Intestine
Liver
Lactate
Plasma glucose 70 mg/dL (3.9 mmol/L)
Kidney
Brain
Fat
Muscle and other tissues
23
the intestines and liver, so its value in replenishing carbohydrate elsewhere in the body is limited. Fructose 6-phosphate can also be phosphorylated in the 2 position, forming fructose 2,6-diphosphate. This compound is an important regulator of hepatic gluconeogenesis. When the fructose 2,6-diphosphate level is high, conversion of fructose 6-phosphate to fructose 1,6-diphosphate is facilitated, and thus breakdown of glucose to pyruvate is increased. A decreased level of fructose 2,6-diphosphate facilitates the reverse reaction and consequently aids gluconeogenesis.
Urine (when plasma glucose > 180 mg/dL)
FIGURE 1–25
Plasma glucose homeostasis. Notice the glucostatic function of the liver, as well as the loss of glucose in the urine when the renal threshold is exceeded (dashed arrows).
METABOLISM OF HEXOSES OTHER THAN GLUCOSE Other hexoses that are absorbed from the intestine include galactose, which is liberated by the digestion of lactose and converted to glucose in the body; and fructose, part of which is ingested and part produced by hydrolysis of sucrose. After phosphorylation, galactose reacts with uridine diphosphoglucose (UDPG) to form uridine diphosphogalactose. The uridine diphosphogalactose is converted back to UDPG, and the UDPG functions in glycogen synthesis. This reaction is reversible, and conversion of UDPG to uridine diphosphogalactose provides the galactose necessary for formation of glycolipids and mucoproteins when dietary galactose intake is inadequate. The utilization of galactose, like that of glucose, depends on insulin. In the inborn error of metabolism known as galactosemia, there is a congenital deficiency of galactose 1-phosphate uridyl transferase, the enzyme responsible for the reaction between galactose 1-phosphate and UDPG, so that ingested galactose accumulates in the circulation. Serious disturbances of growth and development result. Treatment with galactose-free diets improves this condition without leading to galactose deficiency, because the enzyme necessary for the formation of uridine diphosphogalactose from UDPG is present. Fructose is converted in part to fructose 6-phosphate and then metabolized via fructose 1,6-diphosphate. The enzyme catalyzing the formation of fructose 6-phosphate is hexokinase, the same enzyme that catalyzes the conversion of glucose to glucose 6-phosphate. However, much more fructose is converted to fructose 1-phosphate in a reaction catalyzed by fructokinase. Most of the fructose 1-phosphate is then split into dihydroxyacetone phosphate and glyceraldehyde. The glyceraldehyde is phosphorylated, and it and the dihydroxyacetone phosphate enter the pathways for glucose metabolism. Because the reactions proceeding through phosphorylation of fructose in the 1 position can occur at a normal rate in the absence of insulin, it has been recommended that fructose be given to diabetics to replenish their carbohydrate stores. However, most of the fructose is metabolized in
FATTY ACIDS & LIPIDS The biologically important lipids are the fatty acids and their derivatives, the neutral fats (triglycerides), the phospholipids and related compounds, and the sterols. The triglycerides are made up of three fatty acids bound to glycerol (Table 1–4). Naturally occurring fatty acids contain an even number of carbon atoms. They may be saturated (no double bonds) or unsaturated (dehydrogenated, with various numbers of double bonds). The phospholipids are constituents of cell membranes and provide structural components of the cell membrane, as well as an important source of intra- and intercellular signaling molecules. Fatty acids also are an important source of energy in the body.
FATTY ACID OXIDATION & SYNTHESIS In the body, fatty acids are broken down to acetyl-CoA, which enters the citric acid cycle. The main breakdown occurs in the mitochondria by β-oxidation. Fatty acid oxidation begins with activation (formation of the CoA derivative) of the fatty acid, a reaction that occurs both inside and outside the mitochondria. Medium- and short-chain fatty acids can enter the mitochondria without difficulty, but long-chain fatty acids must be bound to carnitine in ester linkage before they can cross the inner mitochondrial membrane. Carnitine is β-hydroxy-γ-trimethylammonium butyrate, and it is synthesized in the body from lysine and methionine. A translocase moves the fatty acid–carnitine ester into the matrix space. The ester is hydrolyzed, and the carnitine recycles. β-oxidation proceeds by serial removal of two carbon fragments from the fatty acid (Figure 1–26). The energy yield of this process is large. For example, catabolism of 1 mol of a six-carbon fatty acid through the citric acid cycle to CO2 and H2O generates 44 mol of ATP, compared with the 38 mol generated by catabolism of 1 mol of the six-carbon carbohydrate glucose.
KETONE BODIES In many tissues, acetyl-CoA units condense to form acetoacetylCoA (Figure 1–27). In the liver, which (unlike other tissues) contains a deacylase, free acetoacetate is formed. This β-keto acid is converted to β-hydroxybutyrate and acetone, and because these compounds are metabolized with difficulty in
24
SECTION I Cellular & Molecular Basis of Medical Physiology
CELLULAR LIPIDS
TABLE 1–4. Lipids. Typical fatty acids: O Palmitic acid:
CH5(CH2)14—C—OH O
Stearic acid:
CH5(CH2)16—C—OH O
Oleic acid:
CH5(CH2)7CH=CH(CH2)7—C—OH (Unsaturated)
Triglycerides (triacylglycerols): Esters of glycerol and three fatty acids. O CH2—O—C—R
CH2OH
O CH2—O—C—R + 3H2O
O CHOH + 3HO—C—R
O CH2—O—C—R Triglyceride
CH2OH Glycerol
R = Aliphatic chain of various lengths and degrees of saturation. Phospholipids: A. Esters of glycerol, two fatty acids, and 1. Phosphate = phosphatidic acid 2. Phosphate plus inositol = phosphatidylinositol 3. Phosphate plus choline = phosphatidylcholine (lecithin) 4. Phosphate plus ethanolamine = phosphatidyl-ethanolamine (cephalin) 5. Phosphate plus serine = phosphatidylserine B. Other phosphate-containing derivatives of glycerol C. Sphingomyelins: Esters of fatty acid, phosphate, choline, and the amino alcohol sphingosine. Cerebrosides: Compounds containing galactose, fatty acid, and sphingosine. Sterols: Cholesterol and its derivatives, including steroid hormones, bile acids, and various vitamins.
the liver, they diffuse into the circulation. Acetoacetate is also formed in the liver via the formation of 3-hydroxy-3-methylglutaryl-CoA, and this pathway is quantitatively more important than deacylation. Acetoacetate, β-hydroxybutyrate, and acetone are called ketone bodies. Tissues other than liver transfer CoA from succinyl-CoA to acetoacetate and metabolize the “active” acetoacetate to CO2 and H2O via the citric acid cycle. Ketone bodies are also metabolized via other pathways. Acetone is discharged in the urine and expired air. An imbalance of ketone bodies can lead to serious health problems (Clinical Box 1–3).
The lipids in cells are of two main types: structural lipids, which are an inherent part of the membranes and other parts of cells; and neutral fat, stored in the adipose cells of the fat depots. Neutral fat is mobilized during starvation, but structural lipid is preserved. The fat depots obviously vary in size, but in nonobese individuals they make up about 15% of body weight in men and 21% in women. They are not the inert structures they were once thought to be but, rather, active dynamic tissues undergoing continuous breakdown and resynthesis. In the depots, glucose is metabolized to fatty acids, and neutral fats are synthesized. Neutral fat is also broken down, and free fatty acids are released into the circulation. A third, special type of lipid is brown fat, which makes up a small percentage of total body fat. Brown fat, which is somewhat more abundant in infants but is present in adults as well, is located between the scapulas, at the nape of the neck, along the great vessels in the thorax and abdomen, and in other scattered locations in the body. In brown fat depots, the fat cells as well as the blood vessels have an extensive sympathetic innervation. This is in contrast to white fat depots, in which some fat cells may be innervated but the principal sympathetic innervation is solely on blood vessels. In addition, ordinary lipocytes have only a single large droplet of white fat, whereas brown fat cells contain several small droplets of fat. Brown fat cells also contain many mitochondria. In these mitochondria, an inward proton conductance that generates ATP takes places as usual, but in addition there is a second proton conductance that does not generate ATP. This “shortcircuit” conductance depends on a 32-kDa uncoupling protein (UCP1). It causes uncoupling of metabolism and generation of ATP, so that more heat is produced.
PLASMA LIPIDS & LIPID TRANSPORT The major lipids are relatively insoluble in aqueous solutions and do not circulate in the free form. Free fatty acids (FFAs) are bound to albumin, whereas cholesterol, triglycerides, and phospholipids are transported in the form of lipoprotein complexes. The complexes greatly increase the solubility of the lipids. The six families of lipoproteins (Table 1–5) are graded in size and lipid content. The density of these lipoproteins is inversely proportionate to their lipid content. In general, the lipoproteins consist of a hydrophobic core of triglycerides and cholesteryl esters surrounded by phospholipids and protein. These lipoproteins can be transported from the intestine to the liver via an exogenous pathway, and between other tissues via an endogenous pathway. Dietary lipids are processed by several pancreatic lipases in the intestine to form mixed micelles of predominantly FFA, 2-monoglycerols, and cholesterol derivatives (see Chapter 27). These micelles additionally can contain important water-insoluble molecules such as vitamins A, D, E, and K. These mixed micelles are taken up into cells of the intestinal
CHAPTER 1 General Principles & Energy Production in Medical Physiology
Fatty acid
"Active" fatty acid
O
OH + HS-CoA
C
CH 2CH 2
ATP
— —
— —
R
O
Mg2 +
H 2O + R
ADP
C
CH 2CH 2
S
CoA
S
CoA
Oxidized flavoprotein Reduced flavoprotein OH
O
C
C
CH 2
S
H 2O + R
CoA
H β-Hydroxy fatty acid–CoA NAD+ O
α,β-Unsaturated fatty acid –CoA
O
CH 2
O
— —
— —
C
C
NADH + H+ O
— —
R
CH — — CH
C
CoA + HS-CoA
S
R
— —
R
— —
— —
O
C
β-Keto fatty acid –CoA
S
CoA + CH3
C
S
CoA
"Active" fatty acid + Acetyl –CoA
R = Rest of fatty acid chain.
Fatty acid oxidation. This process, splitting off two carbon fragments at a time, is repeated to the end of the chain.
C
CoA + CH3
S
C
S
CoA
CH 3
— —
— —
C
CH2
C
C
CH 2
S
CoA + HS-CoA
Acetoacetyl-CoA O
S
CoA + H2O
O
— —
O
C
CH3
2 Acetyl-CoA O
O
Deacylase
C
CH 3
(liver only)
— —
CH 3
O
β-Ketothiolase
— —
— —
O
— —
O
— —
FIGURE 1–26
CH 2
C
O − + H+ + HS-CoA
Acetoacetate
Acetoacetyl-CoA
O
—
— —
OH Acetyl-CoA + Acetoacetyl-CoA
C
CH3
CH2
CH 2
C
CoA + H+
S
COO −
3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) Acetoacetate + H+ + Acetyl-CoA
HMG-CoA Acetoacetate O
— —
— —
O CH 3
C
CH2
C
O − + H+
Tissues except liver CO2 + ATP –CO2 O
–2H
— —
+2H
CH3 — —
CHOH
CH2
C
CH3
Acetone
O
CH 3
C
O − + H+
β-Hydroxybutyrate
FIGURE 1–27
Formation and metabolism of ketone bodies. Note the two pathways for the formation of acetoacetate.
25
26
SECTION I Cellular & Molecular Basis of Medical Physiology
CLINICAL BOX 1–3 Diseases Associated with Imbalance of β-oxidation of Fatty Acids Ketoacidosis The normal blood ketone level in humans is low (about 1 mg/dL) and less than 1 mg is excreted per 24 h, because the ketones are normally metabolized as rapidly as they are formed. However, if the entry of acetyl-CoA into the citric acid cycle is depressed because of a decreased supply of the products of glucose metabolism, or if the entry does not increase when the supply of acetyl-CoA increases, acetyl-CoA accumulates, the rate of condensation to acetoacetyl-CoA increases, and more acetoacetate is formed in the liver. The ability of the tissues to oxidize the ketones is soon exceeded, and they accumulate in the bloodstream (ketosis). Two of the three ketone bodies, acetoacetate and β-hydroxybutyrate, are anions of the moderately strong acids acetoacetic acid and β-hydroxybutyric acid. Many of their protons are buffered, reducing the decline in pH that would otherwise occur. However, the buffering capacity can be exceeded, and the metabolic acidosis that develops in conditions such as diabetic ketosis can be severe
mucosa where large lipoprotein complexes, chylomicrons, are formed. The chylomicrons and their remnants constitute a transport system for ingested exogenous lipids (exogenous pathway). Chylomicrons can enter the circulation via the lymphatic ducts. The chylomicrons are cleared from the circulation by the action of lipoprotein lipase, which is located on the surface of the endothelium of the capillaries. The enzyme catalyzes the breakdown of the triglyceride in the chylomicrons to FFA and glycerol, which then enter adipose
and even fatal. Three conditions lead to deficient intracellular glucose supplies, and hence to ketoacidosis: starvation; diabetes mellitus; and a high-fat, low-carbohydrate diet. The acetone odor on the breath of children who have been vomiting is due to the ketosis of starvation. Parenteral administration of relatively small amounts of glucose abolishes the ketosis, and it is for this reason that carbohydrate is said to be antiketogenic.
Carnitine Deficiency Deficient β-oxidation of fatty acids can be produced by carnitine deficiency or genetic defects in the translocase or other enzymes involved in the transfer of long-chain fatty acids into the mitochondria. This causes cardiomyopathy. In addition, it causes hypoketonemic hypoglycemia with coma, a serious and often fatal condition triggered by fasting, in which glucose stores are used up because of the lack of fatty acid oxidation to provide energy. Ketone bodies are not formed in normal amounts because of the lack of adequate CoA in the liver.
cells and are reesterified. Alternatively, the FFA can remain in the circulation bound to albumin. Lipoprotein lipase, which requires heparin as a cofactor, also removes triglycerides from circulating very low density lipoproteins (VLDL). Chylomicrons depleted of their triglyceride remain in the circulation as cholesterol-rich lipoproteins called chylomicron remnants, which are 30 to 80 nm in diameter. The remnants are carried to the liver, where they are internalized and degraded.
TABLE 1–5 The principal lipoproteins.* Composition (%)
Lipoprotein
Size (nm)
Protein
Free Cholesteryl
Cholesterol Esters
Triglyceride
Phospholipid
Chylomicrons
75–1000
2
2
3
90
3
Intestine
Chylomicron remnants
30–80
…
…
…
…
…
Capillaries
Very low density lipoproteins (VLDL)
30–80
8
4
16
55
17
Liver and intestine
Intermediate-density lipoproteins (IDL)
25–40
10
5
25
40
20
VLDL
Low-density lipoproteins (LDL)
20
20
7
46
6
21
IDL
High-density lipoproteins (HDL)
7.5–10
50
4
16
5
25
Liver and intestine
*The plasma lipids include these components plus free fatty acids from adipose tissue, which circulate bound to albumin.
Origin
CHAPTER 1 General Principles & Energy Production in Medical Physiology The endogenous system, made up of VLDL, intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL), also transports triglycerides and cholesterol throughout the body. VLDL are formed in the liver and transport triglycerides formed from fatty acids and carbohydrates in the liver to extrahepatic tissues. After their triglyceride is largely removed by the action of lipoprotein lipase, they become IDL. The IDL give up phospholipids and, through the action of the plasma enzyme lecithin-cholesterol acyltransferase (LCAT), pick up cholesteryl esters formed from cholesterol in the HDL. Some IDL are taken up by the liver. The remaining IDL then lose more triglyceride and protein, probably in the sinusoids of the liver, and become LDL. LDL provide cholesterol to the tissues. The cholesterol is an essential constituent in cell membranes and is used by gland cells to make steroid hormones.
FREE FATTY ACID METABOLISM In addition to the exogenous and endogenous pathways described above, FFA are also synthesized in the fat depots in which they are stored. They can circulate as lipoproteins bound to albumin and are a major source of energy for many organs. They are used extensively in the heart, but probably all tissues can oxidize FFA to CO2 and H2O. The supply of FFA to the tissues is regulated by two lipases. As noted above, lipoprotein lipase on the surface of the endothelium of the capillaries hydrolyzes the triglycerides in chylomicrons and VLDL, providing FFA and glycerol, which are reassembled into new triglycerides in the fat cells. The intracellular hormone-sensitive lipase of adipose tissue catalyzes the breakdown of stored triglycerides into glycerol and fatty acids, with the latter entering the circulation. Hormone-sensitive lipase is increased by fasting and stress and decreased by feeding and insulin. Conversely, feeding increases and fasting and stress decrease the activity of lipoprotein lipase.
Acetyl-CoA 3-Hydroxy-3methylglutaryl-CoA
Acetoacetyl-CoA Acetoacetate
Acetoacetate
HMG-CoA reductase
27
CHOLESTEROL METABOLISM Cholesterol is the precursor of the steroid hormones and bile acids and is an essential constituent of cell membranes. It is found only in animals. Related sterols occur in plants, but plant sterols are not normally absorbed from the gastrointestinal tract. Most of the dietary cholesterol is contained in egg yolks and animal fat. Cholesterol is absorbed from the intestine and incorporated into the chylomicrons formed in the intestinal mucosa. After the chylomicrons discharge their triglyceride in adipose tissue, the chylomicron remnants bring cholesterol to the liver. The liver and other tissues also synthesize cholesterol. Some of the cholesterol in the liver is excreted in the bile, both in the free form and as bile acids. Some of the biliary cholesterol is reabsorbed from the intestine. Most of the cholesterol in the liver is incorporated into VLDL and circulates in lipoprotein complexes. The biosynthesis of cholesterol from acetate is summarized in Figure 1–28. Cholesterol feeds back to inhibit its own synthesis by inhibiting HMG-CoA reductase, the enzyme that converts 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) to mevalonic acid. Thus, when dietary cholesterol intake is high, hepatic cholesterol synthesis is decreased, and vice versa. However, the feedback compensation is incomplete, because a diet that is low in cholesterol and saturated fat leads to only a modest decline in circulating plasma cholesterol. The most effective and most commonly used cholesterol-lowering drugs are lovastatin and other statins, which reduce cholesterol synthesis by inhibiting HMG-CoA. The relationship between cholesterol and vascular disease is discussed in Clinical Box 1–4.
ESSENTIAL FATTY ACIDS Animals fed a fat-free diet fail to grow, develop skin and kidney lesions, and become infertile. Adding linolenic, linoleic, and arachidonic acids to the diet cures all the deficiency symptoms. These three acids are polyunsaturated fatty acids and because of their action are called essential fatty acids. Similar deficiency symptoms have not been unequivocally demonstrated in humans, but there is reason to believe that some unsaturated fats are essential dietary constituents, especially for children.
Mevalonic acid Squalene Cholesterol
FIGURE 1–28
CH 3 HOOC
CH 2
C
CH2
OH Mevalonic acid
CH2
OH
Squalene (C30 H 50 )
Biosynthesis of cholesterol. Six mevalonic acid molecules condense to form squalene, which is then hydroxylated to cholesterol. The dashed arrow indicates feedback inhibition by cholesterol of HO HMG-CoA reductase, the enzyme that catalyzes mevaCholesterol (C 27 H46 O) lonic acid formation.
28
SECTION I Cellular & Molecular Basis of Medical Physiology
CLINICAL BOX 1–4
CLINICAL BOX 1–5
Cholesterol & Atherosclerosis
Pharmacology of Prostaglandins
The interest in cholesterol-lowering drugs stems from the role of cholesterol in the etiology and course of atherosclerosis. This extremely widespread disease predisposes to myocardial infarction, cerebral thrombosis, ischemic gangrene of the extremities, and other serious illnesses. It is characterized by infiltration of cholesterol and oxidized cholesterol into macrophages, converting them into foam cells in lesions of the arterial walls. This is followed by a complex sequence of changes involving platelets, macrophages, smooth muscle cells, growth factors, and inflammatory mediators that produces proliferative lesions which eventually ulcerate and may calcify. The lesions distort the vessels and make them rigid. In individuals with elevated plasma cholesterol levels, the incidence of atherosclerosis and its complications is increased. The normal range for plasma cholesterol is said to be 120 to 200 mg/dL, but in men, there is a clear, tight, positive correlation between the death rate from ischemic heart disease and plasma cholesterol levels above 180 mg/dL. Furthermore, it is now clear that lowering plasma cholesterol by diet and drugs slows and may even reverse the progression of atherosclerotic lesions and the complications they cause. In evaluating plasma cholesterol levels in relation to atherosclerosis, it is important to analyze the LDL and HDL levels as well. LDL delivers cholesterol to peripheral tissues, including atheromatous lesions, and the LDL plasma concentration correlates positively with myocardial infarctions and ischemic strokes. On the other hand, HDL picks up cholesterol from peripheral tissues and transports it to the liver, thus lowering plasma cholesterol. It is interesting that women, who have a lower incidence of myocardial infarction than men, have higher HDL levels. In addition, HDL levels are increased in individuals who exercise and those who drink one or two alcoholic drinks per day, whereas they are decreased in individuals who smoke, are obese, or live sedentary lives. Moderate drinking decreases the incidence of myocardial infarction, and obesity and smoking are risk factors that increase it. Plasma cholesterol and the incidence of cardiovascular diseases are increased in familial hypercholesterolemia, due to various loss-of-function mutations in the genes for LDL receptors.
Because prostaglandins play a prominent role in the genesis of pain, inflammation, and fever, pharmacologists have long sought drugs to inhibit their synthesis. Glucocorticoids inhibit phospholipase A2 and thus inhibit the formation of all eicosanoids. A variety of nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit both cyclooxygenases, inhibiting the production of PGH2 and its derivatives. Aspirin is the bestknown of these, but ibuprofen, indomethacin, and others are also used. However, there is evidence that prostaglandins synthesized by COX2 are more involved in the production of pain and inflammation, and prostaglandins synthesized by COX1 are more involved in protecting the gastrointestinal mucosa from ulceration. Drugs such as celecoxib and rofecoxib that selectively inhibit COX2 have been developed, and in clinical use they relieve pain and inflammation, possibly with a significantly lower incidence of gastrointestinal ulceration and its complications than is seen with nonspecific NSAIDs. However, rofecoxib has been withdrawn from the market in the United States because of a reported increase of strokes and heart attacks in individuals using it. More research is underway to better understand all the effects of the COX enzymes, their products, and their inhibitors.
Dehydrogenation of fats is known to occur in the body, but there does not appear to be any synthesis of carbon chains with the arrangement of double bonds found in the essential fatty acids.
EICOSANOIDS One of the reasons that essential fatty acids are necessary for health is that they are the precursors of prostaglandins, prostacyclin, thromboxanes, lipoxins, leukotrienes, and related compounds. These substances are called eicosanoids, reflecting
their origin from the 20-carbon (eicosa-) polyunsaturated fatty acid arachidonic acid (arachidonate) and the 20-carbon derivatives of linoleic and linolenic acids. The prostaglandins are a series of 20-carbon unsaturated fatty acids containing a cyclopentane ring. They were first isolated from semen but are now known to be synthesized in most and possibly in all organs in the body. Prostaglandin H2 (PGH2) is the precursor for various other prostaglandins, thromboxanes, and prostacyclin. Arachidonic acid is formed from tissue phospholipids by phospholipase A2. It is converted to prostaglandin H2 (PGH2) by prostaglandin G/H synthases 1 and 2. These are bifunctional enzymes that have both cyclooxygenase and peroxidase activity, but they are more commonly known by the names cyclooxygenase 1 (COX1) and cyclooxygenase 2 (COX2). Their structures are very similar, but COX1 is constitutive whereas COX2 is induced by growth factors, cytokines, and tumor promoters. PGH2 is converted to prostacyclin, thromboxanes, and prostaglandins by various tissue isomerases. The effects of prostaglandins are multitudinous and varied. They are particularly important in the female reproductive cycle, in parturition, in the cardiovascular system, in inflammatory responses, and in the causation of pain. Drugs that target production of prostaglandins are among the most common over the counter drugs available (Clinical Box 1–5). Arachidonic acid also serves as a substrate for the production of several physiologically important leukotrienes and lipoxins. The leukotrienes, thromboxanes, lipoxins, and
CHAPTER 1 General Principles & Energy Production in Medical Physiology prostaglandins have been called local hormones. They have short half-lives and are inactivated in many different tissues. They undoubtedly act mainly in the tissues at sites in which they are produced. The leukotrienes are mediators of allergic responses and inflammation. Their release is provoked when specific allergens combine with IgE antibodies on the surfaces of mast cells (see Chapter 3). They produce bronchoconstriction, constrict arterioles, increase vascular permeability, and attract neutrophils and eosinophils to inflammatory sites. Diseases in which they may be involved include asthma, psoriasis, adult respiratory distress syndrome, allergic rhinitis, rheumatoid arthritis, Crohn’s disease, and ulcerative colitis.
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Cells contain approximately one third of the body fluids, while the remaining extracellular fluid is found between cells (interstitial fluid) or in the circulating blood plasma. The number of molecules, electrical charges, and particles of substances in solution are important in physiology. The high surface tension, high heat capacity, and high electrical capacity allow H2O to function as an ideal solvent in physiology. Biological buffers including bicarbonate, proteins, and phosphates can bind or release protons in solution to help maintain pH. Biological buffering capacity of a weak acid or base is greatest when pKa = pH. Fluid and electrolyte balance in the body is related to plasma osmolality. Isotonic solutions have the same osmolality as blood plasma, hypertonic have higher osmolality, while hypotonic have lower osmolality. Although the osmolality of solutions can be similar across a plasma membrane, the distribution of individual molecules and distribution of charge across the plasma membrane can be quite different. These are affected by the Gibbs-Donnan equilibrium and can be calculated using the Nernst potential equation. There is a distinct difference in concentration of ions in the extracellular and intracellular fluids (concentration gradient). The separation of concentrations of charged species sets up an electrical gradient at the plasma membrane (inside negative). The electrochemical gradient is in large part maintained by the Na, K ATPase. Cellular energy can be stored in high-energy phosphate compounds, including adenosine triphosphate (ATP). Coordinated oxidation-reduction reactions allow for production of a proton gradient at the inner mitochondrial membrane that ultimately yields to the production of ATP in the cell. Nucleotides made from purine or pyrimidine bases linked to ribose or 2-deoxyribose sugars with inorganic phosphates are the basic building blocks for nucleic acids, DNA, and RNA. DNA is a double-stranded structure that contains the fundamental information for an organism. During cell division, DNA is faithfully replicated and a full copy of DNA is in every cell. The fundamental unit of DNA is the gene, which encodes information to make proteins in the cell. Genes are transcribed into messenger RNA, and with the help of ribosomal RNA and transfer RNAs, translated into proteins. Amino acids are the basic building blocks for proteins in the cell and can also serve as sources for several biologically active
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molecules. They exist in an “amino acid pool” that is derived from the diet, protein degradation, and de novo and resynthesis. Translation is the process of protein synthesis. After synthesis, proteins can undergo a variety of posttranslational modifications prior to obtaining their fully functional cell state. Carbohydrates are organic molecules that contain equal amounts of C and H2O. Carbohydrates can be attached to proteins (glycoproteins) or fatty acids (glycolipids) and are critically important for the production and storage of cellular and body energy, with major supplies in the form of glycogen in the liver and skeletal muscle. The breakdown of glucose to generate energy, or glycolysis, can occur in the presence or absence of O2 (aerobic or anaerobically). The net production of ATP during aerobic glycolysis is 19 times higher than anaerobic glycolysis. Fatty acids are carboxylic acids with extended hydrocarbon chains. They are an important energy source for cells and their derivatives, including triglycerides, phospholipids and sterols, and have additional important cellular applications. Free fatty acids can be bound to albumin and transported throughout the body. Triglycerides, phospholipids, and cholesterol are transported as lipoprotein complexes.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. The membrane potential of a particular cell is at the K+ equilibrium. The intracellular concentration for K+ is at 150 mmol/L and the extracellular concentration for K+ is at 5.5 mmol/L. What is the resting potential? A) –70 mv B) –90 mv C) +70 mv D) +90 mv 2. The difference in concentration of H+ in a solution of pH 2.0 compared with one of pH 7.0 is A) 5-fold. B) 1/5 as much. C) 105 fold. D) 10–5 as much. 3. Transcription refers to A) the process where an mRNA is used as a template for protein production. B) the process where a DNA sequence is copied into RNA for the purpose of gene expression. C) the process where DNA wraps around histones to form a nucleosome. D) the process of replication of DNA prior to cell division. 4. The primary structure of a protein refers to A) the twist, folds, or twist and folds of the amino acid sequence into stabilized structures within the protein (ie, α-helices and β-sheets). B) the arrangement of subunits to form a functional structure. C) the amino acid sequence of the protein. D) the arrangement of twisted chains and folds within a protein into a stable structure.
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SECTION I Cellular & Molecular Basis of Medical Physiology
5. Fill in the blanks: Glycogen is a storage form of glucose. _______ refers to the process of making glycogen and _______ refers to the process of breakdown of glycogen. A) Glycogenolysis, glycogenesis B) Glycolysis, glycogenolysis C) Glycogenesis, glycogenolysis D) Glycogenolysis, glycolysis 6. The major lipoprotein source of the cholesterol used in cells is A) chylomicrons. B) intermediate-density lipoproteins (IDLs). C) albumin-bound free fatty acids. D) LDL. E) HDL. 7. Which of the following produces the most high-energy phosphate compounds? A) aerobic metabolism of 1 mol of glucose B) anaerobic metabolism of 1 mol of glucose C) metabolism of 1 mol of galactose D) metabolism of 1 mol of amino acid E) metabolism of 1 mol of long-chain fatty acid 8. When LDL enters cells by receptor-mediated endocytosis, which of the following does not occur? A) Decrease in the formation of cholesterol from mevalonic acid. B) Increase in the intracellular concentration of cholesteryl esters. C) Increase in the transfer of cholesterol from the cell to HDL. D) Decrease in the rate of synthesis of LDL receptors. E) Decrease in the cholesterol in endosomes.
CHAPTER RESOURCES Alberts B, et al: Molecular Biology of the Cell, 5th ed. Garland Science, 2007. Hille B: Ionic Channels of Excitable Membranes, 3rd ed. Sinauer Associates, 2001. Kandel ER, Schwartz JH, Jessell TM: Principles of Neural Science, 4th ed. McGraw-Hill, 2000. Macdonald RG, Chaney WG: USMLE Road Map, Biochemistry. McGraw- Hill, 2007. Murray RK, et al: Harper’s Biochemistry, 26th ed. McGraw-Hill, 2003. Pollard TD, Earnshaw WC: Cell Biology, 2nd ed. Saunders, Elsevier, 2008. Sack GH, Jr. USMLE Road Map, Genetics. McGraw Hill, 2008. Scriver CR, et al (editors): The Metabolic and Molecular Bases of Inherited Disease, 8th ed. McGraw-Hill, 2001. Sperelakis N (editor): Cell Physiology Sourcebook, 3rd ed. Academic Press, 2001.
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Overview of Cellular Physiology in Medical Physiology
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O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■ ■
Name the prominent cellular organelles and state their functions in cells. Name the building blocks of the cellular cytoskeleton and state their contributions to cell structure and function. Name the intercellular and cellular to extracellular connections. Define the processes of exocytosis and endocytosis, and describe the contribution of each to normal cell function. Define proteins that contribute to membrane permeability and transport. Describe specialized transport and filtration across the capillary wall. Recognize various forms of intercellular communication and describe ways in which chemical messengers (including second messengers) affect cellular physiology. Define cellular homeostasis.
INTRODUCTION The cell is the fundamental working unit of all organisms. In humans, cells can be highly specialized in both structure and function; alternatively, cells from different organs can share features and function. In the previous chapter, we examined some basic principles of biophysics and the catabolism and metabolism of building blocks found in the cell. In some of those discussions, we examined how the building blocks
could contribute to basic cellular physiology (eg, DNA replication, transcription, and translation). In this chapter, we will briefly review more of the fundamental aspects of cellular and molecular physiology. Additional aspects that concern specialization of cellular and molecular physiology are considered in the next chapter concerning immune function and in the relevant chapters on the various organs.
FUNCTIONAL MORPHOLOGY OF THE CELL
confocal, and other microscopy along with specialized probes for both static and dynamic cellular structures further expanded the examination of cell structure and function. Equally revolutionary advances in the modern biophysical, biochemical, and molecular biology techniques have also greatly contributed to our knowledge of the cell. The specialization of the cells in the various organs is considerable, and no cell can be called “typical” of all cells in the body. However, a number of structures (organelles) are common to most cells. These structures are shown in Figure 2–1. Many of them can be isolated by ultracentrifugation combined with
A basic knowledge of cell biology is essential to an understanding of the organ systems in the body and the way they function. A key tool for examining cellular constituents is the microscope. A light microscope can resolve structures as close as 0.2 μm, while an electron microscope can resolve structures as close as 0.002 μm. Although cell dimensions are quite variable, this resolution can give us a good look at the inner workings of the cell. The advent of common access to fluorescent,
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SECTION I Cellular & Molecular Basis of Medical Physiology
Secretory granules Golgi apparatus
Centrioles
Rough endoplasmic reticulum
Smooth endoplasmic reticulum
Lysosomes Nuclear envelope
Lipid droplets Mitochondrion
Nucleolus
Globular heads
FIGURE 2–1 Diagram showing a hypothetical cell in the center as seen with the light microscope. Individual organelles are expanded for closer examination. (Adapted from Bloom and Fawcett. Reproduced with permission from Junqueira LC, Carneiro J, Kelley RO: Basic Histology, 9th ed. McGraw-Hill, 1998.)
other techniques. When cells are homogenized and the resulting suspension is centrifuged, the nuclei sediment first, followed by the mitochondria. High-speed centrifugation that generates forces of 100,000 times gravity or more causes a fraction made up of granules called the microsomes to sediment. This fraction includes organelles such as the ribosomes and peroxisomes.
CELL MEMBRANES The membrane that surrounds the cell is a remarkable structure. It is made up of lipids and proteins and is semipermeable, allowing some substances to pass through it and excluding others. However, its permeability can also be varied because it contains numerous regulated ion channels and other transport proteins that can change the amounts of substances moving across it. It is generally referred to as the plasma membrane. The nucleus and other organelles in the cell are bound by similar membranous structures. Although the chemical structures of membranes and their properties vary considerably from one location to another, they have certain common features. They are generally about 7.5 nm (75 Å) thick. The major lipids are phospholipids such as phosphatidylcholine and phosphatidylethanolamine. The shape of the phospholipid molecule reflects its solubility
properties: the head end of the molecule contains the phosphate portion and is relatively soluble in water (polar, hydrophilic) and the tails are relatively insoluble (nonpolar, hydrophobic). The possession of both hydrophilic and hydrophobic properties make the lipid an amphipathic molecule. In the membrane, the hydrophilic ends of the molecules are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane (Figure 2–2). In prokaryotes (ie, bacteria in which there is no nucleus), the membranes are relatively simple, but in eukaryotes (cells containing nuclei), cell membranes contain various glycosphingolipids, sphingomyelin, and cholesterol in addition to phospholipids and phosphatidylcholine. Many different proteins are embedded in the membrane. They exist as separate globular units and many pass through the membrane (integral proteins), whereas others (peripheral proteins) stud the inside and outside of the membrane (Figure 2–2). The amount of protein varies significantly with the function of the membrane but makes up on average 50% of the mass of the membrane; that is, there is about one protein molecule per 50 of the much smaller phospholipid molecules. The proteins in the membranes carry out many functions. Some are cell adhesion molecules that anchor cells to their neighbors or to basal laminas. Some proteins function as pumps, actively
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
gradients by facilitated diffusion. Still others are ion channels, which, when activated, permit the passage of ions into or out of the cell. The role of the pumps, carriers, and ion channels in transport across the cell membrane is discussed below. Proteins in another group function as receptors that bind ligands or messenger molecules, initiating physiologic changes inside the cell. Proteins also function as enzymes, catalyzing reactions at the surfaces of the membrane. Examples from each of these groups are discussed later in this chapter. The uncharged, hydrophobic portions of the proteins are usually located in the interior of the membrane, whereas the charged, hydrophilic portions are located on the surfaces. Peripheral proteins are attached to the surfaces of the membrane in various ways. One common way is attachment to glycosylated forms of phosphatidylinositol. Proteins held by these glycosylphosphatidylinositol (GPI) anchors (Figure 2–3) include enzymes such as alkaline phosphatase, various antigens, a number of cell adhesion molecules, and three proteins that combat cell lysis by complement. Over 45 GPIlinked cell surface proteins have now been described in humans. Other proteins are lipidated, that is, they have specific lipids attached to them (Figure 2–3). Proteins may be myristolated, palmitoylated, or prenylated (ie, attached to geranylgeranyl or farnesyl groups). The protein structure—and particularly the enzyme content—of biologic membranes varies not only from cell to cell, but also within the same cell. For example, some of the enzymes embedded in cell membranes are different from those in mitochondrial membranes. In epithelial cells, the enzymes in the cell membrane on the mucosal surface differ from those in the
Extracellular fluid Transmembrane Phospholipids proteins
Carbohydrate portion of glycoprotein Channel Intregral proteins
Peripheral protein Polar regions
Nonpolar regions
Intracellular fluid
FIGURE 2–2
Organization of the phospholipid bilayer and associated proteins in a biological membrane. The phospholipid molecules each have two fatty acid chains (wavy lines) attached to a phosphate head (open circle). Proteins are shown as irregular colored globules. Many are integral proteins, which extend into the membrane, but peripheral proteins are attached to the inside or outside (not shown) of the membrane. Specific protein attachments and cholesterol commonly found in the bilayer are omitted for clarity. (Reproduced with permission from Widmaier EP, Raff H, Strang K: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
transporting ions across the membrane. Other proteins function as carriers, transporting substances down electrochemical Lipid membrane
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Cytoplasmic or external face of membrane O N
N-Myristoyl
Protein
Gly
COOH
H Protein
S-Cys S-Palmitoyl
NH2
O S-Cys
Protein
NH2
S-Cys
Protein
NH2
Geranylgeranyl
Farnesyl O C
C
CH2
C
C
CH
O
C
GPI anchor (Glycosylphosphatidylinositol) Hydrophobic domain
FIGURE 2–3
H2
O
O O
P
O
Inositol
O
C
Protein
O Hydrophilic domain
Protein linkages to membrane lipids. Some are linked by their amino terminals, others by their carboxyl terminals. Many are attached via glycosylated forms of phosphatidylinositol (GPI anchors). (Reproduced with permission from Fuller GM, Shields D: Molecular Basis of Medical Cell Biology. McGraw-Hill, 1998.)
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SECTION I Cellular & Molecular Basis of Medical Physiology
H+
Intramemb space
H+ CoQ
Inner mito membrane
H+
H+
Cyt c ADP
Matrix space
AS
Complex
I
II
III
IV
V
Subunits from mDNA Subunits from nDNA
7
0
1
3
2
39
4
10
10
14
ATP
FIGURE 2–4
Components involved in oxidative phosphorylation in mitochondria and their origins. As enzyme complexes I through IV convert 2-carbon metabolic fragments to CO 2 and H2O, protons (H+) are pumped into the intermembrane space. The proteins diffuse back to the matrix space via complex V, ATP synthase (AS), in which ADP is converted to ATP. The enzyme complexes are made up of subunits coded by mitochondrial DNA (mDNA) and nuclear DNA (nDNA), and the figures document the contribution of each DNA to the complexes.
cell membrane on the basal and lateral margins of the cells; that is, the cells are polarized. Such polarization makes transport across epithelia possible. The membranes are dynamic structures, and their constituents are being constantly renewed at different rates. Some proteins are anchored to the cytoskeleton, but others move laterally in the membrane. Underlying most cells is a thin, “fuzzy” layer plus some fibrils that collectively make up the basement membrane or, more properly, the basal lamina. The basal lamina and, more generally, the extracellular matrix are made up of many proteins that hold cells together, regulate their development, and determine their growth. These include collagens, laminins, fibronectin, tenascin, and various proteoglycans.
MITOCHONDRIA Over a billion years ago, aerobic bacteria were engulfed by eukaryotic cells and evolved into mitochondria, providing the eukaryotic cells with the ability to form the energy-rich compound ATP by oxidative phosphorylation. Mitochondria perform other functions, including a role in the regulation of apoptosis (programmed cell death), but oxidative phosphorylation is the most crucial. Each eukaryotic cell can have hundreds to thousands of mitochondria. In mammals, they are generally depicted as sausage-shaped organelles (Figure 2–1), but their shape can be quite dynamic. Each has an outer membrane, an intermembrane space, an inner membrane, which is folded to form shelves (cristae), and a central matrix space. The enzyme complexes responsible for oxidative phosphorylation are lined up on the cristae (Figure 2–4). Consistent with their origin from aerobic bacteria, the mitochondria have their own genome. There is much less DNA in the mitochondrial genome than in the nuclear genome, and 99% of the proteins in the mitochondria are the products of nuclear genes, but mitochondrial DNA is responsible for certain key components of the pathway for oxidative phosphorylation. Specifically, human mitochondrial DNA is a double-stranded circular molecule containing approximately
16,500 base pairs (compared with over a billion in nuclear DNA). It codes for 13 protein subunits that are associated with proteins encoded by nuclear genes to form four enzyme complexes plus two ribosomal and 22 transfer RNAs that are needed for protein production by the intramitochondrial ribosomes. The enzyme complexes responsible for oxidative phosphorylation illustrate the interactions between the products of the mitochondrial genome and the nuclear genome. For example, complex I, reduced nicotinamide adenine dinucleotide dehydrogenase (NADH), is made up of 7 protein subunits coded by mitochondrial DNA and 39 subunits coded by nuclear DNA. The origin of the subunits in the other complexes is shown in Figure 2–4. Complex II, succinate dehydrogenase-ubiquinone oxidoreductase; complex III, ubiquinonecytochrome c oxidoreductase; and complex IV, cytochrome c oxidase, act with complex I, coenzyme Q, and cytochrome c to convert metabolites to CO2 and water. Complexes I, III, and IV pump protons (H+) into the intermembrane space during this electron transfer. The protons then flow down their electrochemical gradient through complex V, ATP synthase, which harnesses this energy to generate ATP. As zygote mitochondria are derived from the ovum, their inheritance is maternal. This maternal inheritance has been used as a tool to track evolutionary descent. Mitochondria have an ineffective DNA repair system, and the mutation rate for mitochondrial DNA is over 10 times the rate for nuclear DNA. A large number of relatively rare diseases have now been traced to mutations in mitochondrial DNA. These include for the most part disorders of tissues with high metabolic rates in which energy production is defective as a result of abnormalities in the production of ATP.
LYSOSOMES In the cytoplasm of the cell there are large, somewhat irregular structures surrounded by membrane. The interior of these structures, which are called lysosomes, is more acidic than the
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
TABLE 2–1 Some of the enzymes found in lysosomes and the cell components that are their substrates.
CLINICAL BOX 2–1
Enzyme
Substrate
Lysosomal Diseases
Ribonuclease
RNA
Deoxyribonuclease
DNA
Phosphatase
Phosphate esters
Glycosidases
Complex carbohydrates; glycosides and polysaccharides
Arylsulfatases
Sulfate esters
Collagenase
Collagens
Cathepsins
Proteins
When a lysosomal enzyme is congenitally absent, the lysosomes become engorged with the material the enzyme normally degrades. This eventually leads to one of the lysosomal storage diseases. For example, α-galactosidase A deficiency causes Fabry disease, and β-galactocerebrosidase deficiency causes Gaucher disease. These diseases are rare, but they are serious and can be fatal. Another example is the lysosomal storage disease called Tay–Sachs disease, which causes mental retardation and blindness. Tay–Sachs is caused by the loss of hexosaminidase A, a lysosomal enzyme that catalyzes the biodegradation of gangliosides (fatty acid derivatives).
rest of the cytoplasm, and external material such as endocytosed bacteria, as well as worn-out cell components, are digested in them. The interior is kept acidic by the action of a proton pump, or H+, ATPase. This integral membrane protein uses the energy of ATP to move protons from the cytosol up their electrochemical gradient and keep the lysosome relatively acidic, near pH 5.0. Lysosomes can contain over 40 types of hydrolytic enzymes, some of which are listed in Table 2–1. Not surprisingly, these enzymes are all acid hydrolases, in that they function best at the acidic pH of the lysosomal compartment. This can be a safety feature for the cell; if the lysosome were to break open and release its contents, the enzymes would not be efficient at the near neutral cytosolic pH (7.2), and thus would be unable to digest cytosolic enzymes they may encounter. Diseases associated with lysosomal dysfunction are discussed in Clinical Box 2–1.
PEROXISOMES Peroxisomes are 0.5 μm in diameter, are surrounded by a membrane, and contain enzymes that can either produce H2O2 (oxidases) or break it down (catalases). Proteins are directed to the peroxisome by a unique signal sequence with the help of protein chaperones, peroxins. The peroxisome membrane contains a number of peroxisome-specific proteins that are concerned with transport of substances into and out of the matrix of the peroxisome. The matrix contains more than 40 enzymes, which operate in concert with enzymes outside the peroxisome to catalyze a variety of anabolic and catabolic reactions (eg, breakdown of lipids). Peroxisomes can form by budding of endoplasmic reticulum, or by division. A number of synthetic compounds were found to cause proliferation of peroxisomes by acting on receptors in the nuclei of cells. These peroxisome proliferation activated receptors (PPARs) are members of the nuclear receptor superfamily. When activated, they bind to DNA, producing changes in the production of mRNAs. The known effects for PPARs are extensive and can affect most tissues and organs.
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CYTOSKELETON All cells have a cytoskeleton, a system of fibers that not only maintains the structure of the cell but also permits it to change shape and move. The cytoskeleton is made up primarily of microtubules, intermediate filaments, and microfilaments (Figure 2–5), along with proteins that anchor them and tie them together. In addition, proteins and organelles move along microtubules and microfilaments from one part of the cell to another, propelled by molecular motors. Microtubules (Figures 2–5 and 2–6) are long, hollow structures with 5-nm walls surrounding a cavity 15 nm in diameter. They are made up of two globular protein subunits: αand β-tubulin. A third subunit, γ-tubulin, is associated with the production of microtubules by the centrosomes. The α and β subunits form heterodimers, which aggregate to form long tubes made up of stacked rings, with each ring usually containing 13 subunits. The tubules interact with GTP to facilitate their formation. Although microtubule subunits can be added to either end, microtubules are polar with assembly predominating at the “+” end and disassembly predominating at the “–” end. Both processes occur simultaneously in vitro. The growth of microtubules is temperature sensitive (disassembly is favored under cold conditions) as well as under the control of a variety of cellular factors that can directly interact with microtubules in the cell. Because of their constant assembly and disassembly, microtubules are a dynamic portion of the cell skeleton. They provide the tracks along which several different molecular motors move transport vesicles, organelles such as secretory granules, and mitochondria, from one part of the cell to another. They also form the spindle, which moves the chromosomes in mitosis. Cargo can be transported in either direction on microtubules. There are several drugs available that disrupt cellular function through interaction with microtubules. Microtubule assembly is prevented by colchicine and vinblastine. The anticancer drug paclitaxel (Taxol) binds to microtubules and
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SECTION I Cellular & Molecular Basis of Medical Physiology
Cytoskeletal filaments Microfilament
Diameter (nm) 7
Protein subunit Actin
Intermediate filament
10
Several proteins
Microtubule
25
Tubulin
FIGURE 2–5 Cytoskeletal elements of the cell. Artistic impressions that depict the major cytoskeletal elements are shown on the left, with basic properties of these elements on the right. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
makes them so stable that organelles cannot move. Mitotic spindles cannot form, and the cells die. Intermediate filaments (Figures 2–5 and 2–6) are 8 to 14 nm in diameter and are made up of various subunits. Some of these filaments connect the nuclear membrane to the cell membrane. They form a flexible scaffolding for the cell and help it resist external pressure. In their absence, cells rupture more easily, and when they are abnormal in humans, blistering of the skin is common. The proteins that make up intermediate filaments are celltype specific, and are thus frequently used as cellular markers. For example, vimentin is a major intermediate filament in fibroblasts, whereas cytokeratin is expressed in epithelial cells. Microfilaments (Figures 2–5 and 2–6) are long solid fibers with a 4 to 6 nm diameter that are made up of actin. Although actin is most often associated with muscle contraction, it is
FIGURE 2–6
present in all types of cells. It is the most abundant protein in mammalian cells, sometimes accounting for as much as 15% of the total protein in the cell. Its structure is highly conserved; for example, 88% of the amino acid sequences in yeast and rabbit actin are identical. Actin filaments polymerize and depolymerize in vivo, and it is not uncommon to find polymerization occurring at one end of the filament while depolymerization is occurring at the other end. Filamentous (F) actin refers to intact microfilaments and globular (G) actin refers to the unpolymerized protein actin subunits. F-actin fibers attach to various parts of the cytoskeleton and can interact directly or indirectly with membrane-bound proteins. They reach to the tips of the microvilli on the epithelial cells of the intestinal mucosa. They are also abundant in the lamellipodia that cells put out when they crawl along surfaces. The actin filaments interact with integrin
Microfilaments and microtubules. Electron micrograph (Left) of the cytoplasm of a fibroblast, displaying actin microfilaments (MF) and microtubules (MT). (Reproduced, with permission, from Junqueira LC, Carneiro J: Basic Histology, 10th ed. McGraw-Hill, 2003.) Fluorescent micrographs of airway epithelial cells displaying actin microfilaments stained with phalloidin (Middle) and microtubules visualized with an antibody to β-tubulin (Right). Both fluorescent micrographs are counterstained with Hoechst dye (blue) to visualize nuclei. Note the distinct differences in cytoskeletal structure.
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology receptors and form focal adhesion complexes, which serve as points of traction with the surface over which the cell pulls itself. In addition, some molecular motors use microfilaments as tracks.
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they perform functions as diverse as contraction of muscle and cell migration.
CENTROSOMES
MOLECULAR MOTORS The molecular motors that move proteins, organelles, and other cell parts (collectively referred to as “cargo”) to all parts of the cell are 100 to 500 kDa ATPases. They attach to their cargo at one end of the molecule and to microtubules or actin polymers with the other end, sometimes referred to as the “head.” They convert the energy of ATP into movement along the cytoskeleton, taking their cargo with them. There are three super families of molecular motors: kinesin, dynein, and myosin. Examples of individual proteins from each superfamily are shown in Figure 2–7. It is important to note that there is extensive variation among superfamily members, allowing for specialization of function (eg, choice of cargo, cytoskeletal filament type, and/or direction of movement). The conventional form of kinesin is a doubleheaded molecule that tends to move its cargo toward the “+” ends of microtubules. One head binds to the microtubule and then bends its neck while the other head swings forward and binds, producing almost continuous movement. Some kinesins are associated with mitosis and meiosis. Other kinesins perform different functions, including, in some instances, moving cargo to the “–” end of microtubules. Dyneins have two heads, with their neck pieces embedded in a complex of proteins. Cytoplasmic dyneins have a function like that of conventional kinesin, except they tend to move particles and membranes to the “–” end of the microtubules. The multiple forms of myosin in the body are divided into 18 classes. The heads of myosin molecules bind to actin and produce motion by bending their neck regions (myosin II) or walking along microfilaments, one head after the other (myosin V). In these ways,
Near the nucleus in the cytoplasm of eukaryotic animal cells is a centrosome. The centrosome is made up of two centrioles and surrounding amorphous pericentriolar material. The centrioles are short cylinders arranged so that they are at right angles to each other. Microtubules in groups of three run longitudinally in the walls of each centriole (Figure 2–1). Nine of these triplets are spaced at regular intervals around the circumference. The centrosomes are microtubule-organizing centers (MTOCs) that contain γ-tubulin. The microtubules grow out of this γ-tubulin in the pericentriolar material. When a cell divides, the centrosomes duplicate themselves, and the pairs move apart to the poles of the mitotic spindle, where they monitor the steps in cell division. In multinucleate cells, a centrosome is near each nucleus.
CILIA Cilia are specialized cellular projections that are used by unicellular organisms to propel themselves through liquid and by multicellular organisms to propel mucus and other substances over the surface of various epithelia. Cilia are functionally indistinct from the eukaryotic flagella of sperm cells. Within the cilium there is an axoneme that comprises a unique arrangement of nine outer microtubule doublets and two inner microtubules (“9+2” arrangement). Along this cytoskeleton is axonemal dynein. Coordinated dynein-microtubule interactions within the axoneme are the basis of ciliary and sperm movement. At the base of the axoneme and just inside lies the basal body. It has nine circumferential triplet microtubules, like a centriole, and there is evidence that basal bodies and centrioles are interconvertible.
Cargo
Light chains
Conventional kinesin
4 nm
Cytoplasmic dynein
80 nm Cargo-binding domain
Head 1
Head 2
Head 2 ADP
ADP
Head 1 ATP Actin
Myosin V
FIGURE 2–7 Three examples of molecular motors. Conventional kinesin is shown attached to cargo, in this case a membrane-bound organelle. The way that myosin V “walks” along a microtubule is also shown. Note that the heads of the motors hydrolyze ATP and use the energy to produce motion.
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SECTION I Cellular & Molecular Basis of Medical Physiology
CELL ADHESION MOLECULES Cells are attached to the basal lamina and to each other by cell adhesion molecules (CAMs) that are prominent parts of the intercellular connections described below. These adhesion proteins have attracted great attention in recent years because of their unique structural and signaling functions found to be important in embryonic development and formation of the nervous system and other tissues, in holding tissues together in adults, in inflammation and wound healing, and in the metastasis of tumors. Many CAMs pass through the cell membrane and are anchored to the cytoskeleton inside the cell. Some bind to like molecules on other cells (homophilic binding), whereas others bind to nonself molecules (heterophilic binding). Many bind to laminins, a family of large crossshaped molecules with multiple receptor domains in the extracellular matrix. Nomenclature in the CAM field is somewhat chaotic, partly because the field is growing so rapidly and partly because of the extensive use of acronyms, as in other areas of modern biology. However, the CAMs can be divided into four broad families: (1) integrins, heterodimers that bind to various receptors; (2) adhesion molecules of the IgG superfamily of immunoglobulins; (3) cadherins, Ca2+-dependent molecules that mediate cell-to-cell adhesion by homophilic reactions; and (4) selectins, which have lectin-like domains that bind carbohydrates. Specific functions of some of these molecules are addressed in later chapters. The CAMs not only fasten cells to their neighbors, but they also transmit signals into and out of the cell. For example, cells that lose their contact with the extracellular matrix via integrins have a higher rate of apoptosis than anchored cells, and interactions between integrins and the cytoskeleton are involved in cell movement.
INTERCELLULAR CONNECTIONS Intercellular junctions that form between the cells in tissues can be broadly split into two groups: junctions that fasten the cells to one another and to surrounding tissues, and junctions that permit transfer of ions and other molecules from one cell to another. The types of junctions that tie cells together and endow tissues with strength and stability include tight junctions, which are also known as the zonula occludens (Figure 2–8). The desmosome and zonula adherens also help to hold cells together, and the hemidesmosome and focal adhesions attach cells to their basal laminas. The gap junction forms a cytoplasmic “tunnel” for diffusion of small molecules (< 1000 Da) between two neighboring cells. Tight junctions characteristically surround the apical margins of the cells in epithelia such as the intestinal mucosa, the walls of the renal tubules, and the choroid plexus. They are also important to endothelial barrier function. They are made up of ridges—half from one cell and half from the other—which adhere so strongly at cell junctions that they
Tight junction (zonula occludens) Zonula adherens Desmosomes
Gap junctions
Hemidesmosome
FIGURE 2–8
Intercellular junctions in the mucosa of the small intestine. Tight junctions (zonula occludens), adherens junctions (zonula adherens), desmosomes, gap junctions, and hemidesmosomes are all shown in relative positions in a polarized epithelial cell.
almost obliterate the space between the cells. There are three main families of transmembrane proteins that contribute to tight junctions: occludin, junctional adhesion molecules (JAMs), and claudins; and several more proteins that interact from the cytosolic side. Tight junctions permit the passage of some ions and solute in between adjacent cells (paracellular pathway) and the degree of this “leakiness” varies, depending in part on the protein makeup of the tight junction. Extracellular fluxes of ions and solute across epithelia at these junctions are a significant part of overall ion and solute flux. In addition, tight junctions prevent the movement of proteins in the plane of the membrane, helping to maintain the different distribution of transporters and channels in the apical and basolateral cell membranes that make transport across epithelia possible. In epithelial cells, each zonula adherens is usually a continuous structure on the basal side of the zonula occludens, and it is a major site of attachment for intracellular microfilaments. It contains cadherins. Desmosomes are patches characterized by apposed thickenings of the membranes of two adjacent cells. Attached to the thickened area in each cell are intermediate filaments, some running parallel to the membrane and others radiating away from it. Between the two membrane thickenings the intercellular space contains filamentous material that includes cadherins and the extracellular portions of several other transmembrane proteins. Hemidesmosomes look like half-desmosomes that attach cells to the underlying basal lamina and are connected
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
39
A
Presynaptic cytoplasm
20 nm
3.5 nm
Postsynaptic cytoplasm
Normal extracellular space
Channel formed by pores in each membrane
B
Each of the 6 connexins has 4 membrane-spanning regions
6 connexin subunits = 1 connexon (hemichannel)
Presynaptic cytoplasm
Extracellular space
Cytoplasmic loops for regulation
Extracellular loops for homophilic interactions
FIGURE 2–9 Gap junction connecting the cytoplasm of two cells. A) A gap junction plaque, or collection of individual gap junctions, is shown to form multiple pores between cells that allow for the transfer of small molecules. Inset is electron micrograph from rat liver (N. Gilula). B) Topographical depiction of individual connexon and corresponding 6 connexin proteins that traverse the membrane. Note that each connexin traverses the membrane four times. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.) intracellularly to intermediate filaments. However, they contain integrins rather than cadherins. Focal adhesions also attach cells to their basal laminas. As noted previously, they are labile structures associated with actin filaments inside the cell, and they play an important role in cell movement.
GAP JUNCTIONS At gap junctions, the intercellular space narrows from 25 nm to 3 nm, and units called connexons in the membrane of each cell
are lined up with one another (Figure 2–9). Each connexon is made up of six protein subunits called connexins. They surround a channel that, when lined up with the channel in the corresponding connexon in the adjacent cell, permits substances to pass between the cells without entering the ECF. The diameter of the channel is normally about 2 nm, which permits the passage of ions, sugars, amino acids, and other solutes with molecular weights up to about 1000. Gap junctions thus permit the rapid propagation of electrical activity from cell to cell, as well as the exchange of various chemical messengers. However, the
40
SECTION I Cellular & Molecular Basis of Medical Physiology
gap junction channels are not simply passive, nonspecific conduits. At least 20 different genes code for connexins in humans, and mutations in these genes can lead to diseases that are highly selective in terms of the tissues involved and the type of communication between cells produced. For instance, X-linked Charcot–Marie–Tooth disease is a peripheral neuropathy associated with mutation of one particular connexin gene. Experiments in mice in which particular connexins are deleted by gene manipulation or replaced with different connexins confirm that the particular connexin subunits that make up connexons determine their permeability and selectivity. Recently it has been shown that connexons can be used as channels to release small molecules from the cytosol into the ECF.
NUCLEUS & RELATED STRUCTURES A nucleus is present in all eukaryotic cells that divide. If a cell is cut in half, the anucleate portion eventually dies without dividing. The nucleus is made up in large part of the chromosomes, the structures in the nucleus that carry a complete blueprint for all the heritable species and individual characteristics of the animal. Except in germ cells, the chromosomes occur in pairs, one originally from each parent. Each chromosome is made up of a giant molecule of DNA. The DNA strand is about 2 m long, but it can fit in the nucleus because at intervals it is wrapped around a core of histone proteins to form a nucleosome. There are about 25 million nucleosomes in each nucleus. Thus, the structure of the chromosomes has been likened to a string of beads. The beads are the nucleosomes, and the linker DNA between them is the string. The whole complex of DNA and proteins is called chromatin. During cell division, the coiling around histones is loosened, probably by acetylation of the histones, and pairs of chromosomes become visible, but between cell divisions only clumps of chromatin can be discerned in the nucleus. The ultimate units of heredity are the genes on the chromosomes). As discussed in Chapter 1, each gene is a portion of the DNA molecule. The nucleus of most cells contains a nucleolus (Figure 2–1), a patchwork of granules rich in RNA. In some cells, the nucleus contains several of these structures. Nucleoli are most prominent and numerous in growing cells. They are the site of synthesis of ribosomes, the structures in the cytoplasm in which proteins are synthesized. The interior of the nucleus has a skeleton of fine filaments that are attached to the nuclear membrane, or envelope (Figure 2–1), which surrounds the nucleus. This membrane is a double membrane, and spaces between the two folds are called perinuclear cisterns. The membrane is permeable only to small molecules. However, it contains nuclear pore complexes. Each complex has eightfold symmetry and is made up of about 100 proteins organized to form a tunnel through which transport of proteins and mRNA occurs. There are many transport pathways, and proteins called importins and exportins have been isolated and characterized. Much current research is focused on transport into and out of the nucleus, and a more
detailed understanding of these processes should emerge in the near future.
ENDOPLASMIC RETICULUM The endoplasmic reticulum is a complex series of tubules in the cytoplasm of the cell (Figure 2–1). The inner limb of its membrane is continuous with a segment of the nuclear membrane, so in effect this part of the nuclear membrane is a cistern of the endoplasmic reticulum. The tubule walls are made up of membrane. In rough, or granular, endoplasmic reticulum, ribosomes are attached to the cytoplasmic side of the membrane, whereas in smooth, or agranular, endoplasmic reticulum, ribosomes are absent. Free ribosomes are also found in the cytoplasm. The granular endoplasmic reticulum is concerned with protein synthesis and the initial folding of polypeptide chains with the formation of disulfide bonds. The agranular endoplasmic reticulum is the site of steroid synthesis in steroid-secreting cells and the site of detoxification processes in other cells. A modified endoplasmic reticulum, the sarcoplasmic reticulum, plays an important role in skeletal and cardiac muscle. In particular, the endoplasmic or sarcoplasmic reticulum can sequester Ca2+ ions and allow for their release as signaling molecules in the cytosol.
RIBOSOMES The ribosomes in eukaryotes measure approximately 22 × 32 nm. Each is made up of a large and a small subunit called, on the basis of their rates of sedimentation in the ultracentrifuge, the 60S and 40S subunits. The ribosomes are complex structures, containing many different proteins and at least three ribosomal RNAs. They are the sites of protein synthesis. The ribosomes that become attached to the endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes, and endosomes. These proteins typically have a hydrophobic signal peptide at one end (Figure 2–10). The polypeptide chains that form these proteins are extruded into the endoplasmic reticulum. The free ribosomes synthesize cytoplasmic proteins such as hemoglobin and the proteins found in peroxisomes and mitochondria.
GOLGI APPARATUS & VESICULAR TRAFFIC The Golgi apparatus is a collection of membrane-enclosed sacs (cisterns) that are stacked like dinner plates (Figure 2–1). There are usually about six sacs in each apparatus, but there may be more. One or more Golgi apparati are present in all eukaryotic cells, usually near the nucleus. Much of the organization of the Golgi is directed at proper glycosylation of proteins and lipids. There are more than 200 enzymes that function to add, remove, or modify sugars from proteins and lipids in the Golgi apparatus.
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
41
Cytoplasm mRNA from Gene A
mRNA from Gene B
Free ribosome
Signal sequence
Rough endoplasmic reticulum
Carbohydrate group Growing polypeptide chain Cleaved signal sequences
Vesicle
Golgi apparatus
Lysosome
Secretory vesicle
Digestive protein from Gene B
Exocytosis Plasma membrane Secreted protein from Gene A
FIGURE 2–10
Extracellular fluid
Rough endoplasmic reticulum and protein translation. Messenger RNA and ribosomes meet up in the cytosol for translation. Proteins that have appropriate signal peptides begin translation, then associate with the endoplasmic reticulum (ER) to complete translation. The association of ribosomes is what gives the ER its “rough” appearance. (Reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
42
SECTION I Cellular & Molecular Basis of Medical Physiology
ER
Golgi apparatus
Secretory granules Regulated secretion
Constitutive secretion Recycling
Endocytosis Nucleus
FIGURE 2–11
Lysosome
Late endosome
Early endosome
Cellular structures involved in protein processing. See text for details.
The Golgi apparatus is a polarized structure, with cis and trans sides (Figure 2–11). Membranous vesicles containing newly synthesized proteins bud off from the granular endoplasmic reticulum and fuse with the cistern on the cis side of the apparatus. The proteins are then passed via other vesicles to the middle cisterns and finally to the cistern on the trans side, from which vesicles branch off into the cytoplasm. From the trans Golgi, vesicles shuttle to the lysosomes and to the cell exterior via constitutive and nonconstitutive pathways, both involving exocytosis. Conversely, vesicles are pinched off from the cell membrane by endocytosis and pass to endosomes. From there, they are recycled. Vesicular traffic in the Golgi, and between other membranous compartments in the cell, is regulated by a combination of common mechanisms along with special mechanisms that determine where inside the cell they will go. One prominent feature is the involvement of a series of regulatory proteins controlled by GTP or GDP binding (small G proteins) associated with vesicle assembly and delivery. A second prominent feature is the presence of proteins called SNAREs (for soluble N-ethylmaleimide-sensitive factor attachment receptor). The v- (for vesicle) SNAREs on vesicle membranes interact in a lock-and-key fashion with t- (for target) SNAREs. Individual vesicles also contain structural protein or lipids in their membrane that help to target them for specific membrane compartments (eg, Golgi sacs, cell membranes).
QUALITY CONTROL The processes involved in protein synthesis, folding, and migration to the various parts of the cell are so complex that it is remarkable that more errors and abnormalities do not occur. The fact that these processes work as well as they do is because of mechanisms at each level that are responsible for “quality control.” Damaged DNA is detected and repaired or bypassed. The various RNAs are also checked during the translation process. Finally, when the protein chains are in the endoplasmic reticu-
lum and Golgi apparatus, defective structures are detected and the abnormal proteins are degraded in lysosomes and proteasomes. The net result is a remarkable accuracy in the production of the proteins needed for normal body function.
APOPTOSIS In addition to dividing and growing under genetic control, cells can die and be absorbed under genetic control. This process is called programmed cell death, or apoptosis (Gr. apo “away” + ptosis “fall”). It can be called “cell suicide” in the sense that the cell’s own genes play an active role in its demise. It should be distinguished from necrosis (“cell murder”), in which healthy cells are destroyed by external processes such as inflammation. Apoptosis is a very common process during development and in adulthood. In the central nervous system, large numbers of neurons are produced and then die during the remodeling that occurs during development and synapse formation. In the immune system, apoptosis gets rid of inappropriate clones of immunocytes and is responsible for the lytic effects of glucocorticoids on lymphocytes. Apoptosis is also an important factor in processes such as removal of the webs between the fingers in fetal life and regression of duct systems in the course of sexual development in the fetus. In adults, it participates in the cyclic breakdown of the endometrium that leads to menstruation. In epithelia, cells that lose their connections to the basal lamina and neighboring cells undergo apoptosis. This is responsible for the death of the enterocytes sloughed off the tips of intestinal villi. Abnormal apoptosis probably occurs in autoimmune diseases, neurodegenerative diseases, and cancer. It is interesting that apoptosis occurs in invertebrates, including nematodes and insects. However, its molecular mechanism is much more complex than that in vertebrates. One final common pathway bringing about apoptosis is activation of caspases, a group of cysteine proteases. Many of these have been characterized to date in mammals; 11 have been found in humans. They exist in cells as inactive proenzymes
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology until activated by the cellular machinery. The net result is DNA fragmentation, cytoplasmic and chromatin condensation, and eventually membrane bleb formation, with cell breakup and removal of the debris by phagocytes (see Clinical Box 2–2).
TRANSPORT ACROSS CELL MEMBRANES There are several mechanisms of transport across cellular membranes. Primary pathways include exocytosis, endocytosis, movement through ion channels, and primary and secondary active transport. Each of these are discussed below.
EXOCYTOSIS Vesicles containing material for export are targeted to the cell membrane (Figure 2–11), where they bond in a similar manner to that discussed in vesicular traffic between Golgi stacks, via the v-SNARE/t-SNARE arrangement. The area of fusion then breaks down, leaving the contents of the vesicle outside and the cell membrane intact. This is the Ca2+-dependent process of exocytosis (Figure 2–12). Note that secretion from the cell occurs via two pathways (Figure 2–11). In the nonconstitutive pathway, proteins from the Golgi apparatus initially enter secretory granules, where processing of prohormones to the mature hormones occurs before exocytosis. The other pathway, the constitutive pathway, involves the prompt transport of proteins to the cell membrane in vesicles, with little or no processing or storage. The nonconstitutive pathway is sometimes called the regulated pathway, but this term is misleading because the output of proteins by the constitutive pathway is also regulated.
ENDOCYTOSIS Endocytosis is the reverse of exocytosis. There are various types of endocytosis named for the size of particles being ingested as well as the regulatory requirements for the particular process. These include phagocytosis, pinocytosis, clathrinmediated endocytosis, caveolae-dependent uptake, and nonclathrin/noncaveolae endocytosis. Phagocytosis (“cell eating”) is the process by which bacteria, dead tissue, or other bits of microscopic material are engulfed by cells such as the polymorphonuclear leukocytes of the blood. The material makes contact with the cell membrane, which then invaginates. The invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact. Pinocytosis (“cell drinking”) is a similar process with the vesicles much smaller in size and the substances ingested are in solution. The small size membrane that is ingested should not be misconstrued; cells undergoing active pinocytosis (eg, macrophages) can ingest the equivalent of their entire cell membrane in just 1 hour.
43
CLINICAL BOX 2–2 Molecular Medicine Fundamental research on molecular aspects of genetics, regulation of gene expression, and protein synthesis has been paying off in clinical medicine at a rapidly accelerating rate. One early dividend was an understanding of the mechanisms by which antibiotics exert their effects. Almost all act by inhibiting protein synthesis at one or another of the steps described previously. Antiviral drugs act in a similar way; for example, acyclovir and ganciclovir act by inhibiting DNA polymerase. Some of these drugs have this effect primarily in bacteria, but others inhibit protein synthesis in the cells of other animals, including mammals. This fact makes antibiotics of great value for research as well as for treatment of infections. Single genetic abnormalities that cause over 600 human diseases have now been identified. Many of the diseases are rare, but others are more common and some cause conditions that are severe and eventually fatal. Examples include the defectively regulated Cl– channel in cystic fibrosis and the unstable trinucleotide repeats in various parts of the genome that cause Huntington’s disease, the fragile X syndrome, and several other neurologic diseases. Abnormalities in mitochondrial DNA can also cause human diseases such as Leber’s hereditary optic neuropathy and some forms of cardiomyopathy. Not surprisingly, genetic aspects of cancer are probably receiving the greatest current attention. Some cancers are caused by oncogenes, genes that are carried in the genomes of cancer cells and are responsible for producing their malignant properties. These genes are derived by somatic mutation from closely related proto-oncogenes, which are normal genes that control growth. Over 100 oncogenes have been described. Another group of genes produce proteins that suppress tumors, and more than 10 of these tumor suppressor genes have been described. The most studied of these is the p53 gene on human chromosome 17. The p53 protein produced by this gene triggers apoptosis. It is also a nuclear transcription factor that appears to increase production of a 21-kDa protein that blocks two cell cycle enzymes, slowing the cycle and permitting repair of mutations and other defects in DNA. The p53 gene is mutated in up to 50% of human cancers, with the production of p53 proteins that fail to slow the cell cycle and permit other mutations in DNA to persist. The accumulated mutations eventually cause cancer.
Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. Clathrin molecules have the shape of triskelions, with three “legs” radiating from a central hub (Figure 2–13). As endocytosis progresses,
44
SECTION I Cellular & Molecular Basis of Medical Physiology
Exocytosis
Cytoplasm
Endocytosis
FIGURE 2–12
Exocytosis and endocytosis. Note that in exocytosis the cytoplasmic sides of two membranes fuse, whereas in endocytosis two noncytoplasmic sides fuse. (Reproduced with permission from Alberts B et al: Molecular Biology of the Cell, 4th ed. Garland Science, 2002.)
the clathrin molecules form a geometric array that surrounds the endocytotic vesicle. At the neck of the vesicle, the GTP binding protein dynamin is involved, either directly or indirectly, in pinching off the vesicle. Once the complete vesicle is formed, the clathrin falls off and the three-legged proteins recycle to form another vesicle. The vesicle fuses with and dumps its contents into an early endosome (Figure 2–11). From the early endosome, a new vesicle can bud off and return to the cell membrane. Alternatively, the early endosome can become a late endosome and fuse with a lysosome (Figure 2–11) in which the contents are digested by the lysosomal proteases. Clathrin-mediated endocytosis is responsible for the internal-
FIGURE 2–13
Clathrin molecule on the surface of an endocytotic vesicle. Note the characteristic triskelion shape and the fact that with other clathrin molecules it forms a net supporting the vesicle.
ization of many receptors and the ligands bound to them— including, for example, nerve growth factor and low-density lipoproteins. It also plays a major role in synaptic function. It is apparent that exocytosis adds to the total amount of membrane surrounding the cell, and if membrane were not removed elsewhere at an equivalent rate, the cell would enlarge. However, removal of cell membrane occurs by endocytosis, and such exocytosis–endocytosis coupling maintains the surface area of the cell at its normal size.
RAFTS & CAVEOLAE Some areas of the cell membrane are especially rich in cholesterol and sphingolipids and have been called rafts. These rafts are probably the precursors of flask-shaped membrane depressions called caveolae (little caves) when their walls become infiltrated with a protein called caveolin that resembles clathrin. There is considerable debate about the functions of rafts and caveolae, with evidence that they are involved in cholesterol regulation and transcytosis. It is clear, however, that cholesterol can interact directly with caveolin, effectively limiting the protein’s ability to move around in the membrane. Internalization via caveolae involves binding of cargo to caveolin and regulation by dynamin. Caveolae are prominent in endothelial cells, where they help in the uptake of nutrients from the blood.
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
COATS & VESICLE TRANSPORT
MEMBRANE PERMEABILITY & MEMBRANE TRANSPORT PROTEINS An important technique that has permitted major advances in our knowledge about transport proteins is patch clamping. A micropipette is placed on the membrane of a cell and forms a tight seal to the membrane. The patch of membrane under the pipette tip usually contains only a few transport proteins, allowing for their detailed biophysical study (Figure 2–14). The cell can be left intact (cell-attached patch clamp). Alternatively, the patch can be pulled loose from the cell, forming an inside-out patch. A third alternative is to suck out the patch with the micropipette still attached to the rest of the cell membrane, providing direct access to the interior of the cell (whole cell recording). Small, nonpolar molecules (including O2 and N2) and small uncharged polar molecules such as CO2 diffuse across the lipid membranes of cells. However, the membranes have very limited permeability to other substances. Instead, they cross the membranes by endocytosis and exocytosis and by passage through highly specific transport proteins, transmembrane proteins that form channels for ions or transport substances such as glucose, urea, and amino acids. The limited permeability applies even to water, with simple diffusion being supplemented throughout the body with various water channels (aquaporins). For reference, the sizes of ions and other biologically important substances are summarized in Table 2–2. Some transport proteins are simple aqueous ion channels, though many of these have special features that make them selective for a given substance such as Ca2+ or, in the case of aquaporins, for water. These membrane-spanning proteins (or collections of proteins) have tightly regulated pores that can be gated opened or closed in response to local changes
Inside-out patch
Electrode Pipette Cell membrane
Closed pA
It now appears that all vesicles involved in transport have protein coats. In humans, 53 coat complex subunits have been identified. Vesicles that transport proteins from the trans Golgi to lysosomes have assembly protein 1 (AP-1) clathrin coats, and endocytotic vesicles that transport to endosomes have AP-2 clathrin coats. Vesicles that transport between the endoplasmic reticulum and the Golgi have coat proteins I and II (COPI and COPII). Certain amino acid sequences or attached groups on the transported proteins target the proteins for particular locations. For example, the amino acid sequence Asn–Pro–any amino acid–Tyr targets transport from the cell surface to the endosomes, and mannose-6-phosphate groups target transfer from the Golgi to mannose-6-phosphate receptors (MPR) on the lysosomes. Various small G proteins of the Rab family are especially important in vesicular traffic. They appear to guide and facilitate orderly attachments of these vesicles. To illustrate the complexity of directing vesicular traffic, humans have 60 Rab proteins and 35 SNARE proteins.
45
ms Open
FIGURE 2–14 Patch clamp to investigate transport. In a patch clamp experiment, a small pipette is carefully maneuvered to seal off a portion of a cell membrane. The pipette has an electrode bathed in an appropriate solution that allows for recording of electrical changes through any pore in the membrane (shown below). The illustrated setup is termed an “inside-out patch” because of the orientation of the membrane with reference to the electrode. Other configurations include cell attached, whole cell, and outside-out patches. (Modified from Ackerman MJ, Clapham DE: Ion channels: Basic science and clinical disease. N Engl J Med 1997;336:1575.)
(Figure 2–15). Some are gated by alterations in membrane potential (voltage-gated), whereas others are opened or closed in response to a ligand (ligand-gated). The ligand is
TABLE 2–2 Size of hydrated ions and other substances of biologic interest. Substance
Atomic or Molecular Weight
Radius (nm)
Cl–
35
0.12
K+
39
0.12
H2 O
18
0.12
Ca2+
40
0.15
Na+
23
0.18
Urea
60
0.23
7
0.24
Glucose
180
0.38
Sucrose
342
0.48
5000
0.75
69,000
7.50
Li+
Inulin Albumin
Data from Moore EW: Physiology of Intestinal Water and Electrolyte Absorption. American Gastroenterological Association, 1976.
46
SECTION I Cellular & Molecular Basis of Medical Physiology
Closed
Open
A Ligand-gated
Bind ligand
B Phosphorylation-gated Phosphorylate
Dephosphorylate Pi
C Voltage-gated
P
Change membrane potential
++
++
––
––
––
––
++
++
D Stretch or pressure-gated Stretch
Cytoskeleton
FIGURE 2–15
Regulation of gating in ion channels. Several types of gating are shown for ion channels. A) Ligand-gated channels open in response to ligand binding. B) Protein phosphorylation or dephosphorylation regulate opening and closing of some ion channels. C) Changes in membrane potential alter channel openings. D) Mechanical stretch of the membrane results in channel opening. (Repro-
duced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
often external (eg, a neurotransmitter or a hormone). However, it can also be internal; intracellular Ca2+, cAMP, lipids, or one of the G proteins produced in cells can bind directly to channels and activate them. Some channels are also opened by mechanical stretch, and these mechanosensitive channels play an important role in cell movement. Other transport proteins are carriers that bind ions and other molecules and then change their configuration, moving
the bound molecule from one side of the cell membrane to the other. Molecules move from areas of high concentration to areas of low concentration (down their chemical gradient), and cations move to negatively charged areas whereas anions move to positively charged areas (down their electrical gradient). When carrier proteins move substances in the direction of their chemical or electrical gradients, no energy input is required and the process is called facilitated diffusion. A typical example is glucose transport by the glucose transporter, which moves glucose down its concentration gradient from the ECF to the cytoplasm of the cell. Other carriers transport substances against their electrical and chemical gradients. This form of transport requires energy and is called active transport. In animal cells, the energy is provided almost exclusively by hydrolysis of ATP. Not surprisingly, therefore, many carrier molecules are ATPases, enzymes that catalyze the hydrolysis of ATP. One of these ATPases is sodium–potassium adenosine triphosphatase (Na, K ATPase), which is also known as the Na, K pump. There are also H, K ATPases in the gastric mucosa and the renal tubules. Ca2+ATPase pumps Ca2+ out of cells. Proton ATPases acidify many intracellular organelles, including parts of the Golgi complex and lysosomes. Some of the transport proteins are called uniports because they transport only one substance. Others are called symports because transport requires the binding of more than one substance to the transport protein and the substances are transported across the membrane together. An example is the symport in the intestinal mucosa that is responsible for the cotransport by facilitated diffusion of Na+ and glucose from the intestinal lumen into mucosal cells. Other transporters are called antiports because they exchange one substance for another.
ION CHANNELS There are ion channels specific for K+, Na+, Ca2+, and Cl–, as well as channels that are nonselective for cations or anions. Each type of channel exists in multiple forms with diverse properties. Most are made up of identical or very similar subunits. Figure 2–16 shows the multiunit structure of various channels in diagrammatic cross-section. Most K+ channels are tetramers, with each of the four subunits forming part of the pore through which K+ ions pass. Structural analysis of a bacterial voltage-gated K+ channel indicates that each of the four subunits have a paddle-like extension containing four charges. When the channel is closed, these extensions are near the negatively charged interior of the cell. When the membrane potential is reduced, the paddles containing the charges bend through the membrane to its exterior surface, causing the channel to open. The bacterial K+ channel is very similar to the voltage-gated K+ channels in a wide variety of species, including mammals. In the acetylcholine ion channel and other ligand-gated cation or anion channels, five subunits make up the pore. Members of the ClC family of Cl– channels are dimers, but they have two pores, one in each subunit. Finally, aquaporins are tetramers
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
A
B
C
D
FIGURE 2–16
Different ways in which ion channels form pores. Many K+ channels are tetramers (A), with each protein subunit forming part of the channel. In ligand-gated cation and anion channels (B) such as the acetylcholine receptor, five identical or very similar subunits form the channel. Cl– channels from the ClC family are dimers (C), with an intracellular pore in each subunit. Aquaporin water channels (D) are tetramers with an intracellular channel in each subunit.
(Reproduced with permission from Jentsch TJ: Chloride channels are different. Nature 2002;415:276.)
with a water pore in each of the subunits. Recently, a number of ion channels with intrinsic enzyme activity have been cloned. More than 30 different voltage-gated or cyclic nucleotide-gated Na+ and Ca2+ channels of this type have been described. Representative Na+, Ca2+, and K+ channels are shown in extended diagrammatic form in Figure 2–17. Another family of Na+channels with a different structure has been found in the apical membranes of epithelial cells in the kidneys, colon, lungs, and brain. The epithelial sodium channels (ENaCs) are made up of three subunits encoded by three different genes. Each of the subunits probably spans the membrane twice, and the amino terminal and carboxyl terminal are located inside the cell. The α subunit transports Na+, whereas the β and γ subunits do not. However, the addition of the β and γ subunits increases Na+ transport through the α subunit. ENaCs are inhibited by the diuretic amiloride, which binds to the α subunit, and they used to be called amilorideinhibitable Na+channels. The ENaCs in the kidney play an important role in the regulation of ECF volume by aldosterone. ENaC knockout mice are born alive but promptly die because they cannot move Na+, and hence water, out of their lungs. Humans have several types of Cl– channels. The ClC dimeric channels are found in plants, bacteria, and animals, and there are nine different ClC genes in humans. Other Cl– channels have the same pentameric form as the acetylcholine receptor; examples include the γ-aminobutyric acid A (GABAA) and glycine receptors in the central nervous system (CNS). The cystic fibrosis transmembrane conductance regulator (CFTR) that is mutated in cystic fibrosis is also a Cl– channel. Ion channel mutations cause a variety of channelopathies—diseases that mostly affect muscle and brain tissue and produce episodic paralyses or convulsions.
Na, K ATPase As noted previously, Na, K ATPase catalyzes the hydrolysis of ATP to adenosine diphosphate (ADP) and uses the energy to
47
extrude three Na+ from the cell and take two K+ into the cell for each molecule of ATP hydrolyzed. It is an electrogenic pump in that it moves three positive charges out of the cell for each two that it moves in, and it is therefore said to have a coupling ratio of 3:2. It is found in all parts of the body. Its activity is inhibited by ouabain and related digitalis glycosides used in the treatment of heart failure. It is a heterodimer made up of an α subunit with a molecular weight of approximately 100,000 and a β subunit with a molecular weight of approximately 55,000. Both extend through the cell membrane (Figure 2–18). Separation of the subunits eliminates activity. The β subunit is a glycoprotein, whereas Na+ and K+ transport occur through the α subunit. The β subunit has a single membrane-spanning domain and three extracellular glycosylation sites, all of which appear to have attached carbohydrate residues. These residues account for one third of its molecular weight. The α subunit probably spans the cell membrane 10 times, with the amino and carboxyl terminals both located intracellularly. This subunit has intracellular Na+- and ATP-binding sites and a phosphorylation site; it also has extracellular binding sites for K+ and ouabain. The endogenous ligand of the ouabain-binding site is unsettled. When Na+ binds to the α subunit, ATP also binds and is converted to ADP, with a phosphate being transferred to Asp 376, the phosphorylation site. This causes a change in the configuration of the protein, extruding Na+ into the ECF. K+ then binds extracellularly, dephosphorylating the α subunit, which returns to its previous conformation, releasing K+ into the cytoplasm. The α and β subunits are heterogeneous, with α1, α2, and α3 subunits and β1, β2, and β3 subunits described so far. The α1 isoform is found in the membranes of most cells, whereas α2 is present in muscle, heart, adipose tissue, and brain, and α3 is present in heart and brain. The β1 subunit is widely distributed but is absent in certain astrocytes, vestibular cells of the inner ear, and glycolytic fast-twitch muscles. The fast-twitch muscles contain only β2 subunits. The different α and β subunit structures of Na, K ATPase in various tissues probably represent specialization for specific tissue functions.
REGULATION OF Na, K ATPase ACTIVITY The amount of Na+ normally found in cells is not enough to saturate the pump, so if the Na+ increases, more is pumped out. Pump activity is affected by second messenger molecules (eg, cAMP and diacylglycerol [DAG]). The magnitude and direction of the altered pump effects vary with the experimental conditions. Thyroid hormones increase pump activity by a genomic action to increase the formation of Na, K ATPase molecules. Aldosterone also increases the number of pumps, although this effect is probably secondary. Dopamine in the kidney inhibits the pump by phosphorylating it, causing a natriuresis. Insulin increases pump activity, probably by a variety of different mechanisms.
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SECTION I Cellular & Molecular Basis of Medical Physiology
Na+ channel
I
II
III
IV Extracellular side
1 2 3 4 5
P
6
1 2 3 4 5
P
6
1 2 3 4 5
P
6
1 2 3 4 5
P
6 Cytoplasmic side COOH
NH2
Ca2+ channel
1 2 3 4 5
P
6
1 2 3 4 5
P
6
1 2 3 4 5
P
6
1 2 3 4 5
P
6
COOH NH2
K+ channel
1 2 3 4 5
P
6
COOH NH2
FIGURE 2–17
Diagrammatic representation of the pore-forming subunits of three ion channels. The α subunit of the Na+ and Ca2+ channels traverse the membrane 24 times in four repeats of six membrane-spanning units. Each repeat has a “P” loop between membrane spans 5 and 6 that does not traverse the membrane. These P loops are thought to form the pore. Note that span 4 of each repeat is colored in red, representing its net “+” charge. The K+ channel has only a single repeat of the six spanning regions and P loop. Four K + subunits are assembled for a functional K+ channel. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
SECONDARY ACTIVE TRANSPORT In many situations, the active transport of Na+ is coupled to the transport of other substances (secondary active transport). For example, the luminal membranes of mucosal cells in the small intestine contain a symport that transports glucose into the cell only if Na+ binds to the protein and is transported into the cell at the same time. From the cells, the
glucose enters the blood. The electrochemical gradient for Na+ is maintained by the active transport of Na+ out of the mucosal cell into ECF. Other examples are shown in Figure 2– 19. In the heart, Na,K ATPase indirectly affects Ca2+ transport. An antiport in the membranes of cardiac muscle cells normally exchanges intracellular Ca2+ for extracellular Na+. Active transport of Na+ and K+ is one of the major energyusing processes in the body. On the average, it accounts for
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
49
Active transport
2K+
2K+
Ouabain
Ouabain
β 3Na+
3 2
1
α 4
5
3Na+
FIGURE 2–18
Na+–K+ ATPase. The intracellular portion of the α subunit has a Na+-binding site (1), a phosphorylation site (4), and an ATP-binding site (5). The extracellular portion has a K +-binding site (2) and an ouabain-binding site (3). (From Horisberger J-D et al:
Countertransport
Cytoplasm
ATP Na+
K+, 2Cl− Na+
Na+ 15 meq/L K+ 150 − 7 − Cl−
H+ K+
K+ Cl−
H+
− +
− − Vm = −70 mV + +
Sugars or amino acids
− + Na+
Na+ 140 meq/L 4 − K+ Cl− 105 −
Reproduced with permission from the Annual Review of Physiology, vol. 53.
about 24% of the energy utilized by cells, and in neurons it accounts for 70%. Thus, it accounts for a large part of the basal metabolism. A major payoff for this energy use is the establishment of the electrochemical gradient in cells.
Cl− Na+
Ca2+
Structure–function relationship of Na–K-ATPase. Annu Rev Physiol 1991;53:565. Copyright © 1991 by Annual Reviews)
Na+
3Na+ ADP + Pi
Cotransport
ECF
FIGURE 2–19
Composite diagram of main secondary effects of active transport of Na+ and K+. Na,K ATPase converts the chemical energy of ATP hydrolysis into maintenance of an inward gradient for Na+ and an outward gradient for K+. The energy of the gradients is used for countertransport, cotransport, and maintenance of the membrane potential. Some examples of cotransport and countertransport that use these gradients are shown. (Reproduced with permission from Skou
JC: The Na–K pump. News Physiol Sci 1992;7:95.)
TRANSPORT ACROSS EPITHELIA In the gastrointestinal tract, the pulmonary airways, the renal tubules, and other structures, substances enter one side of a cell and exit another, producing movement of the substance from one side of the epithelium to the other. For transepithelial transport to occur, the cells need to be bound by tight junctions and, obviously, have different ion channels and transport proteins in different parts of their membranes. Most of the instances of secondary active transport cited in the preceding paragraph involve transepithelial movement of ions and other molecules.
THE CAPILLARY WALL FILTRATION The capillary wall separating plasma from interstitial fluid is different from the cell membranes separating interstitial fluid from intracellular fluid because the pressure difference across it makes filtration a significant factor in producing movement of water and solute. By definition, filtration is the process by which fluid is forced through a membrane or other barrier because of a difference in pressure on the two sides.
ONCOTIC PRESSURE The structure of the capillary wall varies from one vascular bed to another. However, in skeletal muscle and many other organs,
water and relatively small solutes are the only substances that cross the wall with ease. The apertures in the junctions between the endothelial cells are too small to permit plasma proteins and other colloids to pass through in significant quantities. The colloids have a high molecular weight but are present in large amounts. Small amounts cross the capillary wall by vesicular transport, but their effect is slight. Therefore, the capillary wall behaves like a membrane impermeable to colloids, and these exert an osmotic pressure of about 25 mm Hg. The colloid osmotic pressure due to the plasma colloids is called the oncotic pressure. Filtration across the capillary membrane as a result of the hydrostatic pressure head in the vascular system is opposed by the oncotic pressure. The way the balance between the hydrostatic and oncotic pressures controls exchanges across the capillary wall is considered in detail in Chapter 32.
TRANSCYTOSIS Vesicles are present in the cytoplasm of endothelial cells, and tagged protein molecules injected into the bloodstream have been found in the vesicles and in the interstitium. This indicates that small amounts of protein are transported out of capillaries across endothelial cells by endocytosis on the capillary side followed by exocytosis on the interstitial side of the cells. The transport mechanism makes use of coated vesicles that appear to be coated with caveolin and is called transcytosis, vesicular transport, or cytopempsis.
50
SECTION I Cellular & Molecular Basis of Medical Physiology
GAP JUNCTIONS
SYNAPTIC
PARACRINE AND AUTOCRINE A
ENDOCRINE
P
Message transmission
Directly from cell to cell
Across synaptic cleft
By diffusion in interstitial fluid
By circulating body fluids
Local or general
Local
Local
Locally diffuse
General
Anatomic location and receptors
Receptors
Receptors
Specificity depends on
FIGURE 2–20
Anatomic location
Intercellular communication by chemical mediators. A, autocrine; P, paracrine.
INTERCELLULAR COMMUNICATION Cells communicate with one another via chemical messengers. Within a given tissue, some messengers move from cell to cell via gap junctions without entering the ECF. In addition, cells are affected by chemical messengers secreted into the ECF, or by direct cell–cell contacts. Chemical messengers typically bind to protein receptors on the surface of the cell or, in some instances, in the cytoplasm or the nucleus, triggering sequences of intracellular changes that produce their physiologic effects. Three general types of intercellular communication are mediated by messengers in the ECF: (1) neural communication, in which neurotransmitters are released at synaptic junctions from nerve cells and act across a narrow synaptic cleft on a postsynaptic cell; (2) endocrine communication, in which hormones and growth factors reach cells via the circulating blood or the lymph; and (3) paracrine communication, in which the products of cells diffuse in the ECF to affect neighboring cells that may be some distance away (Figure 2–20). In addition, cells secrete chemical messengers that in some situations bind to receptors on the same cell, that is, the cell that secreted the messenger (autocrine communication). The chemical messengers include amines, amino acids, steroids, polypeptides, and in some instances, lipids, purine nucleotides, and pyrimidine nucleotides. It is worth noting that in various parts of the body, the same chemical messenger can function as a neurotransmitter, a paracrine mediator, a hormone secreted by neurons into the blood (neural hormone), and a hormone secreted by gland cells into the blood. An additional form of intercellular communication is called juxtacrine communication. Some cells express multiple repeats of growth factors such as transforming growth factor alpha (TGFα) extracellularly on transmembrane proteins that provide an anchor to the cell. Other cells have TGFα receptors. Consequently, TGFα anchored to a cell can bind to a TGFα receptor on another cell, linking the two. This could be important in producing local foci of growth in tissues.
RECEPTORS FOR CHEMICAL MESSENGERS The recognition of chemical messengers by cells typically begins by interaction with a receptor at that cell. There have been over 20 families of receptors for chemical messengers characterized. These proteins are not static components of the cell, but their numbers increase and decrease in response to various stimuli, and their properties change with changes in physiological conditions. When a hormone or neurotransmitter is present in excess, the number of active receptors generally decreases (down-regulation), whereas in the presence of a deficiency of the chemical messenger, there is an increase in the number of active receptors (up-regulation). In its actions on the adrenal cortex, angiotensin II is an exception; it increases rather than decreases the number of its receptors in the adrenal. In the case of receptors in the membrane, receptor-mediated endocytosis is responsible for down-regulation in some instances; ligands bind to their receptors, and the ligand– receptor complexes move laterally in the membrane to coated pits, where they are taken into the cell by endocytosis (internalization). This decreases the number of receptors in the membrane. Some receptors are recycled after internalization, whereas others are replaced by de novo synthesis in the cell. Another type of down-regulation is desensitization, in which receptors are chemically modified in ways that make them less responsive.
MECHANISMS BY WHICH CHEMICAL MESSENGERS ACT Receptor–ligand interaction is usually just the beginning of the cell response. This event is transduced into secondary responses within the cell that can be divided into four broad categories: (1) ion channel activation, (2) G-protein activation, (3) activation of enzyme activity within the cell, or (4) direct activation of transcription. Within each of these groups, responses can be quite varied. Some of the common mechanisms by which
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
TABLE 2–3 Common mechanisms by which chemical messengers in the ECF bring about changes in cell function. Mechanism
Examples
Open or close ion channels in cell membrane
Acetylcholine on nicotinic cholinergic receptor; norepinephrine on K+ channel in the heart
Act via cytoplasmic or nuclear receptors to increase transcription of selected mRNAs
Thyroid hormones, retinoic acid, steroid hormones
Activate phospholipase C with intracellular production of DAG, IP3, and other inositol phosphates
Angiotensin II, norepinephrine via α1-adrenergic receptor, vasopressin via V1 receptor
Activate or inhibit adenylyl cyclase, causing increased or decreased intracellular production of cAMP
Norepinephrine via β1-adrenergic receptor (increased cAMP); norepinephrine via α2-adrenergic receptor (decreased cAMP)
Increase cGMP in cell
Atrial natriuretic peptide; nitric oxide
Increase tyrosine kinase activity of cytoplasmic portions of transmembrane receptors
Insulin, epidermal growth factor (EGF), platelet-derived growth factor (PDGF), monocyte colonystimulating factor (M-CSF)
Increase serine or threonine kinase activity
TGFβ, activin, inhibin
51
pathways. Cellular phosphorylation is under the control of two groups of proteins: kinases, enzymes that catalyze the phosphorylation of tyrosine or serine and threonine residues in proteins (or in some cases, in lipids); and phosphatases, proteins that remove phosphates from proteins (or lipids). Some of the larger receptor families are themselves kinases. Tyrosine kinase receptors initiate phosphorylation on tyrosine residues on complementary receptors following ligand binding. Serine/threonine kinase receptors initiate phosphorylation on serines or threonines in complementary receptors following ligand binding. Cytokine receptors are directly associated with a group of protein kinases that are activated following cytokine binding. Alternatively, second messengers changes can lead to phosphorylation further downstream in the signaling pathway. More than 300 protein kinases have been described. Some of the principal ones that are important in mammalian cell signaling are summarized in Table 2–4. In general, addition of phosphate groups changes the conformation of the proteins, altering their functions and consequently the functions of the cell. The close relationship between phosphorylation and dephosphorylation of cellular proteins allows for a temporal control of activation of cell signaling pathways. This is sometimes referred to as a “phosphate timer.”
STIMULATION OF TRANSCRIPTION The activation of transcription, and subsequent translation, is a common outcome of cellular signaling. There are three
chemical messengers exert their intracellular effects are summarized in Table 2–3. Ligands such as acetylcholine bind directly to ion channels in the cell membrane, changing their conductance. Thyroid and steroid hormones, 1,25-dihydroxycholecalciferol, and retinoids enter cells and act on one or another member of a family of structurally related cytoplasmic or nuclear receptors. The activated receptor binds to DNA and increases transcription of selected mRNAs. Many other ligands in the ECF bind to receptors on the surface of cells and trigger the release of intracellular mediators such as cAMP, IP3, and DAG that initiate changes in cell function. Consequently, the extracellular ligands are called “first messengers” and the intracellular mediators are called “second messengers.” Second messengers bring about many short-term changes in cell function by altering enzyme function, triggering exocytosis, and so on, but they also can lead to the alteration of transcription of various genes. A variety of enzymatic changes, protein–protein interactions or second messenger changes can be activated within a cell in an orderly fashion following receptor recognition of the primary messenger. The resulting cell signaling pathway provides amplification of the primary signal and distribution of the signal to appropriate targets within the cell. Extensive cell signaling pathways also provide opportunities for feedback and regulation that can fine tune the signal for the correct physiological response by the cell. The most predominant posttranslation modification of proteins, phosphorylation, is a common theme in cell signaling
TABLE 2–4 Sample protein kinases. Phosphorylate serine or threonine residues, or both Calmodulin-dependent Myosin light-chain kinase Phosphorylase kinase Ca2+/calmodulin kinase I Ca2+/calmodulin kinase II Ca2+/calmodulin kinase III Calcium-phospholipid-dependent Protein kinase C (seven subspecies) Cyclic nucleotide-dependent cAMP-dependent kinase (protein kinase A; two subspecies) cGMP-dependent kinase Phosphorylate tyrosine residues Insulin receptor, EGF receptor, PDGF receptor, and M-CSF receptor
52
SECTION I Cellular & Molecular Basis of Medical Physiology
distinct pathways for primary messengers to alter transcription of cells. First, as is the case with steroid or thyroid hormones, the primary messenger is able to cross the cell membrane and bind to a nuclear receptor, which then can directly interact with DNA to alter gene expression. A second pathway to gene transcription is the activation of cytoplasmic protein kinases that can move to the nucleus to phosphorylate a latent transcription factor for activation. This pathway is a common endpoint of signals that go through the mitogen activated protein (MAP) kinase cascade. MAP kinases can be activated following a variety of receptor ligand interactions through second messenger signaling. They comprise a series of three kinases that coordinate a stepwise phosphorylation to activate each protein in series in the cytosol. Phosphorylation of the last MAP kinase in series allows it to migrate to the nucleus where it phosphorylates a latent transcription factor. A third common pathway is the activation of a latent transcription factor in the cytosol, which then migrates to the nucleus and alters transcription. This pathway is shared by a diverse set of transcription factors that include nuclear factor kappa B (NFκB; activated following tumor necrosis family receptor binding and others), and signal transducers of activated transcription (STATs; activated following cytokine receptor binding). In all cases the binding of the activated transcription factor to DNA increases (or in some cases, decreases) the transcription of mRNAs encoded by the gene to which it binds. The mRNAs are translated in the ribosomes, with the production of increased quantities of proteins that alter cell function.
INTRACELLULAR Ca2+ AS A SECOND MESSENGER Ca2+ regulates a very large number of physiological processes that are as diverse as proliferation, neural signaling, learning, contraction, secretion, and fertilization, so regulation of intracellular Ca2+ is of great importance. The free Ca2+ concentration in the cytoplasm at rest is maintained at about 100 nmol/ L. The Ca2+ concentration in the interstitial fluid is about 12,000 times the cytoplasmic concentration (ie, 1,200,000 nmol/L), so there is a marked inwardly directed concentration gradient as well as an inwardly directed electrical gradient. Much of the intracellular Ca2+ is stored at relatively high concentrations in the endoplasmic reticulum and other organelles (Figure 2–21), and these organelles provide a store from which Ca2+ can be mobilized via ligand-gated channels to increase the concentration of free Ca2+ in the cytoplasm. Increased cytoplasmic Ca2+ binds to and activates calcium-binding proteins. These proteins can have direct effects in cellular physiology, or can activate other proteins, commonly protein kinases, to further cell signaling pathways. Ca2+ can enter the cell from the extracellular fluid, down its electrochemical gradient, through many different Ca2+ channels. Some of these are ligand-gated and others are voltagegated. Stretch-activated channels exist in some cells as well.
Ca2+ (volt)
CaBP
Effects 2H+ ATP
Ca2+ (lig)
Ca2+
Ca2+ (SOCC)
Ca2+ 3Na+ Ca2+
Mitochondrion
Endoplasmic reticulum
FIGURE 2–21
Ca2+ handling in mammalian cells. Ca2+ is stored in the endoplasmic reticulum and, to a lesser extent, mitochondria and can be released from them to replenish cytoplasmic Ca 2+. Calciumbinding proteins (CaBP) bind cytoplasmic Ca2+ and, when activated in this fashion, bring about a variety of physiologic effects. Ca 2+ enters the cells via voltage-gated (volt) and ligand-gated (lig) Ca 2+ channels and store-operated calcium channels ( SOCCs). It is transported out of the cell by Ca, Mg ATPases (not shown), Ca, H ATPase and an Na, Ca antiport. It is also transported into the ER by Ca ATPases.
Many second messengers act by increasing the cytoplasmic Ca2+ concentration. The increase is produced by releasing Ca2+ from intracellular stores—primarily the endoplasmic reticulum—or by increasing the entry of Ca2+ into cells, or by both mechanisms. IP3 is the major second messenger that causes Ca2+ release from the endoplasmic reticulum through the direct activation of a ligand-gated channel, the IP3 receptor. In effect, the generation of one second messenger (IP3) can lead to the release of another second messenger (Ca2+). In many tissues, transient release of Ca2+ from internal stores into the cytoplasm triggers opening of a population of Ca2+ channels in the cell membrane (store-operated Ca2+ channels; SOCCs). The resulting Ca2+ influx replenishes the total intracellular Ca2+ supply and refills the endoplasmic reticulum. The exact identity of the SOCCs is still unknown, and there is debate about the signal from the endoplasmic reticulum that opens them. As with other second messenger molecules, the increase in Ca2+ within the cytosol is rapid, and is followed by a rapid decrease. Because the movement of Ca2+ outside of the cytosol (ie, across the plasma membrane or the membrane of the internal store) requires that it move up its electrochemical gradient, it requires energy. Ca2+ movement out of the cell is facilitated by the plasma membrane Ca2+ ATPase. Alternatively, it can be transported by an antiport that exchanges three Na+ for each Ca2+ driven by the energy stored in the Na+ electrochemical gradient. Ca2+ movement into the internal stores is through the action of the sarcoplasmic or endoplasmic reticulum Ca2+ ATPase, also known as the SERCA pump.
CALCIUM-BINDING PROTEINS Many different Ca2+-binding proteins have been described, including troponin, calmodulin, and calbindin. Troponin is the
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
90 R V F F R D D K K E E P F R G M D I N K Ca L E I Ca E G D R E T D A A Y V T E H A G I S S D D E G N V 100 80 M 60 N T (Me)3 N 50 I N 110 G L M D Q L E A E E K T L P 120 40 N T Q D E E G L V S D R E M M V G D G T I I T E T T COOH D 130 V G K 30 R G I K Ca L E N E Ca N F E A N A Y K G T 10 A E E D E 140 M I 20 E K A M Q V F Q F L S F E L
70 T M M
A
A
F
E T L Q D A NH Ac
FIGURE 2–22
Structure of calmodulin from bovine brain. Single-letter abbreviations are used for the amino acid residues. Note the four calcium domains (purple residues) flanked on either side by stretches of α helix. (Reproduced with permission from Cheung WY:
Calmodulin: An overview. Fed Proc 1982;41:2253.)
Ca2+-binding protein involved in contraction of skeletal muscle (Chapter 5). Calmodulin contains 148 amino acid residues (Figure 2–22) and has four Ca2+-binding domains. It is unique in that amino acid residue 115 is trimethylated, and it is extensively conserved, being found in plants as well as animals. When calmodulin binds Ca2+, it is capable of activating five different calmodulin-dependent kinases (CaMKs; Table 2–4), among other proteins. One of the kinases is myosin light-chain kinase, which phosphorylates myosin. This brings about contraction in smooth muscle. CaMKI and CaMKII are concerned with synaptic function, and CaMKIII is concerned with protein synthesis. Another calmodulin-activated protein is calcineurin, a phosphatase that inactivates Ca2+ channels by dephosphorylating them. It also plays a prominent role in activating T cells and is inhibited by some immunosuppressants.
MECHANISMS OF DIVERSITY OF Ca2+ ACTIONS It may seem difficult to understand how intracellular Ca2+ can have so many varied effects as a second messenger. Part of the explanation is that Ca2+ may have different effects at low and at high concentrations. The ion may be at high concentration at the site of its release from an organelle or a channel (Ca2+ sparks) and at a subsequent lower concentration after it diffuses throughout the cell. Some of the changes it produces can outlast the rise in intracellular Ca2+ concentration because of the way it binds to some of the Ca2+-binding proteins. In addition, once released, intracellular Ca2+ concentrations fre-
53
quently oscillate at regular intervals, and there is evidence that the frequency and, to a lesser extent, the amplitude of those oscillations codes information for effector mechanisms. Finally, increases in intracellular Ca2+ concentration can spread from cell to cell in waves, producing coordinated events such as the rhythmic beating of cilia in airway epithelial cells.
G PROTEINS A common way to translate a signal to a biologic effect inside cells is by way of nucleotide regulatory proteins that are activated after binding GTP (G proteins). When an activating signal reaches a G protein, the protein exchanges GDP for GTP. The GTP–protein complex brings about the activating effect of the G protein. The inherent GTPase activity of the protein then converts GTP to GDP, restoring the G protein to an inactive resting state. G proteins can be divided into two principal groups involved in cell signaling: small G proteins and heterotrimeric G proteins. Other groups that have similar regulation and are also important to cell physiology include elongation factors, dynamin, and translocation GTPases. There are six different families of small G proteins (or small GTPases) that are all highly regulated. GTPase activating proteins (GAPs) tend to inactivate small G proteins by encouraging hydrolysis of GTP to GDP in the central binding site. Guanine exchange factors (GEFs) tend to activate small G proteins by encouraging exchange of GDP for GTP in the active site. Some of the small G proteins contain lipid modifications that help to anchor them to membranes, while others are free to diffuse throughout the cytosol. Small G proteins are involved in many cellular functions. Members of the Rab family regulate the rate of vesicle traffic between the endoplasmic reticulum, the Golgi apparatus, lysosomes, endosomes, and the cell membrane. Another family of small GTPbinding proteins, the Rho/Rac family, mediates interactions between the cytoskeleton and cell membrane; and a third family, the Ras family, regulates growth by transmitting signals from the cell membrane to the nucleus. Another family of G proteins, the larger heterotrimeric G proteins, couple cell surface receptors to catalytic units that catalyze the intracellular formation of second messengers or couple the receptors directly to ion channels. Despite the knowledge of the small G proteins described above, the heteromeric G proteins are frequently referred to in the shortened “G protein” form because they were the first to be identified. Heterotrimeric G proteins are made up of three subunits designated α, β, and γ (Figure 2–23). Both the α and the γ subunits have lipid modifications that anchor these proteins to plasma membrane. The α subunit is bound to GDP. When a ligand binds to a G protein-coupled receptor (GPCR), this GDP is exchanged for GTP and the α subunit separates from the combined β and γ subunits. The separated α subunit brings about many biologic effects. The β and γ subunits are tightly bound in the cell and together form a signaling molecule that can also activate a variety of effectors. The intrinsic
54
SECTION I Cellular & Molecular Basis of Medical Physiology
TABLE 2–5 Some of the ligands for receptors
Nucleotide exchange Input
coupled to heterotrimeric G proteins.
GDP
GTP
Output
GTPase activity
Class
Ligand
Neurotransmitters
Epinephrine Norepinephrine Dopamine 5-Hydroxytryptamine
α
β
γ
α
β
Histamine
γ
Acetylcholine Effectors
Adenosine
FIGURE 2–23
Heterotrimeric G proteins. Top Summary of overall reaction that occurs in the Gα subunit. Bottom: When the ligand (square) binds to the G protein-coupled receptor in the cell membrane, GTP replaces GDP on the α subunit. GTP-α separates from the βγ subunit and GTP-α and βγ both activate various effectors, producing physiologic effects. The intrinsic GTPase activity of GTP-α then converts GTP to GDP, and the α, β, and γ subunits reassociate.
GTPase activity of the α subunit then converts GTP to GDP, and this leads to reassociation of the α with the βγ subunit and termination of effector activation. The GTPase activity of the α subunit can be accelerated by a family of regulators of G protein signaling (RGS). Heterotrimeric G proteins relay signals from over 1000 GPCRs, and their effectors in the cells include ion channels and enzymes (Table 2–5). There are 20 α, 6 β, and 12 γ genes, which allow for over 1400 α, β, and γ combinations. Not all combinations occur in the cell, but over 20 different heterotrimeric G proteins have been well documented in cell signaling. They can be divided into five families, each with a relatively characteristic set of effectors.
Opioids Tachykinins
Substance P Neurokinin A Neuropeptide K
Other peptides
Angiotensin II Arginine vasopressin Oxytocin VIP, GRP, TRH, PTH
Glycoprotein hormones
TSH, FSH, LH, hCG
Arachidonic acid derivatives
Thromboxane A2
Other
Odorants Tastants Endothelins Platelet-activating factor Cannabinoids
G PROTEIN-COUPLED RECEPTORS All the heterotrimeric G protein-coupled receptors (GPCRs) that have been characterized to date are proteins that span the cell membrane seven times. Because of this structure they are alternatively referred to as seven-helix receptors or serpentine receptors. A very large number have been cloned, and their functions are multiple and diverse. The topological structures of two of them are shown in Figure 2–24. These receptors further assemble into a barrel-like structure. Upon ligand binding, a conformational change activates a resting heterotrimeric G protein associated with the cytoplasmic leaf of the plasma membrane. Activation of a single receptor can result in 1, 10, or more active heterotrimeric G proteins, providing amplification as well as transduction of the first messenger. Bound receptors can be inactivated to limit the amount of cellular signaling. This frequently occurs through phosphorylation of the cytoplasmic side of the receptor.
Light
INOSITOL TRISPHOSPHATE & DIACYLGLYCEROL AS SECOND MESSENGERS The link between membrane binding of a ligand that acts via Ca2+ and the prompt increase in the cytoplasmic Ca2+ concentration is often inositol trisphosphate (inositol 1,4,5-trisphosphate; IP3). When one of these ligands binds to its receptor, activation of the receptor produces activation of phospholipase C (PLC) on the inner surface of the membrane. Ligands bound to G protein-coupled receptor can do this through the Gq heterotrimeric G proteins, while ligands bound to tyrosine kinase receptors can do this through other cell signaling pathways. PLC has at least eight isoforms; PLCβ
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
H D V T E E R D E A W V V
D P V H S G N T T L L F D S D N G P P G M
NH2
2-Adrenergic receptor
C Y H K D E T I C A C K D F N G Q F N W H F M F T N L I W T K D P A N Q C R M K E V Y G M A L I N E F W Y W H Q A Y I V H I L L N A I A S V I N V I L M S B A G F T S I D MQ I P W L G V I V P V V V L C L F S S I V I F F Y V N S A L G M L A I V V T A S T L Q S S F Y V P L W C A F N F G N V L D I E T V I W P L V L T F P L I Y V L V I A C A L L C V I V M L I V M V F T G M I C R S A V D V M R V Y S I G L T A I S T I P T A F R R A D F K R I A F Y Y K V K L Q I A K N N F F H E E K A T K L C E R Q V F L A K R Q L Q T L Q T V I K K L L T I S C L S D S S P K L H S Q F R G E F K Y S R R P R R N L G Q V E Q D G R S G H G L S S S K S A E G M Y D T K G N G N S S Y G N G Y A G C Q L G Q E K E S E R L C E D P P G T E S F V N C Q HOOC L P S D N T S C N R G Q S D L S L S P V T G
Extracellular surface
Cytoplasmic surface
F E A P Q Y Y L A E P W Q
R V V C T K N S F P V Y F N P G E T G N M P S
Intradiskal surface
C G I D S C Q M G E P I
55
NH2
Rhodopsin Y
Y T P G V H C F E N Y E L G S G Q E T D H Y F G H N L R G N F S T S P I F M F S M T Y L F A T E S F F I Y WG V T I P A V I Y M F A V Q L A A Y T T T F L G G E L P P A A C A F F A A Y P M F L Q G F I A L F V V K T S A L I M L V M F L L A M V H F I I L W C I W S L A V Y N G F P D A V V L A P L I L F A W T F A V G M P V I Y I E R Y I N F L A L N L V I F F I V M I I M M V V V I A H C Y G I V M T L Y L I Y N R C N Q N V K Q T F R N C L E L K T M V E P K E A G V V P V K Q F M T F Q H R T S R F L N K K T T V T K T E A L A A A S Q Q Q E C C G HOOC A P A V Q S T E T K S V T T S A E D D G L P N K G P F T
Cytoplasmic surface
FIGURE 2–24
Structures of two G protein-coupled receptors. The individual amino acid residues are identified by their single-letter codes, and the orange residues are sites of phosphorylation. The Y-shaped symbols identify glycosylation sites. Note the extracellular amino terminal, the intracellular carboxyl terminal, and the seven membrane-spanning portions of each protein. (Reproduced with permission from Benovic JL et al: Lightdependent phosphorylation of rhodopsin by β-adrenergic receptor kinase. Reprinted by permission from Nature 1986;321:869. Copyright © 1986 by Macmillan Magazines)
is activated by heterotrimeric G proteins, while PLCγ forms are activated through tyrosine kinase receptors. PLC isoforms can catalyze the hydrolysis of the membrane lipid phosphatidylinositol 4,5-diphosphate (PIP2) to form IP3 and diacylglycerol (DAG) (Figure 2–25). The IP3 diffuses to the endoplasmic
Phosphatidylinositol (PI)
PIP
reticulum, where it triggers the release of Ca2+ into the cytoplasm by binding the IP3 receptor, a ligand-gated Ca2+ channel (Figure 2–26). DAG is also a second messenger; it stays in the cell membrane, where it activates one of several isoforms of protein kinase C.
PIP2
Diacylglycerol
Phospholipase C
P 1 4
P 1 4
P 1
P
4
P 1
5 P
P
5
4 P
Inositol
+ CDP-diacylglycerol
IP
IP3
P
IP2 Phosphatidic acid
FIGURE 2–25 Metabolism of phosphatidylinositol in cell membranes. Phosphatidylinositol is successively phosphorylated to form phosphatidylinositol 4-phosphate (PIP), then phosphatidylinositol 4,5-bisphosphate (PIP 2). Phospholipase Cβ and phospholipase Cγ catalyze the breakdown of PIP2 to inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. Other inositol phosphates and phosphatidylinositol derivatives can also be formed. IP3 is dephosphorylated to inositol, and diacylglycerol is metabolized to cytosine diphosphate (CDP)-diacylglycerol. CDP-diacylglycerol and inositol then combine to form phosphatidylinositol, completing the cycle. (Modified from Berridge MJ: Inositol triphosphate and diacylglycerol as second messengers. Biochem J 1984;220:345.)
56
SECTION I Cellular & Molecular Basis of Medical Physiology
O
Stimulatory receptor
HO
ISF PIP2
PKC
Tyrosine kinase
O
OH
P
O
Adenine
CH2 O
OH H
IP3 Phosphoproteins
H
H
OH
OH
H
Adenylyl cyclase
Cytoplasm CaBP
P
O
ATP
β
γ α Gq, etc
O
OH
DAG PLC
P
O
Ca2+
Physiologic effects
ER
Physiologic effects
PP
FIGURE 2–26
Diagrammatic representation of release of inositol triphosphate (IP3) and diacylglycerol (DAG) as second messengers. Binding of ligand to G protein-coupled receptor activates phospholipase C (PLC)β. Alternatively, activation of receptors with intracellular tyrosine kinase domains can activate PLCγ. The resulting hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP 2) produces IP3, which releases Ca2+ from the endoplasmic reticulum (ER), and DAG, which activates protein kinase C (PKC). CaBP, Ca 2+-binding proteins. ISF, interstitial fluid.
O
O cAMP H
PRODUCTION OF cAMP BY ADENLYL CYCLASE Adenylyl cyclase is a transmembrane protein, and it crosses the membrane 12 times. Ten isoforms of this enzyme have been described and each can have distinct regulatory properties, permitting the cAMP pathway to be customized to specific tissue needs. Notably, stimulatory heterotrimeric G proteins (Gs) activate, while inhibitory heterotrimeric G proteins (Gi) inactivate adenylyl cyclase (Figure 2–28). When the appropriate ligand binds to a stimulatory receptor, a Gs α subunit activates one of the adenylyl cyclases. Conversely, when the
O
H2O
P
H
H
O
OH
H
OH Phosphodiesterase
CYCLIC AMP Another important second messenger is cyclic adenosine 3',5'monophosphate (cyclic AMP or cAMP; Figure 2–27). Cyclic AMP is formed from ATP by the action of the enzyme adenylyl cyclase and converted to physiologically inactive 5'AMP by the action of the enzyme phosphodiesterase. Some of the phosphodiesterase isoforms that break down cAMP are inhibited by methylxanthines such as caffeine and theophylline. Consequently, these compounds can augment hormonal and transmitter effects mediated via cAMP. Cyclic AMP activates one of the cyclic nucleotide-dependent protein kinases (protein kinase A, PKA) that, like protein kinase C, catalyzes the phosphorylation of proteins, changing their conformation and altering their activity. In addition, the active catalytic subunit of PKA moves to the nucleus and phosphorylates the cAMP-responsive element-binding protein (CREB). This transcription factor then binds to DNA and alters transcription of a number of genes.
Adenine
CH2
AMP
O HO
P
O
Adenine
CH2 O
OH H
H
H
OH
OH
H
FIGURE 2–27
Formation and metabolism of cAMP. The second messenger cAMP is a made from ATP by adenylyl cyclase and broken down into cAMP by phosphodiesterase.
appropriate ligand binds to an inhibitory receptor, a Gi α subunit inhibits adenylyl cyclase. The receptors are specific, responding at low threshold to only one or a select group of related ligands. However, heterotrimeric G proteins mediate the stimulatory and inhibitory effects produced by many different ligands. In addition, cross-talk occurs between the phospholipase C system and the adenylyl cyclase system, as several of the isoforms of adenylyl cyclase are stimulated by calmodulin. Finally, the effects of protein kinase A and protein kinase C are very widespread and can also affect directly, or indirectly, the activity at adenylyl cyclase. The close relationship between activation of G proteins and adenylyl cyclases also allows for spatial regulation of cAMP production. All of these events, and others, allow for fine-tuning the cAMP response for a particular physiological outcome in the cell. Two bacterial toxins have important effects on adenylyl cyclase that are mediated by G proteins. The A subunit of cholera toxin catalyzes the transfer of ADP ribose to an arginine residue in the middle of the α subunit of Gs. This inhibits
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
Stimulatory receptor
Adenylyl cyclase
ISF
ANP NH2
Inhibitory receptor
NH2 NH2 EGF
ST β
α
γ
α
PDGF ISF M C
γ
Gi
GS Cytoplasm
β
ATP
57
PDE CAMP
NH2
PTK
NH2
PTK
PTP
5' AMP
PTK PTP
cyc
Protein kinase A
cyc
cyc
COOH
Phosphoproteins Physiologic effects
FIGURE 2–28
The cAMP system. Activation of adenylyl cyclase catalyzes the conversion of ATP to cAMP. Cyclic AMP activates protein kinase A, which phosphorylates proteins, producing physiologic effects. Stimulatory ligands bind to stimulatory receptors and activate adenylyl cyclase via Gs. Inhibitory ligands inhibit adenylyl cyclase via inhibitory receptors and Gi. ISF, interstitial fluid.
COOH COOH
COOH
NH2 PTP COOH
COOH COOH
Guanylyl cyclases
Tyrosine kinases
Tyrosine phosphatases
FIGURE 2–29
Diagrammatic representation of guanylyl cyclases, tyrosine kinases, and tyrosine phosphatases. ANP, atrial natriuretic peptide; C, cytoplasm; cyc, guanylyl cyclase domain; EGF, epidermal growth factor; ISF, interstitial fluid; M, cell membrane; PDGF, platelet-derived growth factor; PTK, tyrosine kinase domain; PTP, tyrosine phosphatase domain; ST, E. coli enterotoxin. (Modified from
Koesling D, Böhme E, Schultz G: Guanylyl cyclases, a growing family of signal
its GTPase activity, producing prolonged stimulation of adenylyl cyclase. Pertussis toxin catalyzes ADP-ribosylation of a cysteine residue near the carboxyl terminal of the α subunit of Gi. This inhibits the function of Gi. In addition to the implications of these alterations in disease, both toxins are used for fundamental research on G protein function. The drug forskolin also stimulates adenylyl cyclase activity by a direct action on the enzyme.
GUANYLYL CYCLASE Another cyclic nucleotide of physiologic importance is cyclic guanosine monophosphate (cyclic GMP or cGMP). Cyclic GMP is important in vision in both rod and cone cells. In addition, there are cGMP-regulated ion channels, and cGMP activates cGMP-dependent kinase, producing a number of physiologic effects. Guanylyl cyclases are a family of enzymes that catalyze the formation of cGMP. They exist in two forms (Figure 2–29). One form has an extracellular amino terminal domain that is a receptor, a single transmembrane domain, and a cytoplasmic portion with guanylyl cyclase catalytic activity. Three such guanylyl cyclases have been characterized. Two are receptors for atrial natriuretic peptide (ANP; also known as atrial natriuretic factor), and a third binds an Escherichia coli enterotoxin and the gastrointestinal polypeptide guanylin. The other form of guanylyl cyclase is soluble, contains heme, and is not bound to the membrane. There appear to be several isoforms of the intracellular enzyme. They are activated by nitric oxide (NO) and NO-containing compounds.
transducing enzymes. FASEB J 1991;5:2785.)
GROWTH FACTORS Growth factors have become increasingly important in many different aspects of physiology. They are polypeptides and proteins that are conveniently divided into three groups. One group is made up of agents that foster the multiplication or development of various types of cells; nerve growth factor (NGF), insulin-like growth factor I (IGF-I), activins and inhibins, and epidermal growth factor (EGF) are examples. More than 20 have been described. The cytokines are a second group. These factors are produced by macrophages and lymphocytes, as well as other cells, and are important in regulation of the immune system (see Chapter 3). Again, more than 20 have been described. The third group is made up of the colony-stimulating factors that regulate proliferation and maturation of red and white blood cells. Receptors for EGF, platelet-derived growth factor (PDGF), and many of the other factors that foster cell multiplication and growth have a single membrane-spanning domain with an intracellular tyrosine kinase domain (Figure 2–29). When ligand binds to a tyrosine kinase receptor, it first causes a dimerization of two similar receptors. The dimerization results in partial activation of the intracellular tyrosine kinase domains and a cross-phosphorylation to fully activate each other. One of the pathways activated by phosphorylation leads, through the small G protein Ras, to MAP kinases, and eventually to the production of transcription factors in the nucleus that alter gene expression (Figure 2–30). Receptors for cytokines and colony-stimulating factors differ from the other growth factors in that most of them do not have tyrosine kinase domains in their cytoplasmic portions and
SECTION I Cellular & Molecular Basis of Medical Physiology
Growth factor
Ligand
Receptor
A
Inactive Ras Ras
Ras Active Ras
GDP
T K Grb2
Receptor
ISF Cytoplasm
JAK
Cell membrane
JAK
58
GTP
SOS
Raf STAT
STAT
MAP KK B
Ligand JAK
TF
JAK
MAP K
P
P
P
P
Nucleus STAT
STAT
Altered gene activity
One of the direct pathways by which growth factors alter gene activity. TK, tyrosine kinase domain; Grb2, Ras activator controller; Sos, Ras activator; Ras, product of the ras gene; MAP K, mitogen-activated protein kinase; MAP KK, MAP kinase kinase; TF, transcription factors. There is cross-talk between this pathway and the cAMP pathway, as well as cross-talk with the IP 3–DAG pathway.
Ligand JAK
FIGURE 2–30
JAK
C
P
P
P
P
STAT P P STAT
JAK
Ligand JAK
D
P
P
P
P STAT P
some have little or no cytoplasmic tail. However, they initiate tyrosine kinase activity in the cytoplasm. In particular, they activate the so-called Janus tyrosine kinases (JAKs) in the cytoplasm (Figure 2–31). These in turn phosphorylate STAT proteins. The phosphorylated STATs form homo- and heterodimers and move to the nucleus, where they act as transcription factors. There are four known mammalian JAKs and seven known STATs. Interestingly, the JAK–STAT pathway can also be activated by growth hormone and is another important direct path from the cell surface to the nucleus. However, it should be emphasized that both the Ras and the JAK–STAT pathways are complex and there is cross-talk between them and other signaling pathways discussed previously. Finally, note that the whole subject of second messengers and intracellular signaling has become immensely complex, with multiple pathways and interactions. It is only possible in a book such as this to list highlights and present general themes that will aid the reader in understanding the rest of physiology (see Clinical Box 2–3).
P STAT
Nucleus DNA
FIGURE 2–31
Signal transduction via the JAK–STAT pathway. A) Ligand binding leads to dimerization of receptor. B) Activation and tyrosine phosphorylation of JAKs. C) JAKs phosphorylate STATs. D) STATs dimerize and move to nucleus, where they bind to response elements on DNA. (Modified from Takeda K, Kishimoto T, Akira S: STAT6:
Its role in interleukin 4-mediated biological functions. J Mol Med 1997;75:317.)
HOMEOSTASIS The actual environment of the cells of the body is the interstitial component of the ECF. Because normal cell function
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
CLINICAL BOX 2–3 Receptor & G Protein Diseases Many diseases are being traced to mutations in the genes for receptors. For example, loss-of-function receptor mutations that cause disease have been reported for the 1,25dihydroxycholecalciferol receptor and the insulin receptor. Certain other diseases are caused by production of antibodies against receptors. Thus, antibodies against thyroidstimulating hormone (TSH) receptors cause Graves’ disease, and antibodies against nicotinic acetylcholine receptors cause myasthenia gravis. An example of loss of function of a receptor is the type of nephrogenic diabetes insipidus that is due to loss of the ability of mutated V2 vasopressin receptors to mediate concentration of the urine. Mutant receptors can gain as well as lose function. A gain-of-function mutation of the Ca2+ receptor causes excess inhibition of parathyroid hormone secretion and familial hypercalciuric hypocalcemia. G proteins can also undergo loss-of-function or gain-of-function mutations that cause disease (Table 2–6), In one form of pseudohypoparathyroidism, a mutated Gsα fails to respond to parathyroid hormone, producing the symptoms of hypoparathyroidism without any decline in circulating parathyroid hormone. Testotoxicosis is an interesting disease that combines gain and loss of function. In this condition, an activating mutation of Gsα causes excess testosterone secretion and prepubertal sexual maturation. However, this mutation is temperature-sensitive and is active only at the relatively low temperature of the testes (33 °C). At 37 °C, the normal temperature of the rest of the body, it is replaced by loss of function, with the production of hypoparathyroidism and decreased responsiveness to TSH. A different activating mutation in Gsα is associated with the rough-bordered areas of skin pigmentation and hypercortisolism of the McCune–Albright syndrome. This mutation occurs during fetal development, creating a mosaic of normal and abnormal cells. A third mutation in Gsα reduces its intrinsic GTPase activity. As a result, it is much more active than normal, and excess cAMP is produced. This causes hyperplasia and eventually neoplasia in somatotrope cells of the anterior pituitary. Forty percent of somatotrope tumors causing acromegaly have cells containing a somatic mutation of this type.
depends on the constancy of this fluid, it is not surprising that in multicellular animals, an immense number of regulatory mechanisms have evolved to maintain it. To describe “the various physiologic arrangements which serve to restore the normal state, once it has been disturbed,” W.B. Cannon coined the term homeostasis. The buffering properties of the body fluids and the renal and respiratory adjustments to the presence of excess acid or alkali are examples of homeostatic
59
TABLE 2–6 Examples of abnormalities caused by loss- or gain-of-function mutations of heterotrimeric G protein-coupled receptors and G proteins. Type of Mutation
Disease
Cone opsins
Loss
Color blindness
Rhodopsin
Loss
Congenital night blindness; two forms of retinitis pigmentosa
V2 vasopressin
Loss
X-linked nephrogenic diabetes insipidus
ACTH
Loss
Familial glucocorticoid deficiency
LH
Gain
Familial male precocious puberty
TSH
Gain
Familial nonautoimmune hyperthyroidism
TSH
Loss
Familial hypothyroidism
Ca2+
Gain
Familial hypercalciuric hypocalcemia
Thromboxane A2
Loss
Congenital bleeding
Endothelin B
Loss
Hirschsprung disease
Gs α
Loss
Pseudohypothyroidism type 1a
Gs α
Gain/loss
Testotoxicosis
Gs α
Gain (mosaic)
McCune–Albright syndrome
Gs α
Gain
Somatotroph adenomas with acromegaly
Gi α
Gain
Ovarian and adrenocortical tumors
Site Receptor
G protein
Modified from Lem J: Diseases of G-protein-coupled signal transduction pathways: The mammalian visual system as a model. Semin Neurosci 1998;9:232.
mechanisms. There are countless other examples, and a large part of physiology is concerned with regulatory mechanisms that act to maintain the constancy of the internal environment. Many of these regulatory mechanisms operate on the principle of negative feedback; deviations from a given normal set point are detected by a sensor, and signals from the sensor trigger compensatory changes that continue until the set point is again reached.
60
SECTION I Cellular & Molecular Basis of Medical Physiology
CHAPTER SUMMARY ■
■
■
■
■
■
■
The cell and the intracellular organelles are surrounded by a semipermeable membrane. Biological membranes have a lipid bilayer with a hydrophobic core and hydrophilic outer regions that provide a barrier between inside and outside compartments as well as a template for biochemical reactions. The membrane is populated by structural and functional proteins that can be integrated into the membrane or be associated with one side of the lipid bilayer. These proteins contribute greatly to the semipermeable properties of biological membrane. Mitochondria are organelles that allow for oxidative phosphorylation in eukaryotic cells. They contain their own DNA, however, proteins in the mitochondria are encoded by both mitochondrial and cellular DNA. Mitochondria also are important in specialized cellular signaling. Lysosomes and peroxisomes are membrane-bound organelles that contribute to protein and lipid processing. They do this in part by creating acidic (lysosomes) or oxidative (peroxisomes) contents relative to the cell cytosol. The cytoskeleton is a network of three types of filaments that provide structural integrity to the cell as well as a means for trafficking of organelles and other structures. Actin is the fundamental building block for thin filaments and represents as much as 15% of cellular protein. Actin filaments are important in cellular contraction, migration, and signaling. Actin filaments also provide the backbone for muscle contraction. Intermediate filaments are primarily structural. Proteins that make up intermediate filaments are cell-type specific. Microtubules are made up of tubulin subunits. Microtubules provide a dynamic structure in cells that allows for movement of cellular components around the cell. There are three superfamilies of molecular motor proteins in the cell that use the energy of ATP to generate force, movement, or both. Myosin is the force generator for muscle cell contraction. There are also cellular myosins that interact with the cytoskeleton (primarily thin filaments) to participate in contraction as well as movement of cell contents. Kinesins and cellular dyneins are motor proteins that primarily interact with microtubules to move cargo around the cells. Cellular adhesion molecules aid in tethering cells to each other or to the extracellular matrix as well as providing for initiation of cellular signaling. There are four main families of these proteins: integrins, immunoglobulins, cadherins, and selectins. Cells contain distinct protein complexes that serve as cellular connections to other cells or the extracellular matrix. Tight junctions provide intercellular connections that link cells into a regulated tissue barrier. Tight junctions also provide a barrier to movement of proteins in the cell membrane and thus, are important to cellular polarization. Gap junctions provide contacts between cells that allow for direct passage of small molecules between two cells. Desmosomes and adherens junctions are spe-
■
■
■
■
■
cialized structures that hold cells together. Hemidesmosomes and focal adhesions attach cells to their basal lamina. The nucleus is an organelle that contains the cellular DNA and is the site of transcription. There are several organelles that emanate from the nucleus, including the endoplasmic reticulum and the Golgi apparatus. These two organelles are important in protein processing and the targeting of proteins to correct compartments within the cell. Exocytosis and endocytosis are vesicular fusion events that allow for movement of proteins and lipids between the cell interior, the plasma membrane, and the cell exterior. Exocytosis can be constitutive or nonconstitutive; both are regulated processes that require specialized proteins for vesicular fusion. Endocytosis is the formation of vesicles at the plasma membrane to take material from the extracellular space into the cell interior. Some endocytoses are defined in part by the size of the vesicles formed whereas others are defined by membrane structures that contribute to the endocytosis. All are tightly regulated processes. Membranes contain a variety of proteins and protein complexes that allow for transport of small molecules. Aqueous ion channels are membrane-spanning proteins that can be gated open to allow for selective diffusion of ions across membranes and down their electrochemical gradient. Carrier proteins bind to small molecules and undergo conformational changes to deliver small molecules across the membrane. This facilitated transport can be passive or active. Active transport requires energy for transport and is typically provided by ATP hydrolysis. Cells can communicate with one another via chemical messengers. Individual messengers (or ligands) typically bind to a plasma membrane receptor to initiate intracellular changes that lead to physiologic changes. Plasma membrane receptor families include ion channels, G protein-coupled receptors, or a variety of enzyme-linked receptors (eg, tyrosine kinase receptors). There are additional cytosolic receptors (eg, steroid receptors) that can bind membrane-permeant compounds. Activation of receptors lead to cellular changes that include changes in membrane potential, activation of heterotrimeric G proteins, increase in second messenger molecules, or initiation of transcription. Second messengers are molecules that undergo a rapid concentration changes in the cell following primary messenger recognition. Common second messenger molecules include Ca2+, cyclic adenosine monophosphate (cAMP), cyclic guanine monophosphate (cGMP), inositol trisphosphate (IP3) and nitric oxide (NO).
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. The electrogenic Na, K ATPase plays a critical role in cellular physiology by A) using the energy in ATP to extrude 3 Na+ out of the cell in exchange for taking two K+ into the cell. B) using the energy in ATP to extrude 3 K+ out of the cell in exchange for taking two Na+ into the cell. C) using the energy in moving Na+ into the cell or K+ outside the cell to make ATP. D) using the energy in moving Na+ outside of the cell or K+ inside the cell to make ATP. 2. Cell membranes A) contain relatively few protein molecules. B) contain many carbohydrate molecules. C) are freely permeable to electrolytes but not to proteins. D) have variable protein and lipid contents depending on their location in the cell. E) have a stable composition throughout the life of the cell. 3. Second messengers A) are substances that interact with first messengers outside cells. B) are substances that bind to first messengers in the cell membrane. C) are hormones secreted by cells in response to stimulation by another hormone. D) mediate the intracellular responses to many different hormones and neurotransmitters. E) are not formed in the brain. 4. The Golgi complex A) is an organelle that participates in the breakdown of proteins and lipids. B) is an organelle that participates in posttranslational processing of proteins. C) is an organelle that participates in energy production. D) is an organelle that participates in transcription and translation. E) is a subcellular compartment that stores proteins for trafficking to the nucleus.
61
5. Endocytosis A) includes phagocytosis and pinocytosis, but not clathrinmediated or caveolae-dependent uptake of extracellular contents. B) refers to the merging of an intracellular vesicle with the plasma membrane to deliver intracellular contents to the extracellular milieu. C) refers to the invagination of the plasma membrane to uptake extracellular contents into the cell. D) refers to vesicular trafficking between Golgi stacks. 6. G protein-coupled receptors A) are intracellular membrane proteins that help to regulate movement within the cell. B) are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to activation of small G proteins. C) are plasma membrane proteins that couple the extracellular binding of primary signaling molecules to the activation of heterotrimeric G proteins. D) are intracellular proteins that couple the binding of primary messenger molecules with transcription. 7. Gap junctions are intercellular connections that A) primarily serve to keep cells separated and allow for transport across a tissue barrier. B) serve as a regulated cytoplasmic bridge for sharing of small molecules between cells. C) serve as a barrier to prevent protein movement within the cellular membrane. D) are cellular components for constitutive exocytosis that occurs between adjacent cells.
CHAPTER RESOURCES Alberts B et al: Molecular Biology of the Cell, 5th ed. Garland Science, 2007. Cannon WB: The Wisdom of the Body. Norton, 1932. Junqueira LC, Carneiro J, Kelley RO: Basic Histology, 9th ed. McGraw-Hill, 1998. Kandel ER, Schwartz JH, Jessell TM (editors): Principles of Neural Science, 4th ed. McGraw-Hill, 2000. Pollard TD, Earnshaw WC: Cell Biology, 2nd ed. Saunders, Elsevier, 2008. Sperelakis N (editor): Cell Physiology Sourcebook, 3rd ed. Academic Press, 2001.
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C
Immunity, Infection, & Inflammation
3
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■
Understand the significance of immunity, particularly with respect to defending the body against microbial invaders. Define the circulating and tissue cell types that contribute to immune and inflammatory responses. Describe how phagocytes are able to kill internalized bacteria. Identify the functions of hematopoietic growth factors, cytokines, and chemokines. Delineate the roles and mechanisms of innate, acquired, humoral, and cellular immunity. Understand the basis of inflammatory responses and wound healing.
INTRODUCTION As an open system, the body is continuously called upon to defend itself from potentially harmful invaders such as bacteria, viruses, and other microbes. This is accomplished by the immune system, which is subdivided into innate and adaptive (or acquired) branches. The immune system is composed of specialized effector cells that sense and respond to foreign antigens and other molecular patterns not found in human tissues. Likewise, the immune system clears the body’s own cells that have become senescent or abnormal, such as cancer cells. Finally, occasionally, normal host tissues become the subject of inappropriate immune attack, such as in autoim-
mune diseases or in settings where normal cells are harmed as innocent bystanders when the immune system mounts an inflammatory response to an invader. It is beyond the scope of this volume to provide a full treatment of all aspects of modern immunology. Nevertheless, the student of physiology should have a working knowledge of immune functions and their regulation, due to a growing appreciation for the ways in which the immune system can contribute to normal physiological regulation in a variety of tissues, as well as contributions of immune effectors to pathophysiology.
IMMUNE EFFECTOR CELLS
monocytes. Immune responses in the tissues are further amplified by these cells following their extravascular migration, as well as tissue macrophages (derived from monocytes) and mast cells (related to basophils). Acting together, these cells provide the body with powerful defenses against tumors and viral, bacterial, and parasitic infections.
Many immune effector cells circulate in the blood as the white blood cells. In addition, the blood is the conduit for the precursor cells that eventually develop into the immune cells of the tissues. The circulating immunologic cells include granulocytes (polymorphonuclear leukocytes, PMNs), comprising neutrophils, eosinophils, and basophils; lymphocytes; and
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SECTION I Cellular & Molecular Basis for Medical Physiology
GRANULOCYTES All granulocytes have cytoplasmic granules that contain biologically active substances involved in inflammatory and allergic reactions. The average half-life of a neutrophil in the circulation is 6 hours. To maintain the normal circulating blood level, it is therefore necessary to produce over 100 billion neutrophils per day. Many neutrophils enter the tissues, particularly if triggered to do so by an infection or by inflammatory cytokines. They are attracted to the endothelial surface by cell adhesion molecules known as selectins, and they roll along it. They then bind firmly to neutrophil adhesion molecules of the integrin family. They next insinuate themselves through the walls of the capillaries between endothelial cells by a process called diapedesis. Many of those that leave the circulation enter the gastrointestinal tract and are eventually lost from the body. Invasion of the body by bacteria triggers the inflammatory response. The bone marrow is stimulated to produce and release large numbers of neutrophils. Bacterial products interact with plasma factors and cells to produce agents that attract neutrophils to the infected area (chemotaxis). The chemotactic agents, which are part of a large and expanding family of chemokines (see following text), include a component of the complement system (C5a); leukotrienes; and polypeptides from lymphocytes, mast cells, and basophils. Other plasma factors act on the bacteria to make them “tasty” to the phagocytes (opsonization). The principal opsonins that coat the bacteria are immunoglobulins of a particular class (IgG) and complement proteins (see following text). The coated bacteria then bind to receptors on the neutrophil cell membrane. This triggers, via heterotrimeric G protein-mediated responses, increased motor activity of the cell, exocytosis, and the socalled respiratory burst. The increased motor activity leads to prompt ingestion of the bacteria by endocytosis (phagocytosis). By exocytosis, neutrophil granules discharge their contents into the phagocytic vacuoles containing the bacteria and also into the interstitial space (degranulation). The granules contain various proteases plus antimicrobial proteins called defensins. In addition, the cell membrane-bound enzyme NADPH oxidase is activated, with the production of toxic oxygen metabolites. The combination of the toxic oxygen metabolites and the proteolytic enzymes from the granules makes the neutrophil a very effective killing machine. Activation of NADPH oxidase is associated with a sharp increase in O2 uptake and metabolism in the neutrophil (the respiratory burst) and generation of O2– by the following reaction: NADPH + H+ + 2O2 + → NADP+ + 2H+ + 2O2– O2– is a free radical formed by the addition of one electron to O2. Two O2– react with two H+ to form H2O2 in a reaction catalyzed by the cytoplasmic form of superoxide dismutase (SOD-1): O2–+ O2– + H+ + H+ SOD-1
→ H2O2 + O2
O2– and H2O2 are both oxidants that are effective bactericidal agents, but H2O2 is converted to H2O and O2 by the enzyme catalase. The cytoplasmic form of SOD contains both Zn and Cu. It is found in many parts of the body. It is defective as a result of genetic mutation in a familial form of amyotrophic lateral sclerosis (ALS; see Chapter 19). Therefore, it may be that O2– accumulates in motor neurons and kills them in at least one form of this progressive, fatal disease. Two other forms of SOD encoded by at least one different gene are also found in humans. Neutrophils also discharge the enzyme myeloperoxidase, which catalyzes the conversion of Cl–, Br–, I–, and SCN– to the corresponding acids (HOCl, HOBr, etc). These acids are also potent oxidants. Because Cl– is present in greatest abundance in body fluids, the principal product is HOCl. In addition to myeloperoxidase and defensins, neutrophil granules contain an elastase, two metalloproteinases that attack collagen, and a variety of other proteases that help destroy invading organisms. These enzymes act in a cooperative fashion with the O2–, H2O2, and HOCl formed by the action of the NADPH oxidase and myeloperoxidase to produce a killing zone around the activated neutrophil. This zone is effective in killing invading organisms, but in certain diseases (eg, rheumatoid arthritis) the neutrophils may also cause local destruction of host tissue. The movements of the cell in phagocytosis, as well as migration to the site of infection, involve microtubules and microfilaments (see Chapter 1). Proper function of the microfilaments involves the interaction of the actin they contain with myosin-1 on the inside of the cell membrane (see Chapter 1). Like neutrophils, eosinophils have a short half-life in the circulation, are attracted to the surface of endothelial cells by selectins, bind to integrins that attach them to the vessel wall, and enter the tissues by diapedesis. Like neutrophils, they release proteins, cytokines, and chemokines that produce inflammation but are capable of killing invading organisms. However, eosinophils have some selectivity in the way in which they respond and in the killing molecules they secrete. Their maturation and activation in tissues is particularly stimulated by IL-3, IL-5, and GM-CSF (see below). They are especially abundant in the mucosa of the gastrointestinal tract, where they defend against parasites, and in the mucosa of the respiratory and urinary tracts. Circulating eosinophils are increased in allergic diseases such as asthma and in various other respiratory and gastrointestinal diseases. Basophils also enter tissues and release proteins and cytokines. They resemble but are not identical to mast cells, and like mast cells they contain histamine (see below). They release histamine and other inflammatory mediators when activated by binding of specific antigens to cell-fixed IgE molecules, and are essential for immediate-type hypersensitivity reactions. These range from mild urticaria and rhinitis to severe anaphylactic shock. The antigens that trigger IgE formation and basophil (and mast cell) activation are innocuous to most individuals, and are referred to as allergens.
CHAPTER 3 Immunity, Infection, & Inflammation
MAST CELLS Mast cells are heavily granulated cells of the connective tissue that are abundant in tissues that come into contact with the external environment, such as beneath epithelial surfaces. Their granules contain proteoglycans, histamine, and many proteases. Like basophils, they degranulate when allergens bind to IgE molecules directed against them that previously coat the mast cell surface. They are involved in inflammatory responses initiated by immunoglobulins IgE and IgG (see below). The inflammation combats invading parasites. In addition to this involvement in acquired immunity, they release TNF-α in response to bacterial products by an antibody-independent mechanism, thus participating in the nonspecific innate immunity that combats infections prior to the development of an adaptive immune response (see following text). Marked mast cell degranulation produces clinical manifestations of allergy up to and including anaphylaxis.
MONOCYTES Monocytes enter the blood from the bone marrow and circulate for about 72 hours. They then enter the tissues and become tissue macrophages (Figure 3–1). Their life span in the tissues is unknown, but bone marrow transplantation data in humans suggest that they persist for about 3 months. It appears that they do not reenter the circulation. Some of them end up as the multinucleated giant cells seen in chronic inflammatory diseases such as tuberculosis. The tissue macrophages include the Kupffer cells of the liver, pulmonary alveolar macrophages (see Chapter 35), and microglia in the brain, all of which come from the circulation. In the past, they have been called the reticuloendothelial system, but the general term tissue macrophage system seems more appropriate. Macrophages are activated by cytokines released from T lymphocytes, among others. Activated macrophages migrate in response to chemotactic stimuli and engulf and kill bacte-
Macrophages
Pseudopods
Bacteria
FIGURE 3–1 Macrophages contacting bacteria and preparing to engulf them. Figure is a colorized version of a scanning electron micrograph.
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ria by processes generally similar to those occurring in neutrophils. They play a key role in immunity (see below). They also secrete up to 100 different substances, including factors that affect lymphocytes and other cells, prostaglandins of the E series, and clot-promoting factors.
GRANULOCYTE & MACROPHAGE COLONY-STIMULATING FACTORS The production of white blood cells is regulated with great precision in healthy individuals, and the production of granulocytes is rapidly and dramatically increased in infections. The proliferation and self-renewal of hematopoietic stem cells (HSCs) depends on stem cell factor (SCF). Other factors specify particular lineages. The proliferation and maturation of the cells that enter the blood from the marrow are regulated by glycoprotein growth factors or hormones that cause cells in one or more of the committed cell lines to proliferate and mature (Table 3–1). The regulation of erythrocyte production by erythropoietin is discussed in Chapter 39. Three additional factors are called colony-stimulating factors (CSFs), because they cause appropriate single stem cells to proliferate in soft agar, forming colonies in this culture medium. The factors stimulating the production of committed stem cells include granulocyte–macrophage CSF (GM-CSF), granulocyte CSF (G-CSF), and macrophage CSF (M-CSF). Interleukins IL-1 and IL-6 followed by IL-3 (Table 3–1) act in sequence to convert pluripotential uncommitted stem cells to committed progenitor cells. IL-3 is also known as multi-CSF. Each of the CSFs has a predominant action, but all the CSFs and interleukins also have other overlapping actions. In addition, they activate and sustain mature blood cells. It is interesting in this regard that the genes for many of these factors are located together on the long arm of chromosome 5 and may have originated by duplication of an ancestral gene. It is also interesting that basal hematopoiesis is normal in mice in which the GMCSF gene is knocked out, indicating that loss of one factor can be compensated for by others. On the other hand, the absence of GM-CSF causes accumulation of surfactant in the lungs (see Chapter 35). As noted in Chapter 39, erythropoietin is produced in part by kidney cells and is a circulating hormone. The other factors are produced by macrophages, activated T cells, fibroblasts, and endothelial cells. For the most part, the factors act locally in the bone marrow (Clinical Box 3–1).
LYMPHOCYTES Lymphocytes are key elements in the production of immunity (see below). After birth, some lymphocytes are formed in the bone marrow. However, most are formed in the lymph nodes (Figure 3–2), thymus, and spleen from precursor cells that originally came from the bone marrow and were processed in the thymus or bursal equivalent (see below). Lymphocytes enter the bloodstream for the most part via the lymphatics. At
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TABLE 3–1 Hematopoietic growth factors. Cytokine
Cell Lines Stimulated
Cytokine Source
IL-1
Erythrocyte
Multiple cell types
Granulocyte Megakaryocyte Monocyte IL-3
Erythrocyte
T lymphocytes
Granulocyte Megakaryocyte Monocyte IL-4
Basophil
T lymphocytes
IL-5
Eosinophil
T lymphocytes
IL-6
Erythrocyte
Endothelial cells
Granulocyte
IL-11
Megakaryocyte
Fibroblasts
Monocyte
Macrophages
Erythrocyte
Fibroblasts
Granulocyte
Osteoblasts
Megakaryocyte Erythropoietin
Erythrocyte
SCF
Erythrocyte
CLINICAL BOX 3–1 Disorders of Phagocytic Function More than 15 primary defects in neutrophil function have been described, along with at least 30 other conditions in which there is a secondary depression of the function of neutrophils. Patients with these diseases are prone to infections that are relatively mild when only the neutrophil system is involved, but which can be severe when the monocyte-tissue macrophage system is also involved. In one syndrome (neutrophil hypomotility), actin in the neutrophils does not polymerize normally, and the neutrophils move slowly. In another, there is a congenital deficiency of leukocyte integrins. In a more serious disease (chronic granulomatous disease), there is a failure to generate O2– in both neutrophils and monocytes and consequent inability to kill many phagocytosed bacteria. In severe congenital glucose 6-phosphate dehydrogenase deficiency, there are multiple infections because of failure to generate the NADPH necessary for O2– production. In congenital myeloperoxidase deficiency, microbial killing power is reduced because hypochlorous acid is not formed.
Kidney Kupffer cells of liver Multiple cell types
Granulocyte Megakaryocyte Monocyte G-CSF
Granulocyte
Endothelial cells Fibroblasts Monocytes
GM-CSF
Erythrocyte
Endothelial cells
any given time, only about 2% of the body lymphocytes are in the peripheral blood. Most of the rest are in the lymphoid organs. It has been calculated that in humans, 3.5 × 1010 lymphocytes per day enter the circulation via the thoracic duct alone; however, this count includes cells that reenter the lymphatics and thus traverse the thoracic duct more than once. The effects of adrenocortical hormones on the lymphoid organs, the circulating lymphocytes, and the granulocytes are discussed in Chapter 22.
Fibroblasts
M-CSF
Granulocyte
Monocytes
Megakaryocyte
T lymphocytes
Monocyte
Endothelial cells Fibroblasts
Cortical follicles, B cells
Monocytes Thrombopoietin
Megakaryocyte
Liver, kidney Paracortex, T cells
Key: IL = interleukin; CSF = colony stimulating factor; G = granulocyte; M = macrophage; SCF = stem cell factor. Reproduced with permission from McPhee SJ, Lingappa VR, Ganong WF (editors): Pathophysiology of Disease, 4th ed. McGraw-Hill, 2003.
Medullary cords, plasma cells
FIGURE 3–2
Anatomy of a normal lymph node. (After
Chandrasoma. Reproduced with permission from McPhee SJ, Lingappa VR, Ganong WF [editors]: Pathophysiology of Disease, 4th ed. McGraw-Hill, 2003.)
CHAPTER 3 Immunity, Infection, & Inflammation
IMMUNITY OVERVIEW Insects and other invertebrates have only innate immunity. This system is triggered by receptors that bind sequences of sugars, fats, or amino acids in common bacteria and activate various defense mechanisms. The receptors are coded in the germ line, and their fundamental structure is not modified by exposure to antigen. The activated defenses include, in various species, release of interferons, phagocytosis, production of antibacterial peptides, activation of the complement system, and several proteolytic cascades. Even plants release antibacterial peptides in response to infection. In vertebrates, innate immunity is also present, but is complemented by adaptive or acquired immunity, a system in which T and B lymphocytes are activated by very specific antigens. In both innate and acquired immunity, the receptors involved recognize the shape of antigens, not their specific chemical composition. In acquired immunity, activated B lymphocytes form clones that produce more antibodies which attack foreign proteins. After the invasion is repelled, small numbers persist as memory cells so that a second exposure to the same antigen provokes a prompt and magnified immune attack. The genetic event that led to acquired immunity occurred 450 million years ago in the ancestors of jawed vertebrates and was probably insertion of a transposon into the genome in a way that made possible the generation of the immense repertoire of T cell receptors that are present in the body.
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In vertebrates, including humans, innate immunity provides the first line of defense against infections, but it also triggers the slower but more specific acquired immune response (Figure 3–3). In vertebrates, natural and acquired immune mechanisms also attack tumors and tissue transplanted from other animals. Once activated, immune cells communicate by means of cytokines and chemokines. They kill viruses, bacteria, and other foreign cells by secreting other cytokines and activating the complement system.
CYTOKINES Cytokines are hormonelike molecules that act—generally in a paracrine fashion—to regulate immune responses. They are secreted not only by lymphocytes and macrophages but by endothelial cells, neurons, glial cells, and other types of cells. Most of the cytokines were initially named for their actions, for example, B cell-differentiating factor, B cell-stimulating factor 2. However, the nomenclature has since been rationalized by international agreement to that of the interleukins. For example, the name of B cell-differentiating factor was changed to interleukin-4. A number of cytokines selected for their biological and clinical relevance are listed in Table 3–2, but it would be beyond the scope of this text to list all cytokines, which now number more than 100. Many of the receptors for cytokines and hematopoietic growth factors (see above), as well as the receptors for prolactin
Plasma cell B γδT cell Chemokines TH 2 N
IL-4
M
Bacteria Viruses Tumors
Naive T cell APC
Cytotoxic lymphocyte
TH 1
FIGURE 3–3 How bacteria, viruses, and tumors trigger innate immunity and initiate the acquired immune response. Arrows indicate mediators/cytokines that act on the target cell shown and/or pathways of differentiation. APC, antigen-presenting cell; M, monocyte; N, neutrophil; TH1 and TH2, helper T cells type 1 and type 2, respectively.
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SECTION I Cellular & Molecular Basis for Medical Physiology
TABLE 3–2 Examples of cytokines and their clinical relevance. Cytokine
Cellular Sources
Major Activities
Clinical Relevance
Interleukin-1
Macrophages
Activation of T cells and macrophages; promotion of inflammation
Implicated in the pathogenesis of septic shock, rheumatoid arthritis, and atherosclerosis
Interleukin-2
Type 1 (TH1) helper T cells
Activation of lymphocytes, natural killer cells, and macrophages
Used to induce lymphokine-activated killer cells; used in the treatment of metastatic renal-cell carcinoma, melanoma, and various other tumors
Interleukin-4
Type 2 (TH2) helper T cells, mast cells, basophils, and eosinophils
Activation of lymphocytes, monocytes, and IgE class switching
As a result of its ability to stimulate IgE production, plays a part in mast-cell sensitization and thus in allergy and in defense against nematode infections
Interleukin-5
Type 2 (TH2) helper T cells, mast cells, and eosinophils
Differentiation of eosinophils
Monoclonal antibody against interleukin-5 used to inhibit the antigen-induced late-phase eosinophilia in animal models of allergy
Interleukin-6
Type 2 (TH2) helper T cells and macrophages
Activation of lymphocytes; differentiation of B cells; stimulation of the production of acute-phase proteins
Overproduced in Castleman’s disease; acts as an autocrine growth factor in myeloma and in mesangial proliferative glomerulonephritis
Interleukin-8
T cells and macrophages
Chemotaxis of neutrophils, basophils, and T cells
Levels are increased in diseases accompanied by neutrophilia, making it a potentially useful marker of disease activity
Interleukin-11
Bone marrow stromal cells
Stimulation of the production of acutephase proteins
Used to reduce chemotherapy-induced thrombocytopenia in patients with cancer
Interleukin-12
Macrophages and B cells
Stimulation of the production of interferon γ by type 1 (TH1) helper T cells and by natural killer cells; induction of type 1 (TH1) helper T cells
May be useful as an adjuvant for vaccines
Tumor necrosis factor α
Macrophages, natural killer cells, T cells, B cells, and mast cells
Promotion of inflammation
Treatment with antibodies against tumor necrosis factor α beneficial in rheumatoid arthritis
Lymphotoxin (tumor necrosis factor β)
Type 1 (TH1) helper T cells and B cells
Promotion of inflammation
Implicated in the pathogenesis of multiple sclerosis and insulin-dependent diabetes mellitus
Transforming growth factor β
T cells, macrophages, B cells, and mast cells
Immunosuppression
May be useful therapeutic agent in multiple sclerosis and myasthenia gravis
Granulocytemacrophage colonystimulating factor
T cells, macrophages, natural killer cells, and B cells
Promotion of the growth of granulocytes and monocytes
Used to reduce neutropenia after chemotherapy for tumors and in ganciclovir-treated patients with AIDS; used to stimulate cell production after bone marrow transplantation
Interferon-α
Virally infected cells
Induction of resistance of cells to viral infection
Used to treat AIDS-related Kaposi sarcoma, melanoma, chronic hepatitis B infection, and chronic hepatitis C infection
Interferon-β
Virally infected cells
Induction of resistance of cells to viral infection
Used to reduce the frequency and severity of relapses in multiple sclerosis
Interferon-γ
Type 1 (TH1) helper T cells and natural killer cells
Activation of macrophages; inhibition of type 2 (TH2) helper T cells
Used to enhance the killing of phagocytosed bacteria in chronic granulomatous disease
Reproduced with permission from Delves PJ, Roitt IM: The immune system. First of two parts. N Engl J Med 2000;343:37.
(see Chapter 25), and growth hormone (see Chapter 24) are members of a cytokine-receptor superfamily that has three subfamilies (Figure 3–4). The members of subfamily 1, which includes the receptors for IL-4 and IL-7, are homodimers. The members of subfamily 2, which includes the receptors for IL-3, IL-5, and IL-6, are heterodimers. The receptor for IL-2 and
several other cytokines is unique in that it consists of a heterodimer plus an unrelated protein, the so-called Tac antigen. The other members of subfamily 3 have the same γ chain as IL2R. The extracellular domain of the homodimer and heterodimer subunits all contain four conserved cysteine residues plus a conserved Trp-Ser-X-Trp-Ser domain, and although the
CHAPTER 3 Immunity, Infection, & Inflammation
Erythropoietin G-CSF IL-4 IL-7 Growth hormone PRL
IL-3 GM-CSF IL-5 IL-6 IL-11 LIF OSM CNTF
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IL-2 IL-4 IL-7 IL-9 IL-15
Shared β subunit
Shared gp130 subunit
ECF Cytoplasm
α α
Subfamily 1
β Subfamily 2
γ
β Subfamily 3
FIGURE 3–4 Members of one of the cytokine receptor superfamilies, showing shared structural elements. Note that all the subunits except the α subunit in subfamily 3 have four conserved cysteine residues (open boxes at top) and a Trp-Ser-X-Trp-Ser motif (pink). Many subunits also contain a critical regulatory domain in their cytoplasmic portions (green). CNTF, ciliary neurotrophic factor; LIF, leukemia inhibitory factor; OSM, oncostatin M; PRL, prolactin. (Modified from D’Andrea AD: Cytokine receptors in congenital hematopoietic disease. N Engl J Med 1994;330:839.)
intracellular portions do not contain tyrosine kinase catalytic domains, they activate cytoplasmic tyrosine kinases when ligand binds to the receptors. The effects of the principal cytokines are listed in Table 3–2. Some of them have systemic as well as local paracrine effects. For example, IL-1, IL-6, and tumor necrosis factor α cause fever, and IL-1 increases slow-wave sleep and reduces appetite. Another superfamily of cytokines is the chemokine family. Chemokines are substances that attract neutrophils (see previous text) and other white blood cells to areas of inflammation or immune response. Over 40 have now been identified, and it is clear that they also play a role in the regulation of cell growth and angiogenesis. The chemokine receptors are G protein-coupled receptors that cause, among other things, extension of pseudopodia with migration of the cell toward the source of the chemokine.
triggered by immune complexes; the mannose-binding lectin pathway, triggered when this lectin binds mannose groups in bacteria; and the alternative or properdin pathway, triggered by contact with various viruses, bacteria, fungi, and tumor cells. The proteins that are produced have three functions: They help kill invading organisms by opsonization, chemotaxis, and eventual lysis of the cells; they serve in part as a bridge from innate to acquired immunity by activating B cells and aiding immune memory; and they help dispose of waste products after apoptosis. Cell lysis, one of the principal ways the complement system kills cells, is brought about by inserting proteins called perforins into their cell membranes. These create holes, which permit free flow of ions and thus disruption of membrane polarity.
THE COMPLEMENT SYSTEM
The cells that mediate innate immunity include neutrophils, macrophages, and natural killer (NK) cells, large lymphocytes that are not T cells but are cytotoxic. All these cells respond to lipid and carbohydrate sequences unique to bacterial cell walls and to other substances characteristic of tumor and transplant cells. Many cells that are not professional immunocytes may nevertheless also contribute to innate immune
The cell-killing effects of innate and acquired immunity are mediated in part by a system of more than 30 plasma proteins originally named the complement system because they “complemented” the effects of antibodies. Three different pathways or enzyme cascades activate the system: the classic pathway,
INNATE IMMUNITY
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SECTION I Cellular & Molecular Basis for Medical Physiology
responses, such as endothelial and epithelial cells. The activated cells produce their effects via the release of cytokines, as well as, in some cases, complement and other systems. An important link in innate immunity in Drosophila is a receptor protein named toll, which binds fungal antigens and triggers activation of genes coding for antifungal proteins. An expanding list of toll-like receptors (TLRs) have now been identified in humans. One of these, TLR4, binds bacterial lipopolysaccharide and a protein called CD14, and this initiates a cascade of intracellular events that activate transcription of genes for a variety of proteins involved in innate immune responses. This is important because bacterial lipopolysaccharide produced by gram-negative organisms is the cause of septic shock. TLR2 mediates the response to microbial lipoproteins, TLR6 cooperates with TLR2 in recognizing certain peptidoglycans, and TLR9 recognizes the DNA of certain bacteria.
ACQUIRED IMMUNITY As noted previously, the key to acquired immunity is the ability of lymphocytes to produce antibodies (in the case of B cells) or cell-surface receptors (in the case of T cells) that are specific for one of the many millions of foreign agents that may invade the body. The antigens stimulating production of T cell receptors or antibodies are usually proteins and polypeptides, but antibodies can also be formed against nucleic acids and lipids if these are presented as nucleoproteins and lipoproteins, and antibodies to smaller molecules can be produced experimentally if the molecules are bound to protein. Acquired immunity has two components: humoral immunity and cellular immunity. Humoral immunity is mediated by circulating immunoglobulin antibodies in the γ-globulin fraction of the plasma proteins. Immunoglobulins are produced by differentiated forms of B lymphocytes known as plasma cells, and they activate the complement system and attack and neutralize antigens. Humoral immunity is a major defense
against bacterial infections. Cellular immunity is mediated by T lymphocytes. It is responsible for delayed allergic reactions and rejection of transplants of foreign tissue. Cytotoxic T cells attack and destroy cells that have the antigen which activated them. They kill by inserting perforins (see above) and by initiating apoptosis. Cellular immunity constitutes a major defense against infections due to viruses, fungi, and a few bacteria such as the tubercle bacillus. It also helps defend against tumors.
DEVELOPMENT OF THE IMMUNE SYSTEM During fetal development, and to a much lesser extent during adult life, lymphocyte precursors come from the bone marrow. Those that populate the thymus (Figure 3–5) become transformed by the environment in this organ into T lymphocytes. In birds, the precursors that populate the bursa of Fabricius, a lymphoid structure near the cloaca, become transformed into B lymphocytes. There is no bursa in mammals, and the transformation to B lymphocytes occurs in bursal equivalents, that is, the fetal liver and, after birth, the bone marrow. After residence in the thymus or liver, many of the T and B lymphocytes migrate to the lymph nodes. T and B lymphocytes are morphologically indistinguishable but can be identified by markers on their cell membranes. B cells differentiate into plasma cells and memory B cells. There are three major types of T cells: cytotoxic T cells, helper T cells, and memory T cells. There are two subtypes of helper T cells: T helper 1 (TH1) cells secrete IL-2 and γinterferon and are concerned primarily with cellular immunity; T helper 2 (TH2) cells secrete IL-4 and IL-5 and interact primarily with B cells in relation to humoral immunity. Cytotoxic T cells destroy transplanted and other foreign cells, with their development aided and directed by helper T cells. Markers on the surface of lymphocytes are assigned CD (clusters of differentiation) numbers on the basis of their reactions to a
Memory T cells Thymus T lymphocytes
Bone marrow lymphocyte precursors
Cytotoxic T cells (mostly CD8 T cells)
Helper T cells (CD4 T cells) Plasma cells
B lymphocytes Bursal equivalent (liver, bone marrow) Memory B cells
FIGURE 3–5
Cellular immunity
Development of the system mediating acquired immunity.
IgG IgA IgM IgD IgE
Humoral immunity
CHAPTER 3 Immunity, Infection, & Inflammation panel of monoclonal antibodies. Most cytotoxic T cells display the glycoprotein CD8, and helper T cells display the glycoprotein CD4. These proteins are closely associated with the T cell receptors and may function as coreceptors. On the basis of differences in their receptors and functions, cytotoxic T cells are divided into αβ and γδ types (see below). Natural killer cells (see above) are also cytotoxic lymphocytes, though they are not T cells. Thus, there are three main types of cytotoxic lymphocytes in the body: αβ T cells, γδ T cells, and NK cells.
MEMORY B CELLS & T CELLS After exposure to a given antigen, a small number of activated B and T cells persist as memory B and T cells. These cells are readily converted to effector cells by a later encounter with the same antigen. This ability to produce an accelerated response to a second exposure to an antigen is a key characteristic of acquired immunity. The ability persists for long periods of time, and in some instances (eg, immunity to measles) it can be lifelong. After activation in lymph nodes, lymphocytes disperse widely throughout the body and are especially plentiful in areas where invading organisms enter the body, for example, the mucosa of the respiratory and gastrointestinal tracts. This puts memory cells close to sites of reinfection and may account in part for the rapidity and strength of their response. Chemokines are involved in guiding activated lymphocytes to these locations.
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Langerhans dendritic cells in the skin. Macrophages and B cells themselves, and likely many other cell types, can also function as APCs. In APCs, polypeptide products of antigen digestion are coupled to protein products of the major histocompatibility complex (MHC) genes and presented on the surface of the cell. The products of the MHC genes are called human leukocyte antigens (HLA). The genes of the MHC, which are located on the short arm of human chromosome 6, encode glycoproteins and are divided into two classes on the basis of structure and function. Class I antigens are composed of a 45-kDa heavy chain associated noncovalently with β2-microglobulin encoded by a gene outside the MHC (Figure 3–6). They are found on all nucleated cells. Class II antigens are heterodimers made up of a 29- to 34-kDa α chain associated noncovalently with a 25to 28-kDa β chain. They are present in antigen-presenting cells, including B cells, and in activated T cells. The class I MHC proteins (MHC-I proteins) are coupled primarily to peptide fragments generated from proteins synthesized within cells. The peptides to which the host is not tolerant (eg, those from mutant or viral proteins) are recognized by T cells. The digestion of these proteins occurs in
α1
α2
ANTIGEN RECOGNITION The number of different antigens recognized by lymphocytes in the body is extremely large. The repertoire develops initially without exposure to the antigen. Stem cells differentiate into many million different T and B lymphocytes, each with the ability to respond to a particular antigen. When the antigen first enters the body, it can bind directly to the appropriate receptors on B cells. However, a full antibody response requires that the B cells contact helper T cells. In the case of T cells, the antigen is taken up by an antigen-presenting cell and partially digested. A peptide fragment of it is presented to the appropriate receptors on T cells. In either case, the cells are stimulated to divide, forming clones of cells that respond to this antigen (clonal selection). Effector cells are also subject to negative selection, during which lymphocyte precursors that are reactive with self antigens are normally deleted. This results in immune tolerance. It is this latter process that presumably goes awry in autoimmune diseases, where the body reacts to and destroys cells expressing normal proteins, with accompanying inflammation that may lead to tissue destruction.
ANTIGEN PRESENTATION Antigen-presenting cells (APCs) include specialized cells called dendritic cells in the lymph nodes and spleen and the
N
N
C C
β2m α3
FIGURE 3–6
Structure of human histocompatibility antigen HLA-A2. The antigen-binding pocket is at the top and is formed by the α1 and α2 parts of the molecule. The α3 portion and the associated β2microglobulin (β2m) are close to the membrane. The extension of the C terminal from α3 that provides the transmembrane domain and the small cytoplasmic portion of the molecule have been omitted. (Reproduced with permission from Bjorkman PJ et al: Structure of the human histocompatibility antigen HLA-A2. Nature 1987;329:506.)
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SECTION I Cellular & Molecular Basis for Medical Physiology
proteasomes, complexes of proteolytic enzymes that may be produced by genes in the MHC group, and the peptide fragments appear to bind to MHC proteins in the endoplasmic reticulum. The class II MHC proteins (MHC-II proteins) are concerned primarily with peptide products of extracellular antigens, such as bacteria, that enter the cell by endocytosis and are digested in the late endosomes.
CD4
TCR
T CELL RECEPTORS The MHC protein–peptide complexes on the surface of the antigen-presenting cells bind to appropriate T cells. Therefore, receptors on the T cells must recognize a very wide variety of complexes. Most of the receptors on circulating T cells are made up of two polypeptide units designated α and β. They form heterodimers that recognize the MHC proteins and the antigen fragments with which they are combined (Figure 3–7). These cells are called αβ T cells. About 10% of the circulating T cells have two different polypeptides designated γ and δ in their receptors, and they are called γδ T cells. These T cells are prominent in the mucosa of the gastrointestinal tract, and there is evidence that they form a link between the innate and acquired immune systems by way of the cytokines they secrete (Figure 3–3). CD8 occurs on the surface of cytotoxic T cells that bind MHC-I proteins, and CD4 occurs on the surface of helper T cells that bind MHC-II proteins (Figure 3–8). The CD8 and CD4 proteins facilitate the binding of the MHC proteins to the T cell receptors, and they also foster lymphocyte development, but how they produce these effects is unsettled. The
Antigen-presenting cell membrane
Cytoplasm
ECF
β 2m α 3
MHC molecular complex
α1/α2 Antigen fragment Variable regions
Constant regions
S–S
T cell membrane
ECF + + Cytoplasm
β
α
T cell receptor heterodimer (α:β)
FIGURE 3–7
Class II MHC CD8
Interaction between antigen-presenting cell (top) and αβ T lymphocyte (bottom). The MHC proteins (in this case, MHC-I) and their peptide antigen fragment bind to the α and β units that combine to form the T cell receptor.
Class I MHC
TCR
FIGURE 3–8
Diagrammatic summary of the structure of CD4 and CD8, and their relation to MHC-I and MHC-II proteins. Note that CD4 is a single protein, whereas CD8 is a heterodimer.
activated CD8 cytotoxic T cells kill their targets directly, whereas the activated CD4 helper T cells secrete cytokines that activate other lymphocytes. The T cell receptors are surrounded by adhesion molecules and proteins that bind to complementary proteins in the antigen-presenting cell when the two cells transiently join to form the “immunologic synapse” that permits T cell activation to occur. It is now generally accepted that two signals are necessary to produce activation. One is produced by the binding of the digested antigen to the T cell receptor. The other is produced by the joining of the surrounding proteins in the “synapse.” If the first signal occurs but the second does not, the T cell is inactivated and becomes unresponsive.
B CELLS As noted above, B cells can bind antigens directly, but they must contact helper T cells to produce full activation and antibody formation. It is the TH2 subtype that is mainly involved. Helper T cells develop along the TH2 lineage in response to IL-4 (see below). On the other hand, IL-12 promotes the TH1 phenotype. IL-2 acts in an autocrine fashion to cause activated T cells to proliferate. The role of various cytokines in B cell and T cell activation is summarized in Figure 3–9. The activated B cells proliferate and transform into memory B cells (see above) and plasma cells. The plasma cells secrete large quantities of antibodies into the general circulation. The antibodies circulate in the globulin fraction of the plasma and, like antibodies elsewhere, are called immunoglobulins. The immunoglobulins are actually the secreted form of antigen-binding receptors on the B cell membrane.
CHAPTER 3 Immunity, Infection, & Inflammation
MHC class II
Antigenbinding site
Macrophage (antigenpresenting cell)
1
2
VH
VH
JH D CH1
Fab
IL-1
VL
CD4 CL
TCR Cytokineinduced IL-2R activation 4
Activated B cell
CD4 4
Activated T cell
Inflammation and delayed hypersensitivity
Fc
IL-2
C H2
Hinge
C H3
4
FIGURE 3–10
IL-2R
Cytotoxic T cell
CD8
MHC class I Antibodyproducing cell
Complement binding Macrophage binding
SS SS
JL
SS
SS
VL
3
73
Typical immunoglobulin G molecule. Fab, portion of the molecule that is concerned with antigen binding; Fc, effector portion of the molecule. The constant regions are pink and purple, and the variable regions are orange. The constant segment of the heavy chain is subdivided into CH1, CH2, and CH3. SS lines indicate intersegmental disulfide bonds. On the right side, the C labels are omitted to show regions JH, D, and JL.
Cell death
FIGURE 3–9
Summary of acquired immunity. (1) An antigenpresenting cell ingests and partially digests an antigen, then presents part of the antigen along with MHC peptides (in this case, MHC II peptides on the cell surface). (2) An “immune synapse” forms with a naive CD4 T cell, which is activated to produce IL-2. (3) IL-2 acts in an autocrine fashion to cause the cell to multiply, forming a clone. (4) The activated CD4 cell may promote B cell activation and production of plasma cells or it may activate a cytotoxic CD8 cell. The CD8 cell can also be activated by forming a synapse with an MCH I antigen-presenting cell. (Reproduced with permission from McPhee SJ, Lingappa VR, Ganong WF
[editors]: Pathophysiology of Disease, 4th ed. McGraw-Hill, 2003.)
IMMUNOGLOBULINS Circulating antibodies protect their host by binding to and neutralizing some protein toxins, by blocking the attachment of some viruses and bacteria to cells, by opsonizing bacteria (see above), and by activating complement. Five general types of immunoglobulin antibodies are produced by the lymphocyte–plasma cell system. The basic component of each is a symmetric unit containing four polypeptide chains (Figure 3–10). The two long chains are called heavy chains, whereas the two short chains are called light chains. There are two types of light chains, k and λ, and eight types of heavy chains. The chains are joined by disulfide bridges that permit mobility, and there are intrachain disulfide bridges as well. In addition, the heavy chains are flexible in a region called the hinge. Each heavy chain has a variable (V) segment in which the amino acid sequence is highly variable, a diversity (D) segment in which the amino acid segment is also highly variable, a joining (J) segment in which the sequence is moderately variable, and a constant (C) segment in which the sequence is constant. Each light
chain has a V, a J, and a C segment. The V segments form part of the antigen-binding sites (Fab portion of the molecule [Figure 3–10]). The Fc portion of the molecule is the effector portion, which mediates the reactions initiated by antibodies. Two of the classes of immunoglobulins contain additional polypeptide components (Table 3–3). In IgMs, five of the basic immunoglobulin units join around a polypeptide called the J chain to form a pentamer. In IgAs, the secretory immunoglobulins, the immunoglobulin units form dimers and trimers around a J chain and a polypeptide that comes from epithelial cells, the secretory component (SC). In the intestine, bacterial and viral antigens are taken up by M cells (see Chapter 27) and passed on to underlying aggregates of lymphoid tissue (Peyer’s patches), where they activate naive T cells. These lymphocytes then form B cells that infiltrate mucosa of the gastrointestinal, respiratory, genitourinary, and female reproductive tracts and the breast. There they secrete large amounts of IgAs when exposed again to the antigen that initially stimulated them. The epithelial cells produce the SC, which acts as a receptor for and binds the IgA. The resulting secretory immunoglobulin passes through the epithelial cell and is secreted by exocytosis. This system of secretory immunity is an important and effective defense mechanism.
GENETIC BASIS OF DIVERSITY IN THE IMMUNE SYSTEM The genetic mechanism for the production of the immensely large number of different configurations of immunoglobulins produced by human B cells is a fascinating biologic problem.
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SECTION I Cellular & Molecular Basis for Medical Physiology
TABLE 3–3 Human immunoglobulins.a
a
Immunoglobulin
Function
Heavy Chain
Additional Chain
IgG
Complement activation
γ1 , γ 2 , γ 3 , γ 4
IgA
Localized protection in external secretions (tears, intestinal secretions, etc)
α1 , α2
IgM
Complement activation
μ
IgD
Antigen recognition by B cells
δ
Monomer
3
IgE
Reagin activity; releases histamine from basophils and mast cells
ε
Monomer
0.05
Structure
Plasma Concentration (mg/dL)
Monomer
1000
J, SC
Monomer; dimer with J or SC chain; trimer with J chain
200
J
Pentamer with J chain
120
In all instances, the light chains are k or γ.
Diversity is brought about in part by the fact that in immune globulin molecules there are two kinds of light chains and eight kinds of heavy chains. As noted previously, there are areas of great variability (hypervariable regions) in each chain. The variable portion of the heavy chains consists of the V, D, and J segments. In the gene family responsible for this region, there are several hundred different coding regions for the V segment, about 20 for the D segment, and 4 for the J segment. During B cell development, one V coding region, one D coding region, and one J coding region are selected at random and recombined to form the gene that produces that particular variable portion. A similar variable recombination takes place in the coding regions responsible for the two variable segments (V and J) in the light chain. In addition, the J segments are variable because the gene segments join in an imprecise and variable fashion (junctional site diversity) and nucleotides are sometimes added (junctional insertion diversity). It has been calculated that these mechanisms permit the production of about 1015 different immunoglobulin molecules. Additional variability is added by somatic mutation. Similar gene rearrangement and joining mechanisms operate to produce the diversity in T cell receptors. In humans, the α subunit has a V region encoded by 1 of about 50 different genes and a J region encoded by 1 of another 50 different genes. The β subunits have a V region encoded by 1 of about 50 genes, a D region encoded by 1 of 2 genes, and a J region encoded by 1 of 13 genes. These variable regions permit the generation of up to an estimated 1015 different T cell receptors (Clinical Box 3–2 and Clinical Box 3–3). A variety of immunodeficiency states can arise from defects in these various stages of B and T lymphocyte maturation. These are summarized in Figure 3–12.
PLATELETS Platelets are circulating cells that are important mediators of hemostasis. While not immune cells, per se, they often participate in the response to tissue injury in cooperation with in-
flammatory cell types (see below). They have a ring of microtubules around their periphery and an extensively invaginated membrane with an intricate canalicular system in contact with the ECF. Their membranes contain receptors for collagen, ADP, vessel wall von Willebrand factor (see below), and fibrinogen. Their cytoplasm contains actin, myosin, glycogen, lysosomes, and two types of granules: (1) dense granules, which contain the nonprotein substances that are secreted in response to platelet activation, including serotonin, ADP, and other adenine nucleotides; and (2) α-granules, which contain secreted proteins other than the hydrolases in lysosomes. These proteins include clotting factors and plateletderived growth factor (PDGF). PDGF is also produced by macrophages and endothelial cells. It is a dimer made up of A and B subunit polypeptides. Homodimers (AA and BB), as well as the heterodimer (AB), are produced. PDGF stimulates wound healing and is a potent mitogen for vascular smooth muscle. Blood vessel walls as well as platelets contain von Willebrand factor, which, in addition to its role in adhesion, regulates circulating levels of factor VIII (see below). When a blood vessel wall is injured, platelets adhere to the exposed collagen and von Willebrand factor in the wall via receptors on the platelet membrane. Von Willebrand factor is a very large circulating molecule that is produced by endothelial cells. Binding produces platelet activations which release the contents of their granules. The released ADP acts on the ADP receptors in the platelet membranes to produce further accumulation of more platelets (platelet aggregation). Humans have at least three different types of platelet ADP receptors: P2Y1, P2Y2, and P2X1. These are obviously attractive targets for drug development, and several new inhibitors have shown promise in the prevention of heart attacks and strokes. Aggregation is also fostered by platelet-activating factor (PAF), a cytokine secreted by neutrophils and monocytes as well as platelets. This compound also has inflammatory activity. It is an ether phospholipid, 1-alkyl-2acetylglyceryl-3-phosphorylcholine, which is produced from membrane lipids. It acts via a G protein-coupled receptor to
CHAPTER 3 Immunity, Infection, & Inflammation
CLINICAL BOX 3–2
CLINICAL BOX 3–3
Autoimmunity
Tissue Transplantation
Sometimes the processes that eliminate antibodies against self antigens fail and a variety of different autoimmune diseases are produced. These can be B cell- or T cell-mediated and can be organ-specific or systemic. They include type 1 diabetes mellitus (antibodies against pancreatic islet B cells), myasthenia gravis (antibodies against nicotinic cholinergic receptors), and multiple sclerosis (antibodies against myelin basic protein and several other components of myelin). In some instances, the antibodies are against receptors and are capable of activating those receptors; for example, antibodies against TSH receptors increase thyroid activity and cause Graves’ disease (see Chapter 20). Other conditions are due to the production of antibodies against invading organisms that cross-react with normal body constituents (molecular mimicry). An example is rheumatic fever following a streptococcal infection; a portion of cardiac myosin resembles a portion of the streptococcal M protein, and antibodies induced by the latter attack the former and damage the heart. Some conditions may be due to bystander effects, in which inflammation sensitizes T cells in the neighborhood, causing them to become activated when otherwise they would not respond. However, much is still uncertain about the pathogenesis of autoimmune disease.
The T lymphocyte system is responsible for the rejection of transplanted tissue. When tissues such as skin and kidneys are transplanted from a donor to a recipient of the same species, the transplants “take” and function for a while but then become necrotic and are “rejected” because the recipient develops an immune response to the transplanted tissue. This is generally true even if the donor and recipient are close relatives, and the only transplants that are never rejected are those from an identical twin. A number of treatments have been developed to overcome the rejection of transplanted organs in humans. The goal of treatment is to stop rejection without leaving the patient vulnerable to massive infections. One approach is to kill T lymphocytes by killing all rapidly dividing cells with drugs such as azathioprine, a purine antimetabolite, but this makes patients susceptible to infections and cancer. Another is to administer glucocorticoids, which inhibit cytotoxic T cell proliferation by inhibiting production of IL-2, but these cause osteoporosis, mental changes, and the other facets of Cushing syndrome (see Chapter 22). More recently, immunosuppressive drugs such as cyclosporine or tacrolimus (FK-506) have found favor. Activation of the T cell receptor normally increases intracellular Ca2+, which acts via calmodulin to activate calcineurin (Figure 3-11). Calcineurin dephosphorylates the transcription factor NF-AT, which moves to the nucleus and increases the activity of genes coding for IL-2 and related stimulatory cytokines. Cyclosporine and tacrolimus prevent the dephosphorylation of NF-AT. However, these drugs inhibit all T cell-mediated immune responses, and cyclosporine causes kidney damage and cancer. A new and promising approach to transplant rejection is the production of T cell unresponsiveness by using drugs that block the costimulation that is required for normal activation (see text). Clinically effective drugs that act in this fashion could be of great value to transplant surgeons.
increase the production of arachidonic acid derivatives, including thromboxane A2. The role of this compound in the balance between clotting and anticlotting activity at the site of vascular injury is discussed in Chapter 32. Platelet production is regulated by the colony-stimulating factors that control the production of megakaryocytes, plus thrombopoietin, a circulating protein factor. This factor, which facilitates megakaryocyte maturation, is produced constitutively by the liver and kidneys, and there are thrombopoietin receptors on platelets. Consequently, when the number of platelets is low, less is bound and more is available to stimulate production of platelets. Conversely, when the number of platelets is high, more is bound and less is available, producing a form of feedback control of platelet production. The amino terminal portion of the thrombopoietin molecule has the platelet-stimulating activity, whereas the carboxyl terminal portion contains many carbohydrate residues and is concerned with the bioavailability of the molecule. When the platelet count is low, clot retraction is deficient and there is poor constriction of ruptured vessels. The resulting clinical syndrome (thrombocytopenic purpura) is characterized by easy bruisability and multiple subcutaneous hemorrhages. Purpura may also occur when the platelet count is normal, and in some of these cases, the circulating platelets are abnormal (thrombasthenic purpura). Individuals with thrombocytosis are predisposed to thrombotic events.
75
INFLAMMATION & WOUND HEALING LOCAL INJURY Inflammation is a complex localized response to foreign substances such as bacteria or in some instances to internally produced substances. It includes a sequence of reactions initially involving cytokines, neutrophils, adhesion molecules, complement, and IgG. PAF, an agent with potent inflammatory effects, also plays a role. Later, monocytes and lymphocytes are involved. Arterioles in the inflamed area dilate, and capillary permeability is increased (see Chapters 33 and 34). When the inflammation occurs in or just under the skin (Figure 3–13), it
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SECTION I Cellular & Molecular Basis for Medical Physiology NF-κB by increasing the production of IκBα, and this is probably the main basis of their anti-inflammatory action (see Chapter 22).
T cell receptor
Ca2+
SYSTEMIC RESPONSE TO INJURY
CAM
Cytokines produced in response to inflammation and other injuries also produce systemic responses. These include alterations in plasma acute phase proteins, defined as proteins whose concentration is increased or decreased by at least 25% following injury. Many of the proteins are of hepatic origin. A number of them are shown in Figure 3–14. The causes of the changes in concentration are incompletely understood, but it can be said that many of the changes make homeostatic sense. Thus, for example, an increase in C-reactive protein activates monocytes and causes further production of cytokines. Other changes that occur in response to injury include somnolence, negative nitrogen balance, and fever.
Calcineurin TCLBP CsABP
NF-AT
P IL-2 gene activation Nucleus
FIGURE 3–11
Action of cyclosporine (CsA) and tacrolimus (TCL) in lymphocytes. BP, binding protein; CAM, calmodulin.
WOUND HEALING
is characterized by redness, swelling, tenderness, and pain. Elsewhere, it is a key component of asthma, ulcerative colitis, and many other diseases. Evidence is accumulating that a transcription factor, nuclear factor-κB, plays a key role in the inflammatory response. NF-κB is a heterodimer that normally exists in the cytoplasm of cells bound to IκBα, which renders it inactive. Stimuli such as cytokines, viruses, and oxidants separate NFκB from IκBα, which is then degraded. NF-κB moves to the nucleus, where it binds to the DNA of the genes for numerous inflammatory mediators, resulting in their increased production and secretion. Glucocorticoids inhibit the activation of
When tissue is damaged, platelets adhere to exposed matrix via integrins that bind to collagen and laminin (Figure 3–13). Blood coagulation produces thrombin, which promotes platelet aggregation and granule release. The platelet granules generate an inflammatory response. White blood cells are attracted by selectins and bind to integrins on endothelial cells, leading to their extravasation through the blood vessel walls. Cytokines released by the white blood cells and platelets up-regulate integrins on macrophages, which migrate to the area of injury, and on fibroblasts and epithelial cells, which mediate wound healing
Pluripotent stem cell Autosomal recessive SCID Lymphoid progenitor
BONE MARROW
THYMUS X-linked SCID
pre-B cell Immature T cell
X-linked agammaglobulinemia
Hyper-IgM syndrome
IgM
FIGURE 3–12
MHC class II deficiency
MHC class I deficiency
B cell
IgG
IgA
CD8 cell
CD4 cell
IgE
Sites of congenital blockade of B and T lymphocyte maturation in various immunodeficiency states. SCID, severe combined immune deficiency. (Modified from Rosen FS, Cooper MD, Wedgwood RJP: The primary immunodeficiencies. N Engl J Med 1995;333:431.)
CHAPTER 3 Immunity, Infection, & Inflammation
CHAPTER SUMMARY
Fibrin clot
■
Neutrophil
Macrophage TGF-β1 TGF-α FGF VEGF
Platelet plug
PDGF BB TGF-β1 PDGF AB
■
IGF
Blood vessel
■
VEGF FGF-2
Neutrophil
■
FGF-2 ■
Fibroblast
■
FIGURE 3–13
Cutaneous wound 3 days after injury, showing the multiple cytokines and growth factors affecting the repair process. VEGF, vascular endothelial growth factor. For other abbreviations, see Appendix. Note the epidermis growing down under the fibrin clot, restoring skin continuity. (Modified from Singer AJ, Clark RAF:
Cutaneous wound healing. N Engl J Med 1999;341:738.)
and scar formation. Plasmin aids healing by removing excess fibrin. This aids the migration of keratinocytes into the wound to restore the epithelium under the scab. Collagen proliferates, producing the scar. Wounds gain 20% of their ultimate strength in 3 weeks and later gain more strength, but they never reach more than about 70% of the strength of normal skin. 30,100
Change in plasma concentration (%)
30,000 700 C-reactive protein
600 500
Serum amyloid A
400 300
Haptoglobin Fibrinogen
200 100
C3
0 Transferrin
Albumin 0
7
14
21
Time after inflammatory stimulus (d)
FIGURE 3–14
Time course of changes in some major acute phase proteins. C3, C3 component of complement. (Modified and reproduced with permission from Gitlin JD, Colten HR: Molecular biology of acute phase plasma proteins. In Pick F, et al [editors]: Lymphokines, vol 14, pages 123–153. Academic Press, 1987.)
77
■
■
Immune and inflammatory responses are mediated by several different cell types—granulocytes, lymphocytes, monocytes, mast cells, tissue macrophages, and antigen presenting cells— that arise predominantly from the bone marrow and may circulate or reside in connective tissues. Granulocytes mount phagocytic responses that engulf and destroy bacteria. These are accompanied by the release of reactive oxygen species and other mediators into adjacent tissues that may cause tissue injury. Mast cells and basophils underpin allergic reactions to substances that would be treated as innocuous by nonallergic individuals. A variety of soluble mediators orchestrate the development of immunologic effector cells and their subsequent immune and inflammatory reactions. Innate immunity represents an evolutionarily conserved, primitive response to stereotypical microbial components. Acquired immunity is slower to develop than innate immunity, but long-lasting and more effective. Genetic rearrangements endow B and T lymphocytes with a vast array of receptors capable of recognizing billions of foreign antigens. Self-reactive lymphocytes are normally deleted; a failure of this process leads to autoimmune disease. Disease can also result from abnormal function or development of granulocytes and lymphocytes. In these latter cases, deficient immune responses to microbial threats usually result.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. In normal human blood A) the eosinophil is the most common type of white blood cell. B) there are more lymphocytes than neutrophils. C) the iron is mostly in hemoglobin. D) there are more white cells than red cells. E) there are more platelets than red cells. 2. Lymphocytes A) all originate from the bone marrow after birth. B) are unaffected by hormones. C) convert to monocytes in response to antigens. D) interact with eosinophils to produce platelets. E) are part of the body’s defense against cancer. 3. The ability of the blood to phagocytose pathogens and mount a respiratory burst is increased by A) interleukin-2 (IL-2). B) granulocyte colony-stimulating factor (G-CSF). C) erythropoietin. D) interleukin-4 (IL-4). E) interleukin-5 (IL-5). 4. Cells responsible for innate immunity are activated most commonly by A) glucocorticoids. B) pollen. C) carbohydrate sequences in bacterial cell walls. D) eosinophils. E) cytoplasmic proteins of bacteria.
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SECTION I Cellular & Molecular Basis for Medical Physiology
CHAPTER RESOURCES Delibro G: The Robin Hood of antigen presentation. Science 2004;302:485. Delves PJ, Roitt IM: The immune system. (Two parts.) N Engl J Med 2000;343:37,108. Dhainaut J-K, Thijs LG, Park G (editors): Septic Shock. WB Saunders, 2000. Ganz T: Defensins and host defense. Science 1999;286:420. Samstein B, Emond JC: Liver transplant from living related donors. Annu Rev Med 2001;52:147.
Singer AJ, Clark RAF: Cutaneous wound healing. N Engl J Med 1999;341:738 Tedder TF, et al: The selectins: Vascular adhesion molecules. FASEB J 1995;9:866. Tilney NL: Transplant: From Myth to Reality. Yale University Press, 2003. Walport MJ: Complement. (Two parts) N Engl J Med 2001;344:1058, 1140.
SECTION II PHYSIOLOGY OF NERVE & MUSCLE CELLS
C
Excitable Tissue: Nerve
4
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■
Name the parts of a neuron and their functions. Name the various types of glia and their functions. Describe the chemical nature of myelin, and summarize the differences in the ways in which unmyelinated and myelinated neurons conduct impulses. Define orthograde and retrograde axonal transport and the molecular motors involved in each. Describe the changes in ionic channels that underlie electrotonic potentials, the action potential, and repolarization. List the various nerve fiber types found in the mammalian nervous system. Describe the function of neurotrophins.
INTRODUCTION The human central nervous system (CNS) contains about 1011 (100 billion) neurons. It also contains 10–50 times this number of glial cells. The CNS is a complex organ; it has been calculated that 40% of the human genes participate, at least to a degree, in its formation. The neurons, the basic building blocks of the nervous system, have evolved from primitive neuroeffector cells that respond to various stimuli by contracting. In more complex animals, contraction has become
the specialized function of muscle cells, whereas integration and transmission of nerve impulses have become the specialized functions of neurons. This chapter describes the cellular components of the CNS and the excitability of neurons, which involves the genesis of electrical signals that enable neurons to integrate and transmit impulses (action potentials, receptor potentials, and synaptic potentials).
79
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SECTION II Physiology of Nerve & Muscle Cells
CELLULAR ELEMENTS IN THE CNS GLIAL CELLS For many years following their discovery, glial cells (or glia) were viewed as CNS connective tissue. In fact, the word glia is Greek for glue. However, today theses cells are recognized for their role in communication within the CNS in partnership with neurons. Unlike neurons, glial cells continue to undergo cell division in adulthood and their ability to proliferate is particularly noticeable after brain injury (eg, stroke). There are two major types of glial cells in the vertebrate nervous system: microglia and macroglia. Microglia are scavenger cells that resemble tissue macrophages and remove debris resulting from injury, infection, and disease (eg, multiple sclerosis, AIDS-related dementia, Parkinson disease, and Alzheimer disease). Microglia arise from macrophages outside of the nervous system and are physiologically and embryologically unrelated to other neural cell types. There are three types of macroglia: oligodendrocytes, Schwann cells, and astrocytes (Figure 4–1). Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the CNS and peripheral nervous system, respectively. Astrocytes, which are found throughout the brain, are of two subtypes. Fibrous astrocytes, which contain many intermediate filaments, are found primarily in white matter. Protoplasmic astrocytes are found in gray matter and have a granular cytoplasm. Both types send processes to blood vessels, where they induce capillaries to form the tight junctions making up the blood–brain barrier. They also send processes that
A Oligodendrocyte Oligodendrocyte in white matter
envelop synapses and the surface of nerve cells. Protoplasmic astrocytes have a membrane potential that varies with the external K+ concentration but do not generate propagated potentials. They produce substances that are tropic to neurons, and they help maintain the appropriate concentration of ions and neurotransmitters by taking up K+ and the neurotransmitters glutamate and γ-aminobutyrate (GABA).
NEURONS Neurons in the mammalian central nervous system come in many different shapes and sizes. Most have the same parts as the typical spinal motor neuron illustrated in Figure 4–2. The cell body (soma) contains the nucleus and is the metabolic center of the neuron. Neurons have several processes called dendrites that extend outward from the cell body and arborize extensively. Particularly in the cerebral and cerebellar cortex, the dendrites have small knobby projections called dendritic spines. A typical neuron also has a long fibrous axon that originates from a somewhat thickened area of the cell body, the axon hillock. The first portion of the axon is called the initial segment. The axon divides into presynaptic terminals, each ending in a number of synaptic knobs which are also called terminal buttons or boutons. They contain granules or vesicles in which the synaptic transmitters secreted by the nerves are stored. Based on the number of processes that emanate from their cell body, neurons can be classified as unipolar, bipolar, and multipolar (Figure 4–3).
C Astrocyte
B Schwann cell Perineural oligodendrocytes
Capillary
Nodes of Ranvier
End-foot Neuron
Layers of myelin
Axons Schwann cell
End-foot
Fibrous astrocyte
Nucleus Inner tongue
Axon Neuron
FIGURE 4–1
The principal types of glial cells in the nervous system. A) Oligodendrocytes are small with relatively few processes. Those in the white matter provide myelin, and those in the gray matter support neurons. B) Schwann cells provide myelin to the peripheral nervous system. Each cell forms a segment of myelin sheath about 1 mm long; the sheath assumes its form as the inner tongue of the Schwann cell turns around the axon several times, wrapping in concentric layers. Intervals between segments of myelin are the nodes of Ranvier. C) Astrocytes are the most common glia in the CNS and are characterized by their starlike shape. They contact both capillaries and neurons and are thought to have a nutritive function. They are also involved in forming the blood–brain barrier. (From Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
CHAPTER 4 Excitable Tissue: Nerve
81
Cell body (soma) Initial segment of axon
Node of Ranvier
Schwann cell
Axon hillock Nucleus
Terminal buttons
Dendrites
FIGURE 4–2
Motor neuron with a myelinated axon. A motor neuron is comprised of a cell body (soma) with a nucleus, several processes called dendrites, and a long fibrous axon that originates from the axon hillock. The first portion of the axon is called the initial segment. A myelin sheath forms from Schwann cells and surrounds the axon except at its ending and at the nodes of Ranvier. Terminal buttons (boutons) are located at the terminal endings.
A Unipolar cell
B Bipolar cell
C Pseudo-unipolar cell
Dendrites
Peripheral axon to skin and muscle
Dendrite
Cell body Axon Cell body
Single bifurcated process
Axon
Central axon
Cell body
Axon terminals Invertebrate neuron
Bipolar cell of retina
Ganglion cell of dorsal root
D Three types of multipolar cells
Dendrites Apical dendrite Cell body Cell body Basal dendrite Axon
Dendrites
Axon
Motor neuron of spinal cord
FIGURE 4–3
Pyramidal cell of hippocampus
Cell body
Axon
Purkinje cell of cerebellum
Some of the types of neurons in the mammalian nervous system. A) Unipolar neurons have one process, with different segments serving as receptive surfaces and releasing terminals. B) Bipolar neurons have two specialized processes: a dendrite that carries information to the cell and an axon that transmits information from the cell. C) Some sensory neurons are in a subclass of bipolar cells called pseudo-unipolar cells. As the cell develops, a single process splits into two, both of which function as axons—one going to skin or muscle and another to the spinal cord. D) Multipolar cells have one axon and many dendrites. Examples include motor neurons, hippocampal pyramidal cells with dendrites in the apex and base, and cerebellar Purkinje cells with an extensive dendritic tree in a single plane. (From Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)
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SECTION II Physiology of Nerve & Muscle Cells
The conventional terminology used for the parts of a neuron works well enough for spinal motor neurons and interneurons, but there are problems in terms of “dendrites” and “axons” when it is applied to other types of neurons found in the nervous system. From a functional point of view, neurons generally have four important zones: (1) a receptor, or dendritic zone, where multiple local potential changes generated by synaptic connections are integrated; (2) a site where propagated action potentials are generated (the initial segment in spinal motor neurons, the initial node of Ranvier in cutaneous sensory neurons); (3) an axonal process that transmits propagated impulses to the nerve endings; and (4) the nerve endings, where action potentials cause the release of synaptic transmitters. The cell body is often located at the dendritic zone end of the axon, but it can be within the axon (eg, auditory neurons) or attached to the side of the axon (eg, cutaneous neurons). Its location makes no difference as far as the receptor function of the dendritic zone and the transmission function of the axon are concerned. The axons of many neurons are myelinated, that is, they acquire a sheath of myelin, a protein–lipid complex that is wrapped around the axon (Figure 4–2). In the peripheral nervous system, myelin forms when a Schwann cell wraps its membrane around an axon up to 100 times (Figure 4–1). The myelin is then compacted when the extracellular portions of a membrane protein called protein zero (P0) lock to the extracellular portions of P0 in the apposing membrane. Various mutations in the gene for P0 cause peripheral neuropathies; 29 different mutations have been described that cause symptoms ranging from mild to severe. The myelin sheath envelops the axon except at its ending and at the nodes of Ranvier, periodic 1-μm constrictions that are about 1 mm apart (Figure 4–2). The insulating function of myelin is discussed later in this chapter. Not all neurons are myelinated; some are unmyelinated, that is, simply surrounded by Schwann cells without the wrapping of the Schwann cell membrane that produces myelin around the axon. In the CNS of mammals, most neurons are myelinated, but the cells that form the myelin are oligodendrocytes rather than Schwann cells (Figure 4–1). Unlike the Schwann cell, which forms the myelin between two nodes of Ranvier on a single neuron, oligodendrocytes emit multiple processes that form myelin on many neighboring axons. In multiple sclerosis, a crippling autoimmune disease, patchy destruction of myelin occurs in the CNS (see Clinical Box 4–1). The loss of myelin is associated with delayed or blocked conduction in the demyelinated axons.
AXONAL TRANSPORT Neurons are secretory cells, but they differ from other secretory cells in that the secretory zone is generally at the end of the axon, far removed from the cell body. The apparatus for protein synthesis is located for the most part in the cell body, with transport of proteins and polypeptides to the axonal ending by axoplasmic flow. Thus, the cell body maintains the functional and an-
CLINICAL BOX 4–1 Demyelinating Diseases Normal conduction of action potentials relies on the insulating properties of myelin. Thus, defects in myelin can have major adverse neurological consequences. One example is multiple sclerosis (MS), an autoimmune disease that affects over 3 million people worldwide, usually striking between the ages of 20 and 50 and affecting women about twice as often as men. The cause of MS appears to include both genetic and environmental factors. It is most common among Caucasians living in countries with temperate climates, including Europe, southern Canada, northern United States, and southeastern Australia. Environmental triggers include early exposure to viruses such as Epstein-Barr virus and those that cause measles, herpes, chicken pox, or influenza. In MS, antibodies and white blood cells in the immune system attack myelin, causing inflammation and injury to the sheath and eventually the nerves that it surrounds. Loss of myelin leads to leakage of K+ through voltage-gated channels, hyperpolarization, and failure to conduct action potentials. Typical physiological deficits range from muscle weakness, fatigue, diminished coordination, slurred speech, blurred or hazy vision, bladder dysfunction, and sensory disturbances. Symptoms are often exasperated by increased body temperature or ambient temperature. Progression of the disease is quite variable. In the most common form, transient episodes appear suddenly, last a few weeks or months, and then gradually disappear. Subsequent episodes can appear years later, and eventually full recovery does not occur. Others have a progressive form of the disease in which there are no periods of remission. Diagnosing MS is very difficult and generally is delayed until multiple episodes occur with deficits separated in time and space. Nerve conduction tests can detect slowed conduction in motor and sensory pathways. Cerebral spinal fluid analysis can detect the presence of oligoclonal bands indicative of an abnormal immune reaction against myelin. The most definitive assessment is magnetic resonance imaging (MRI) to visualize multiple scarred (sclerotic) areas in the brain. Although there is no cure for MS, some drugs (eg, β-interferon) that suppress the immune response reduce the severity and slow the progression of the disease.
atomic integrity of the axon; if the axon is cut, the part distal to the cut degenerates (wallerian degeneration). Orthograde transport occurs along microtubules that run along the length of the axon and requires two molecular motors, dynein and kinesin (Figure 4–4). Orthograde transport moves from the cell body toward the axon terminals. It has both fast and slow components; fast axonal transport occurs at about 400 mm/day, and slow axonal transport occurs at 0.5 to 10 mm/day. Retrograde transport, which is in the opposite direction (from the
CHAPTER 4 Excitable Tissue: Nerve
83
FIGURE 4–4
Axonal transport along microtubules by dynein and kinesin. Fast and slow axonal orthograde transport occurs along microtubules that run along the length of the axon from the cell body to the terminal. Retrograde transport occurs from the terminal to the cell body.
(From Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology. McGraw-Hill, 2008.)
nerve ending to the cell body), occurs along microtubules at about 200 mm/day. Synaptic vesicles recycle in the membrane, but some used vesicles are carried back to the cell body and deposited in lysosomes. Some materials taken up at the ending by endocytosis, including nerve growth factor (NGF) and various viruses, are also transported back to the cell body. A potentially important exception to these principles seems to occur in some dendrites. In them, single strands of mRNA transported from the cell body make contact with appropriate ribosomes, and protein synthesis appears to create local protein domains.
EXCITATION & CONDUCTION Nerve cells have a low threshold for excitation. The stimulus may be electrical, chemical, or mechanical. Two types of physicochemical disturbances are produced: local, nonpropagated potentials called, depending on their location, synaptic, generator, or electrotonic potentials; and propagated potentials, the action potentials (or nerve impulses). These are the only electrical responses of neurons and other excitable tissues, and they are the main language of the nervous system. They are due to changes in the conduction of ions across the cell membrane that are produced by alterations in ion channels. The electrical events in neurons are rapid, being measured in milliseconds (ms); and the potential changes are small, being measured in millivolts (mV). The impulse is normally transmitted (conducted) along the axon to its termination. Nerves are not “telephone wires” that
transmit impulses passively; conduction of nerve impulses, although rapid, is much slower than that of electricity. Nerve tissue is in fact a relatively poor passive conductor, and it would take a potential of many volts to produce a signal of a fraction of a volt at the other end of a meter-long axon in the absence of active processes in the nerve. Conduction is an active, self-propagating process, and the impulse moves along the nerve at a constant amplitude and velocity. The process is often compared to what happens when a match is applied to one end of a trail of gunpowder; by igniting the powder particles immediately in front of it, the flame moves steadily down the trail to its end as it is extinguished in its progression. Mammalian neurons are relatively small, but giant unmyelinated nerve cells exist in a number of invertebrate species. Such cells are found, for example, in crabs (Carcinus), cuttlefish (Sepia), and squid (Loligo). The fundamental properties of neurons were first determined in these species and then found to be similar in mammals. The neck region of the muscular mantle of the squid contains single axons up to 1 mm in diameter. The fundamental properties of these long axons are similar to those of mammalian axons.
RESTING MEMBRANE POTENTIAL When two electrodes are connected through a suitable amplifier and placed on the surface of a single axon, no potential difference is observed. However, if one electrode is inserted into the interior of the cell, a constant potential difference is
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SECTION II Physiology of Nerve & Muscle Cells
observed, with the inside negative relative to the outside of the cell at rest. A membrane potential results from separation of positive and negative charges across the cell membrane (Figure 4–5). In neurons, the resting membrane potential is usually about –70 mV, which is close to the equilibrium potential for K+ (Figure 4–6). In order for a potential difference to be present across a membrane lipid bilayer, two conditions must be met. First, there must be an unequal distribution of ions of one or more species across the membrane (ie, a concentration gradient). Two, the membrane must be permeable to one or more of these ion species. The permeability is provided by the existence of channels or pores in the bilayer; these channels are usually permeable to a single species of ions. The resting membrane potential represents an equilibrium situation at which the driving force for the membrane-permeant ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions down their electrical gradients.
Equal +,–
– –
+
–
–
+
+
– –
+
– –
+
–
– +
+
+
+
+
+
+
+
+
+
Extracellular side
–
–
–
–
–
–
–
–
–
Cytoplasmic side
–
–
–
+
+ Equal +,–
–
+
+ +
+
+
–
–
+ +
+ – –
+
–
+
–
–
+
–
–
+ + – –
+
+
– +
FIGURE 4–5
This membrane potential results from separation of positive and negative charges across the cell membrane. The excess of positive charges (red circles) outside the cell and negative charges (blue circles) inside the cell at rest represents a small fraction of the total number of ions present. (From Kandel ER,
Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.).
4
(a) +30
Membrane potential (mV)
+
+
0
5
3
2 7
–70 1
6
Na+
Gated Na+ channel Gated K+ channel K+
K+
Relative membrane permeability
(b) 600
PNa
300
PK 50 1 0
1
2
3
4
Time (ms)
FIGURE 4–6 The changes in (a) membrane potential (mV) and (b) relative membrane permeability (P) to Na+ and K+ during an action potential. (From Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology. McGraw-Hill, 2008.)
CHAPTER 4 Excitable Tissue: Nerve In neurons, the concentration of K+ is much higher inside than outside the cell, while the reverse is the case for Na+. This concentration difference is established by the Na+-K+ ATPase. The outward K+ concentration gradient results in passive movement of K+ out of the cell when K+-selective channels are open. Similarly, the inward Na+ concentration gradient results in passive movement of Na+ into the cell when Na+-selective channels are open. Because there are more open K+ channels than Na+ channels at rest, the membrane permeability to K+ is greater. Consequently, the intracellular and extracellular K+ concentrations are the prime determinants of the resting membrane potential, which is therefore close to the equilibrium potential for K+. Steady ion leaks cannot continue forever without eventually dissipating the ion gradients. This is prevented by the Na+-K+ ATPase, which actively moves Na+ and K+ against their electrochemical gradient.
IONIC FLUXES DURING THE ACTION POTENTIAL The cell membranes of nerves, like those of other cells, contain many different types of ion channels. Some of these are voltage-gated and others are ligand-gated. It is the behavior of these channels, and particularly Na+ and K+ channels, which explains the electrical events in nerves. The changes in membrane conductance of Na+ and K+ that occur during the action potentials are shown in Figure 4–6. The conductance of an ion is the reciprocal of its electrical resistance in the membrane and is a measure of the membrane permeability to that ion. In response to a depolarizing stimulus, some of the voltage-gated Na+ channels become active, and when the threshold potential is reached, the voltage-gated Na+ channels overwhelm the K+ and other channels and an action potential results (a positive feedback loop). The membrane potential moves toward the equilibrium potential for Na+ (+60 mV) but does not reach it during the action potential, primarily because the increase in Na+ conductance is short-lived. The Na+ channels rapidly enter a closed state called the inactivated state and remain in this state for a few milliseconds before returning to the resting state, when they again can be activated. In addition, the direction of the electrical gradient for Na+ is reversed during the overshoot because the membrane potential is reversed, and this limits Na+ influx. A third factor producing repolarization is the opening of voltage-gated K+ channels. This opening is slower and more prolonged than the opening of the Na+ channels, and consequently, much of the increase in K+ conductance comes after the increase in Na+ conductance. The net movement of positive charge out of the cell due to K+ efflux at this time helps complete the process of repolarization. The slow return of the K+ channels to the closed state also explains the after-hyperpolarization, followed by a return to the resting membrane potential. Thus, voltage-gated K+ channels bring the action potential to an end and cause closure of their gates through a negative feedback process.
85
Figure 4–7 shows the sequential feedback control in voltagegated K+ and Na+ channels during the action potential. Decreasing the external Na+ concentration reduces the size of the action potential but has little effect on the resting membrane potential. The lack of much effect on the resting membrane potential would be predicted, since the permeability of the membrane to Na+ at rest is relatively low. Conversely, increasing the external K+ concentration decreases the resting membrane potential. Although Na+ enters the nerve cell and K+ leaves it during the action potential, the number of ions involved is minute relative to the total numbers present. The fact that the nerve gains Na+ and loses K+ during activity has been demonstrated experimentally, but significant differences in ion concentrations can be measured only after prolonged, repeated stimulation. Other ions, notably Ca2+, can affect the membrane potential through both channel movement and membrane interactions. A decrease in extracellular Ca2+ concentration increases the excitability of nerve and muscle cells by decreasing the amount of depolarization necessary to initiate the changes in the Na+ and K+ conductance that produce the action potential. Conversely, an increase in extracellular Ca2+ concentration can stabilize the membrane by decreasing excitability.
DISTRIBUTION OF ION CHANNELS IN MYELINATED NEURONS The spatial distribution of ion channels along the axon plays a key role in the initiation and regulation of the action potential. Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons. The initial segment and, in sensory neurons, the first node of Ranvier are the sites where impulses are normally generated, and the other nodes of Ranvier are the sites to which the impulses jump during saltatory conduction. The number of Na+ channels per square micrometer of membrane in myelinated mammalian neurons has been estimated to be 50–75 in the cell body, 350–500 in the initial segment, less than 25 on the surface of the myelin, 2000–12,000 at the nodes of Ranvier, and 20–75 at the axon terminals. Along the axons of unmyelinated neurons, the number is about 110. In many myelinated neurons, the Na+ channels are flanked by K+ channels that are involved in repolarization.
“ALL-OR-NONE” LAW It is possible to determine the minimal intensity of stimulating current (threshold intensity) that, acting for a given duration, will just produce an action potential. The threshold intensity varies with the duration; with weak stimuli it is long, and with strong stimuli it is short. The relation between the strength and the duration of a threshold stimulus is called the strength–duration curve. Slowly rising currents fail to fire the nerve because the nerve adapts to the applied stimulus, a process called adaptation.
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SECTION II Physiology of Nerve & Muscle Cells
(a) Start Opening of voltage-gated Na+ channels
Depolarizing stimulus
Stop
Inactivation of Na+ channels
+ Depolarization of membrane potential
Positive feedback Increased PNa
Increased flow of Na+ into the cell
(b) Start Depolarization of membrane by Na+ influx
Repolarization of membrane potential
Opening of voltage-gated K+ channels Negative feedback Increased PK
Increased flow of K+ out of the cell
FIGURE 4–7
Feedback control in voltage-gated ion channels in the membrane. (a) Na+ channels exert positive feedback. (b) K+ channels exert negative feedback. (From Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology. McGraw-Hill, 2008.)
Once threshold intensity is reached, a full-fledged action potential is produced. Further increases in the intensity of a stimulus produce no increment or other change in the action potential as long as the other experimental conditions remain constant. The action potential fails to occur if the stimulus is subthreshold in magnitude, and it occurs with constant amplitude and form regardless of the strength of the stimulus if the stimulus is at or above threshold intensity. The action potential is therefore “all or none” in character and is said to obey the all-or-none law.
ELECTROTONIC POTENTIALS, LOCAL RESPONSE, & FIRING LEVEL Although subthreshold stimuli do not produce an action potential, they do have an effect on the membrane potential. This can be demonstrated by placing recording electrodes within a few millimeters of a stimulating electrode and applying subthreshold stimuli of fixed duration. Application of such currents leads to a localized depolarizing potential
change that rises sharply and decays exponentially with time. The magnitude of this response drops off rapidly as the distance between the stimulating and recording electrodes is increased. Conversely, an anodal current produces a hyperpolarizing potential change of similar duration. These potential changes are called electrotonic potentials. As the strength of the current is increased, the response is greater due to the increasing addition of a local response of the membrane (Figure 4–8). Finally, at 7–15 mV of depolarization (potential of –55 mV), the firing level is reached and an action potential occurs.
CHANGES IN EXCITABILITY DURING ELECTROTONIC POTENTIALS & THE ACTION POTENTIAL During the action potential, as well as during electrotonic potentials and the local response, the threshold of the neuron to stimulation changes. Hyperpolarizing responses elevate the threshold, and depolarizing potentials lower it as they move
CHAPTER 4 Excitable Tissue: Nerve
Firing level Local response Resting membrane potential
After-depolarization After-hyperpolarization
−70 0.5
1.0 ms
Local response
1.5
−85
FIGURE 4–8 Electrotonic potentials and local response. The changes in the membrane potential of a neuron following application of stimuli of 0.2, 0.4, 0.6, 0.8, and 1.0 times threshold intensity are shown superimposed on the same time scale. The responses below the horizontal line are those recorded near the anode, and the responses above the line are those recorded near the cathode. The stimulus of threshold intensity was repeated twice. Once it caused a propagated action potential (top line), and once it did not. the membrane potential closer to the firing level. During the local response, the threshold is lowered, but during the rising and much of the falling phases of the spike potential, the neuron is refractory to stimulation. This refractory period is divided into an absolute refractory period, corresponding to the period from the time the firing level is reached until repolarization is about one-third complete, and a relative refractory period, lasting from this point to the start of afterdepolarization. During the absolute refractory period, no stimulus, no matter how strong, will excite the nerve, but during the relative refractory period, stronger than normal stimuli can cause excitation. During after-depolarization, the threshold is again decreased, and during after-hyperpolarization, it is increased. These changes in threshold are correlated with the phases of the action potential in Figure 4–9.
ELECTROGENESIS OF THE ACTION POTENTIAL The nerve cell membrane is polarized at rest, with positive charges lined up along the outside of the membrane and negative charges along the inside. During the action potential, this polarity is abolished and for a brief period is actually reversed (Figure 4–10). Positive charges from the membrane ahead of and behind the action potential flow into the area of negativity represented by the action potential (“current sink”). By drawing off positive charges, this flow decreases the polarity of the membrane ahead of the action potential. Such electrotonic depolarization initiates a local response, and when the firing level is reached, a propagated response occurs that in turn electrotonically depolarizes the membrane in front of it.
Excitability
Membrane potential (mV)
−55
Spike potential
Potential change
Propagated action potential
87
Period of latent addition Supernormal period
Subnormal period
Refractory period Time
FIGURE 4–9
Relative changes in excitability of a nerve cell membrane during the passage of an impulse. Note that excitability is the reciprocal of threshold. (Modified from Morgan CT: Physiological
Psychology. McGraw-Hill, 1943.)
SALTATORY CONDUCTION Conduction in myelinated axons depends on a similar pattern of circular current flow. However, myelin is an effective insulator, and current flow through it is negligible. Instead, depo-
ECF
+ + + + – – – –
– – + +
+ + + – – –
– – – – + + + +
+ + – –
– – – + + +
Axon
ECF Myelin
Active node
Inactive node
_
+ _
+
Axon + _
_ +
Direction of propagation
FIGURE 4–10
Local current flow (movement of positive charges) around an impulse in an axon. Top: Unmyelinated axon. Bottom: Myelinated axon. Positive charges from the membrane ahead of and behind the action potential flow into the area of negativity represented by the action potential (“current sink”). In myelinated axons, depolarization jumps from one node of Ranvier to the next (salutatory conduction).
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SECTION II Physiology of Nerve & Muscle Cells
larization in myelinated axons jumps from one node of Ranvier to the next, with the current sink at the active node serving to electrotonically depolarize the node ahead of the action potential to the firing level (Figure 4–10). This jumping of depolarization from node to node is called saltatory conduction. It is a rapid process that allows myelinated axons to conduct up to 50 times faster than the fastest unmyelinated fibers.
ORTHODROMIC & ANTIDROMIC CONDUCTION An axon can conduct in either direction. When an action potential is initiated in the middle of it, two impulses traveling in opposite directions are set up by electrotonic depolarization on either side of the initial current sink. In the natural situation, impulses pass in one direction only, ie, from synaptic junctions or receptors along axons to their termination. Such conduction is called orthodromic. Conduction in the opposite direction is called antidromic. Because synapses, unlike axons, permit conduction in one direction only, an antidromic impulse will fail to pass the first synapse they encounter and die out at that point.
BIPHASIC ACTION POTENTIALS The descriptions of the resting membrane potential and action potential outlined above are based on recording with two electrodes, one in the extracellular space and the other inside it. If both recording electrodes are placed on the surface of the axon, there is no potential difference between them at rest. When the nerve is stimulated and an impulse is conducted past the two electrodes, a characteristic sequence of potential changes results. As the wave of depolarization reaches the electrode nearest the stimulator, this electrode becomes negative relative to the other electrode (Figure 4–11). When the impulse passes to the portion of the nerve between the two electrodes, the potential returns to zero, and then, as it passes the second electrode, the first electrode becomes positive relative to the second. It is conventional to connect the leads in such a way that when the first electrode becomes negative relative to the second, an upward deflection is recorded. Therefore, the record shows an upward deflection followed by an isoelectric interval and then a downward deflection. This sequence is called a biphasic action potential (Figure 4–11).
PROPERTIES OF MIXED NERVES Peripheral nerves in mammals are made up of many axons bound together in a fibrous envelope called the epineurium. Potential changes recorded extracellularly from such nerves therefore represent an algebraic summation of the all-or-none action potentials of many axons. The thresholds of the individual axons in the nerve and their distance from the stimulat-
+ + − + + + + + + + + _ _ + _ _ _ _ _ _ _ _
+ + + + − + + + + + + _ _ _ _ + _ _ _ _ _ _
+ + + + + + + + − + + _ _ _ _ − _ _ _ + _ _
+ + + + + + + + + + − _ _ _ _ _ _ _ _ _ _ + Nerve mV Time
FIGURE 4–11
Biphasic action potential. Both recording electrodes are on the outside of the nerve membrane. It is conventional to connect the leads in such a way that when the first electrode becomes negative relative to the second, an upward deflection is recorded. Therefore, the record shows an upward deflection followed by an isoelectric interval and then a downward deflection.
ing electrodes vary. With subthreshold stimuli, none of the axons are stimulated and no response occurs. When the stimuli are of threshold intensity, axons with low thresholds fire and a small potential change is observed. As the intensity of the stimulating current is increased, the axons with higher thresholds are also discharged. The electrical response increases proportionately until the stimulus is strong enough to excite all of the axons in the nerve. The stimulus that produces excitation of all the axons is the maximal stimulus, and application of greater, supramaximal stimuli produces no further increase in the size of the observed potential.
NERVE FIBER TYPES & FUNCTION After a stimulus is applied to a nerve, there is a latent period before the start of the action potential. This interval corresponds to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrodes. Its duration is proportionate to the distance between the stimulating and recording electrodes and inversely proportionate to the speed of conduction. If the duration of the latent period and the distance between the stimulating and recording electrodes are known, axonal conduction velocity can be calculated. Erlanger and Gasser divided mammalian nerve fibers into A, B, and C groups, further subdividing the A group into α, β, γ, and δ fibers. In Table 4–1, the various fiber types are listed
CHAPTER 4 Excitable Tissue: Nerve
89
TABLE 4–1 Nerve fiber types in mammalian nerve.a Fiber Type
Function
Fiber Diameter (μm)
Conduction Velocity (m/s)
Spike Duration (ms)
Absolute Refractory Period (ms)
A α
Proprioception; somatic motor
12–20
70–120
β
Touch, pressure
5–12
30–70
γ
Motor to muscle spindles
3–6
15–30
δ
Pain, cold, touch
2–5
12–30
B
Preganglionic autonomic
50 mg estrogen and progestin
0.32
< 50 mg estrogen and progestin
0.27
Progestin only
1.2
IUD Copper 7
1.5
Loop D
1.3
Diaphragm
1.9
Condom
3.6
Withdrawal
6.7
Spermicide
11.9
Rhythm
15.5
Data from Vessey M, Lawless M, Yeates D: Efficacy of different contraceptive methods. Lancet 1982;1:841. Reproduced with permission.
uterus is impeded. IUDs can cause intrauterine infections, but these usually occur in the first month after insertion and in women exposed to sexually transmitted diseases. Women undergoing long-term treatment with relatively large doses of estrogen do not ovulate, probably because they have depressed FSH levels and multiple irregular bursts of LH secretion rather than a single midcycle peak. Women treated with similar doses of estrogen plus a progestational agent do not ovulate because the secretion of both gonadotropins is suppressed. In addition, the progestin makes the cervical mucus thick and unfavorable to sperm migration, and it may also interfere with implantation. For contraception, an orally active estrogen such as ethinyl estradiol is often combined with a synthetic progestin such as norethindrone. The pills are administered for 21 d, then withdrawn for 5 to 7 d to permit menstrual flow, and started again. Like ethinyl estradiol, norethindrone has an ethinyl group on position 17 of the steroid nucleus, so it is resistant to hepatic metabolism and consequently is effective by mouth. In addition to being a progestin, it is partly metabolized to ethinyl estradiol, and for this reason it also has estrogenic activity. Small as well as large doses of estrogen are effective (Table 25–8). Implants made up primarily of progestins such as levonorgestrel are now seeing increased use in some parts of the world. These are inserted under the skin and can prevent pregnancy for up to 5 y. They often produce amenorrhea, but otherwise they appear to be effective and well tolerated.
ABNORMALITIES OF OVARIAN FUNCTION Menstrual Abnormalities Some women who are infertile have anovulatory cycles; they fail to ovulate but have menstrual periods at fairly regular intervals. As noted above, anovulatory cycles are the rule for the first 1 to 2 y after menarche and again before the menopause. Amenorrhea is the absence of menstrual periods. If menstrual bleeding has never occurred, the condition is called primary amenorrhea. Some women with primary amenorrhea have small breasts and other signs of failure to mature sexually. Cessation of cycles in a woman with previously normal periods is called secondary amenorrhea. The most common cause of secondary amenorrhea is pregnancy, and the old clinical maxim that “secondary amenorrhea should be considered to be due to pregnancy until proved otherwise” has considerable merit. Other causes of amenorrhea include emotional stimuli and changes in the environment, hypothalamic diseases, pituitary disorders, primary ovarian disorders, and various systemic diseases. Evidence suggests that in some women with hypothalamic amenorrhea, the frequency of GnRH pulses is slowed as a result of excess opioid activity in the hypothalamus. In encouraging preliminary studies, the frequency of GnRH pulses has been increased by administration of the orally active opioid blocker naltrexone.
CHAPTER 25 The Gonads: Development & Function of the Reproductive System
CLINICAL BOX 25–5 Genetic Defects Causing Reproductive Abnormalities A number of single-gene mutations cause reproductive abnormalities when they occur in women. Examples include (1) Kallmann syndrome, which causes hypogonadotropic hypogonadism; (2) GnRH resistance, FSH resistance, and LH resistance, which are due to defects in the GnRH, FSH, or LH receptors, respectively; and (3) aromatase deficiency, which prevents the formation of estrogens. These are all caused by loss-of-function mutations. An interesting gain-of-function mutation causes the McCune–Albright syndrome, in which Gsα becomes constitutively active in certain cells but not others (mosaicism) because a somatic mutation after initial cell division has occurred in the embryo. It is associated with multiple endocrine abnormalities, including precocious puberty and amenorrhea with galactorrhea.
The terms hypomenorrhea and menorrhagia refer to scanty and abnormally profuse flow, respectively, during regular periods. Metrorrhagia is bleeding from the uterus between periods, and oligomenorrhea is reduced frequency of periods. Dysmenorrhea is painful menstruation. The severe menstrual cramps that are common in young women quite often disappear after the first pregnancy. Most of the symptoms of dysmenorrhea are due to accumulation of prostaglandins in the uterus, and symptomatic relief has been obtained by treatment with inhibitors of prostaglandin synthesis. Some women develop symptoms such as irritability, bloating, edema, decreased ability to concentrate, depression, headache, and constipation during the last 7 to 10 d of their menstrual cycles. These symptoms of the premenstrual syndrome (PMS) have been attributed to salt and water retention. However, it seems unlikely that this or any of the other hormonal alterations that occur in the late luteal phase are responsible because the time course and severity of the symptoms are not modified if the luteal phase is terminated early and menstruation produced by administration of mifepristone. The antidepressant fluoxetine (Prozac), which is a serotonin reuptake inhibitor, and the benzodiazepine alprazolam (Xanax) produce symptomatic relief, and so do GnRH-releasing agonists in doses that suppress the pituitary–ovarian axis. How these diverse clinical observations fit together to produce a picture of the pathophysiology of PMS is still unknown (see Clinical Box 25–5).
PREGNANCY Fertilization & Implantation In humans, fertilization of the ovum by the sperm usually occurs in the ampulla of the uterine tube. Fertilization involves
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(1) chemoattraction of the sperm to the ovum by substances produced by the ovum; (2) adherence to the zona pellucida, the membranous structure surrounding the ovum; (3) penetration of the zona pellucida and the acrosome reaction; and (4) adherence of the sperm head to the cell membrane of the ovum, with breakdown of the area of fusion and release of the sperm nucleus into the cytoplasm of the ovum (Figure 25–33). Millions of sperms are deposited in the vagina during intercourse. Eventually, 50 to 100 sperms reach the ovum, and many of them contact the zona pellucida. Sperms bind to a sperm receptor in the zona, and this is followed by the acrosomal reaction, that is, the breakdown of the acrosome, the lysosome-like organelle on the head of the sperm (Figure 25–14). Various enzymes are released, including the trypsin-like protease acrosin. Acrosin facilitates but is not required for the penetration of the sperm through the zona pellucida. When one sperm reaches the membrane of the ovum, fusion to the ovum membrane is mediated by fertilin, a protein on the surface of the sperm head that resembles the viral fusion proteins that permit some viruses to attack cells. The fusion provides the signal that initiates development. In addition, the fusion sets off a reduction in the membrane potential of the ovum that prevents polyspermy, the fertilization of the ovum by more than one sperm. This transient potential change is followed by a structural change in the zona pellucida that provides protection against polyspermy on a more long-term basis. The developing embryo, now called a blastocyst, moves down the tube into the uterus. This journey takes about 3 d, during which the blastocyst reaches the 8- or 16-cell stage. Once in contact with the endometrium, the blastocyst becomes surrounded by an outer layer of syncytiotrophoblast, a multinucleate mass with no discernible cell boundaries, and an inner layer of cytotrophoblast made up of individual cells. The syncytiotrophoblast erodes the endometrium, and the blastocyst
Sperm tail
Nucleus Acrosome Zona pellucida Egg cytoplasm Egg cell membrane
FIGURE 25–33
Sequential events in fertilization in mammals. Sperm are attracted to the ovum, bind to the zona pellucida, release acrosomal enzymes, penetrate the zona pellucida, and fuse with the membrane of the ovum, releasing the sperm nucleus into its cytoplasm. Current evidence indicates that the side, rather than the tip, of the sperm head fuses with the egg cell membrane. (Modified from
Vacquier VD: Evolution of gamete recognition proteins. Science 1999;281:1995.)
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burrows into it (implantation). The implantation site is usually on the dorsal wall of the uterus. A placenta then develops, and the trophoblast remains associated with it.
TABLE 25–9 Hormone levels in human maternal blood during normal pregnancy. Hormone
Failure to Reject the “Fetal Graft” It should be noted that the fetus and the mother are two genetically distinct individuals, and the fetus is in effect a transplant of foreign tissue in the mother. However, the transplant is tolerated, and the rejection reaction that is characteristically produced when other foreign tissues are transplanted (see Chapter 3) fails to occur. The way the “fetal graft” is protected is unknown. However, one explanation may be that the placental trophoblast, which separates maternal and fetal tissues, does not express the polymorphic class I and class II MHC genes and instead expresses HLA-G, a nonpolymorphic gene. Therefore, antibodies against the fetal proteins do not develop. In addition, there is a Fas ligand on the surface of the placenta, and this bonds to T cells, causing them to undergo apoptosis (see Chapter 3).
Infertility The vexing clinical problem of infertility often requires extensive investigation before a cause is found. In 30% of cases the problem is in the man; in 45%, the problem is in the woman; in 20%, both partners have a problem; and in 5% no cause can be found. In vitro fertilization, that is, removing mature ova, fertilizing them with sperm, and implanting one or more of them in the uterus at the four-cell stage is of some value in these cases. It has a 5–10% chance of producing a live birth.
Endocrine Changes In all mammals, the corpus luteum in the ovary at the time of fertilization fails to regress and instead enlarges in response to stimulation by gonadotropic hormones secreted by the placenta. The placental gonadotropin in humans is called human chorionic gonadotropin (hCG). The enlarged corpus luteum of pregnancy secretes estrogens, progesterone, and relaxin. The relaxin helps maintain pregnancy by inhibiting myometrial contractions. In most species, removal of the ovaries at any time during pregnancy precipitates abortion. In humans, however, the placenta produces sufficient estrogen and progesterone from maternal and fetal precursors to take over the function of the corpus luteum after the sixth week of pregnancy. Ovariectomy before the sixth week leads to abortion, but ovariectomy thereafter has no effect on the pregnancy. The function of the corpus luteum begins to decline after 8 wk of pregnancy, but it persists throughout pregnancy. hCG secretion decreases after an initial marked rise, but estrogen and progesterone secretion increase until just before parturition (Table 25–9).
Human Chorionic Gonadotropin hCG is a glycoprotein that contains galactose and hexosamine. It is produced by the syncytiotrophoblast. Like the pituitary
Approximate Peak Value
Time of Peak Secretion
hCG
5 mg/mL
First trimester
Relaxin
1 ng/mL
First trimester
hCS
15 mg/mL
Term
Estradiol
16 ng/mL
Term
Estriol
14 ng/mL
Term
Progesterone
190 ng/mL
Term
Prolactin
200 ng/mL
Term
glycoprotein hormones, it is made up of α and β subunits. hCG-α is identical to the α subunit of LH, FSH, and TSH. The molecular weight of hCG-α is 18,000, and that of hCG-β is 28,000. hCG is primarily luteinizing and luteotropic and has little FSH activity. It can be measured by radioimmunoassay and detected in the blood as early as 6 d after conception. Its presence in the urine in early pregnancy is the basis of the various laboratory tests for pregnancy, and it can sometimes be detected in the urine as early as 14 d after conception. It appears to act on the same receptor as LH. hCG is not absolutely specific for pregnancy. Small amounts are secreted by a variety of gastrointestinal and other tumors in both sexes, and hCG has been measured in individuals with suspected tumors as a “tumor marker.” It also appears that the fetal liver and kidney normally produce small amounts of hCG.
Human Chorionic Somatomammotropin The syncytiotrophoblast also secretes large amounts of a protein hormone that is lactogenic and has a small amount of growth-stimulating activity. This hormone has been called chorionic growth hormone-prolactin (CGP) and human placental lactogen (hPL), but it is now generally called human chorionic somatomammotropin (hCS). The structure of hCS is very similar to that of human growth hormone (see Figure 24–3), and it appears that these two hormones and prolactin evolved from a common progenitor hormone. Large quantities of hCS are found in maternal blood, but very little reaches the fetus. Secretion of growth hormone from the maternal pituitary is not increased during pregnancy and may actually be decreased by hCS. However, hCS has most of the actions of growth hormone and apparently functions as a “maternal growth hormone of pregnancy” to bring about the nitrogen, potassium, and calcium retention, lipolysis, and decreased glucose utilization seen in this state. These latter two actions divert glucose to the fetus. The amount of hCS secreted is proportionate to the size of the placenta, which normally weighs about one-sixth as much as the fetus, and low hCS levels are a sign of placental insufficiency.
CHAPTER 25 The Gonads: Development & Function of the Reproductive System
Other Placental Hormones In addition to hCG, hCS, progesterone, and estrogens, the placenta secretes other hormones. Human placental fragments probably produce proopiomelanocortin (POMC). In culture, they release corticotropin-releasing hormone (CRH), βendorphin, α-melanocyte-stimulating hormone (MSH), and dynorphin A, all of which appear to be identical to their hypothalamic counterparts. They also secrete GnRH and inhibin, and since GnRH stimulates and inhibin inhibits hCG secretion, locally produced GnRH and inhibin may act in a paracrine fashion to regulate hCG secretion. The trophoblast cells and amnion cells also secrete leptin, and moderate amounts of this satiety hormone enter the maternal circulation. Some also enters the amniotic fluid. Its function in pregnancy is unknown. The placenta also secretes prolactin in a number of forms. Finally, the placenta secretes the α subunits of hCG, and the plasma concentration of free α subunits rises throughout pregnancy. These α subunits acquire a carbohydrate composition that makes them unable to combine with β subunits, and their prominence suggests that they have a function of their own. It is interesting in this regard that the secretion of the prolactin produced by the endometrium also appears to increase throughout pregnancy, and it may be that the circulating α subunits stimulate endometrial prolactin secretion. The cytotrophoblast of the human chorion contains prorenin (see Chapter 39). A large amount of prorenin is also present in amniotic fluid, but its function in this location is unknown.
Fetoplacental Unit The fetus and the placenta interact in the formation of steroid hormones. The placenta synthesizes pregnenolone and progesterone from cholesterol. Some of the progesterone enters the fetal circulation and provides the substrate for the formation of cortisol and corticosterone in the fetal adrenal glands (Figure 25–34). Some of the pregnenolone enters the fetus and, along with pregnenolone synthesized in the fetal liver, is the substrate for the formation of dehydroepiandrosterone sulfate (DHEAS) and 16-hydroxydehydroepiandrosterone sulfate (16-OHDHEAS) in the fetal adrenal. Some 16-hydroxylation also occurs in the fetal liver. DHEAS and 16-OHDHEAS are transported back to the placenta, where DHEAS forms estradiol and 16-OHDHEAS forms estriol. The principal estrogen formed is estriol, and since fetal 16-OHDHEAS is the principal substrate for the estrogens, the urinary estriol excretion of the mother can be monitored as an index of the state of the fetus.
Parturition The duration of pregnancy in humans averages 270 d from fertilization (284 d from the first day of the menstrual period preceding conception). Irregular uterine contractions increase in frequency in the last month of pregnancy. The difference between the body of the uterus and the cervix becomes evident at the time of delivery. The cervix, which
Placenta
425
Fetal Adrenal
Cholesterol Pregnenolone
DHEAS 16-OHDHEAS
Progesterone
Cortisol, corticosterone
Estradiol
DHEAS
Estriol
16-OHDHEAS
FIGURE 25–34
Interactions between the placenta and the fetal adrenal cortex in the production of steroids.
is firm in the nonpregnant state and throughout pregnancy until near the time of delivery, softens and dilates, while the body of the uterus contracts and expels the fetus. There is still considerable uncertainty about the mechanisms responsible for the onset of labor. One factor is the increase in circulating estrogens produced by increased circulating DHEAS. This makes the uterus more excitable, increases the number of gap junctions between myometrial cells, and causes production of more prostaglandins, which in turn cause uterine contractions. In humans, CRH secretion by the fetal hypothalamus increases and is supplemented by increased placental production of CRH. This increases circulating adrenocorticotropic hormone (ACTH) in the fetus, and the resulting increase in cortisol hastens the maturation of the respiratory system. Thus, in a sense, the fetus picks the time to be born by increasing CRH secretion. The number of oxytocin receptors in the myometrium and the decidua (the endometrium of pregnancy) increases more than 100-fold during pregnancy and reaches a peak during early labor. Estrogens increase the number of oxytocin receptors, and uterine distention late in pregnancy may also increase their formation. In early labor, the oxytocin concentration in maternal plasma is not elevated from the prelabor value of about 25 pg/mL. It is possible that the marked increase in oxytocin receptors causes the uterus to respond to normal plasma oxytocin concentrations. However, at least in rats, the amount of oxytocin mRNA in the uterus increases, reaching a peak at term; this suggests that locally produced oxytocin also participates in the process. Premature onset of labor is a problem because premature infants have a high mortality rate and often require intensive, expensive care. Intramuscular 17α-hydroxyprogesterone causes a significant decrease in the incidence of premature labor. The mechanism by which it exerts its effect is uncertain, but it may be that the steroid provides a stable level of circulating progesterone. Progesterone relaxes uterine smooth muscle, inhibits the action of oxytocin on the muscle, and reduces
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Increase in oxytocin receptors
Prostaglandins
Uterine contractions
Dilation of cervix and distention of vagina
Stimuli from cervix and vagina
gens and progesterone are elevated as well, producing full lobuloalveolar development.
Secretion & Ejection of Milk The composition of human and cows’ milk is shown in Table 25–10. In estrogen- and progesterone-primed rodents, injections of prolactin cause the formation of milk droplets and their secretion into the ducts. Oxytocin causes contraction of the myoepithelial cells lining the duct walls, with consequent ejection of the milk through the nipple.
TABLE 25–10 Composition of colostrum and milk.* Human Colostrum
Human Milk
Cows’ Milk
Water, g
...
88
88
Lactose, g
5.3
6.8
5.0
Protein, g
2.7
1.2
3.3
Casein: lactalbumin ratio
...
1:2
3:1
Fat, g
2.9
3.8
3.7
Linoleic acid
...
8.3% of fat
1.6% of fat
Sodium, mg
92
15
58
Potassium, mg
55
55
138
Chloride, mg
117
43
103
Calcium, mg
31
33
125
Magnesium, mg
4
4
12
Phosphorus, mg
14
15
100
0.092
0.15a
0.10a
Vitamin A, μg
89
53
34
Vitamin D, μg
...
0.03a
0.06a
Thiamine, μg
15
16
42
Riboflavin, μg
30
43
157
Nicotinic acid, μg
75
172
85
Ascorbic acid, mg
4.4a
4.3a
1.6a
Component Increased secretion of oxytocin
FIGURE 25–35
Role of oxytocin in parturition.
the formation of gap junctions between the muscle fibers. All these actions would be expected to inhibit the onset of labor. Once labor is started, the uterine contractions dilate the cervix, and this dilation in turn sets up signals in afferent nerves that increase oxytocin secretion (Figure 25–35). The plasma oxytocin level rises and more oxytocin becomes available to act on the uterus. Thus, a positive feedback loop is established that aids delivery and terminates on expulsion of the products of conception. Oxytocin increases uterine contractions in two ways: (1) It acts directly on uterine smooth muscle cells to make them contract, and (2) it stimulates the formation of prostaglandins in the decidua. The prostaglandins enhance the oxytocin-induced contractions. During labor, spinal reflexes and voluntary contractions of the abdominal muscles (“bearing down”) also aid in delivery. However, delivery can occur without bearing down and without a reflex increase in secretion of oxytocin from the posterior pituitary gland, since paraplegic women can go into labor and deliver.
Iron, mg
LACTATION Development of the Breasts
*Weights per deciliter.
Many hormones are necessary for full mammary development. In general, estrogens are primarily responsible for proliferation of the mammary ducts and progesterone for the development of the lobules. In rats, some prolactin is also needed for development of the glands at puberty, but it is not known if prolactin is necessary in humans. During pregnancy, prolactin levels increase steadily until term, and levels of estro-
Reproduced with permission from Findlay ALR: Lactation. Res Reprod (Nov) 1974;6(6).
a
Poor source.
CHAPTER 25 The Gonads: Development & Function of the Reproductive System
Initiation of Lactation after Delivery The breasts enlarge during pregnancy in response to high circulating levels of estrogens, progesterone, prolactin, and possibly hCG. Some milk is secreted into the ducts as early as the fifth month, but the amounts are small compared with the surge of milk secretion that follows delivery. In most animals, milk is secreted within an hour after delivery, but in women it takes 1 to 3 d for the milk to “come in.” After expulsion of the placenta at parturition, the levels of circulating estrogens and progesterone abruptly decline. The drop in circulating estrogen initiates lactation. Prolactin and estrogen synergize in producing breast growth, but estrogen antagonizes the milk-producing effect of prolactin on the breast. Indeed, in women who do not wish to nurse their babies, estrogens may be administered to stop lactation. Suckling not only evokes reflex oxytocin release and milk ejection, it also maintains and augments the secretion of milk because of the stimulation of prolactin secretion produced by suckling.
Effect of Lactation on Menstrual Cycles Women who do not nurse their infants usually have their first menstrual period 6 wk after delivery. However, women who nurse regularly have amenorrhea for 25 to 30 wk. Nursing stimulates prolactin secretion, and evidence suggests that prolactin inhibits GnRH secretion, inhibits the action of GnRH on the pituitary, and antagonizes the action of gonadotropins on the ovaries. Ovulation is inhibited, and the ovaries are inactive, so estrogen and progesterone output falls to low levels. Consequently, only 5–10% of women become pregnant again during the suckling period, and nursing has long been known to be an important, if only partly effective, method of birth control. Furthermore, almost 50% of the cycles in the first 6 mo after resumption of menses are anovulatory (see Clinical Box 25–6).
Gynecomastia Breast development in the male is called gynecomastia. It may be unilateral but is more commonly bilateral. It is common, occurring in about 75% of newborns because of transplacental passage of maternal estrogens. It also occurs in mild, transient form in 70% of normal boys at the time of puberty and in many men over the age of 50. It occurs in androgen resistance. It is a complication of estrogen therapy and is seen in patients with estrogen-secreting tumors. It is found in a wide variety of seemingly unrelated conditions, including eunuchoidism, hyperthyroidism, and cirrhosis of the liver. Digitalis can produce it, apparently because cardiac glycosides are weakly estrogenic. It can also be caused by many other drugs. It has been seen in malnourished prisoners of war, but only after they were liberated and eating an adequate diet. A feature common to many and perhaps all cases of gynecomastia is an increase in the plasma estrogen:androgen ratio due to either increased circulating estrogens or decreased circulating androgens.
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CLINICAL BOX 25–6 Chiari–Frommel Syndrome An interesting, although rare, condition is persistence of lactation (galactorrhea) and amenorrhea in women who do not nurse after delivery. This condition, called the Chiari– Frommel syndrome, may be associated with some genital atrophy and is due to persistent prolactin secretion without the secretion of the FSH and LH necessary to produce maturation of new follicles and ovulation. A similar pattern of galactorrhea and amenorrhea with high circulating prolactin levels is seen in nonpregnant women with chromophobe pituitary tumors and in women in whom the pituitary stalk has been sectioned during treatment of cancer.
HORMONES & CANCER About 35% of carcinomas of the breast in women of childbearing age are estrogen-dependent; their continued growth depends on the presence of estrogens in the circulation. The tumors are not cured by decreasing estrogen secretion, but symptoms are dramatically relieved, and the tumor regresses for months or years before recurring. Women with estrogendependent tumors often have a remission when their ovaries are removed. Inhibition of the action of estrogens with tamoxifen also produces remissions, and inhibition of estrogen formation with drugs that inhibit aromatase (Figure 25–26) is even more effective. Some carcinomas of the prostate are androgen-dependent and regress temporarily after the removal of the testes or treatment with GnRH agonists in doses that are sufficient to produce down-regulation of the GnRH receptors on gonadotropes and decrease LH secretion.
CHAPTER SUMMARY ■
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Differences between males and females depend primarily on a single chromosome (the Y chromosome) and a single pair of endocrine structures (the gonads); testes in the male and ovaries in the female. The gonads have a dual function: the production of germ cells (gametogenesis) and the secretion of sex hormones. The testes secrete large amounts of androgens, principally testosterone, but they also secrete small amounts of estrogens. The ovaries secrete large amounts of estrogens and small amounts of androgens. Spermatogonia develop into mature spermatozoa that start in the seminiferous tubules in a process called spermatogenesis. This is a multistep process that includes maturation of spermatogonia into primary spermatocytes, which undergo meiotic division, resulting in haploid secondary spermatocytes and several further divisions result in spermatids. Each cell division from a spermatogonium to a spermatid is incomplete with cells remaining connected via cytoplasmic bridges. Spermatids eventually mature into motile spermatozoa to
428
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SECTION IV Endocrine & Reproductive Physiology complete spermatogenesis; this last part of maturation is called spermiogenesis. Testosterone is the principal hormone of the testis. It is synthesized from cholesterol in Leydig cells. The secretion of testosterone from Leydig cells is under control of luteinizing hormone at a rate of 4 to 9 mg/day in adult males. Most testosterone is bound to albumin or to gonadal steroid-binding globulin in the plasma. Testosterone plays an important role in the development and maintenance of male secondary sex characteristics, as well as other defined functions. Ovaries also secrete progesterone, a steroid that has special functions in preparing the uterus for pregnancy. During pregnancy the ovaries secrete relaxin, which facilitates the delivery of the fetus. In both sexes, the gonads secrete other polypeptides, including inhibin B, a polypeptide that inhibits FSH secretion. In women, a period called perimenopause precedes menopause, and can last up to ten years; during this time the menstrual cycles become irregular and the level of inhibins decrease. Once in menopause, the ovaries no longer secrete progesterone and 17β-estradiol and estrogen is formed only in small amounts by aromatization of androstenedione in peripheral tissues. The naturally occurring estrogens are 17β-estradiol, estrone, and estriol. They are secreted primarily by the granulosa cells of the ovarian follicles, the corpus luteum, and the placenta. Their biosynthesis depends on the enzyme aromatase (CYP19), which converts testosterone to estradiol and androstenedione to estrone. The latter reaction also occurs in fat, liver, muscle, and the brain.
4. In human males, testosterone is produced mainly by the A) Leydig cells. B) Sertoli cells. C) seminiferous tubules. D) epididymis. E) vas deferens. 5. Home-use kits for determining a woman’s fertile period depend on the detection of one hormone in the urine. This hormone is A) FSH. B) progesterone. C) estradiol. D) hCG. E) LH. 6. Which of the following is not a steroid? A) 17α-hydroxyprogesterone B) estrone C) relaxin D) pregnenolone E) etiocholanolone 7. Which of the following probably triggers the onset of labor? A) ACTH in the fetus B) ACTH in the mother C) prostaglandins D) oxytocin E) placental renin
CHAPTER RESOURCES MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. If a young woman has high plasma levels of T3, cortisol, and renin activity but her blood pressure is only slightly elevated and she has no symptoms or signs of thyrotoxicosis or Cushing syndrome, the most likely explanation is that A) she has been treated with TSH and ACTH. B) she has been treated with T3 and cortisol. C) she is in the third trimester of pregnancy. D) she has an adrenocortical tumor. E) she has been subjected to chronic stress. 2. Full development and function of the seminiferous tubules require A) somatostatin. B) LH. C) oxytocin. D) FSH. E) androgens and FSH. 3. In humans, fertilization usually occurs in the A) vagina. B) cervix. C) uterine cavity. D) uterine tubes. E) abdominal cavity.
Bole-Feysot C et al: Prolactin (PRL) and its receptor: Actions, signal transduction pathways, and phenotypes observed in PRL receptor knockout mice. Endocrinol Rev 1998;19:225. Mather JP, Moore A, Li R-H: Activins, inhibins, and follistatins: Further thoughts on a growing family of regulators. Proc Soc Exper Biol Med 1997;215:209. Matthews J, Gustafson J-A: Estrogen signaling: A subtle balance between ERα and ERβ. Mol Interv 2003;3:281. McLaughlin DT, Donahoe PR: Sex determination and differentiation. N Engl J Med 2004;350:367. Naz RK (editor): Endocrine Disruptors. CRC Press, 1998. Norwitz ER, Robinson JN, Challis JRG: The control of labor. N Engl J Med 1999;341:660. Primakoff P, Nyles DG: Penetration, adhesion, and fusion in mammalian sperm–egg interaction. Science 2002;296:2183. Simpson ER, et al: Aromatose—A brief overview. Annu Rev Physiol 2002;64:93. Yen SSC, Jaffe RB, Barbieri RL: Reproductive Endocrinology: Physiology, Pathophysiology, and Clinical Management, 4th ed. Saunders, 1999.
SECTION V GASTROINTESTINAL PHYSIOLOGY
Overview of Gastrointestinal Function & Regulation
26 C
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■
Understand the functional significance of the gastrointestinal system, and in particular, its roles in nutrient assimilation, excretion, and immunity. Describe the structure of the gastrointestinal tract, the glands that drain into it, and its subdivision into functional segments. List the major gastrointestinal secretions, their components, and the stimuli that regulate their production. Describe water balance in the gastrointestinal tract and explain how the level of luminal fluidity is adjusted to allow for digestion and absorption. Identify the major hormones, other peptides, and key neurotransmitters of the gastrointestinal system. Describe the special features of the enteric nervous system and the splanchnic circulation.
INTRODUCTION The gastrointestinal tract is a continuous tube that stretches from the mouth to the anus. Its primary function is to serve as a portal whereby nutrients and water can be absorbed into the body. In fulfilling this function, the meal is mixed with a variety of secretions that arise from both the gastrointestinal tract itself and organs that drain into it, such as the pancreas, gallbladder, and salivary glands. Likewise, the intestine displays a variety of motility patterns that serve to mix the meal with digestive secretions and move it along the length of the gastrointestinal tract. Ultimately, residues of the meal that cannot be absorbed, along with cellular debris and lipid-soluble metabolic end products that are excreted in the bile rather than
the urine, are expelled from the body. All of these functions are tightly regulated in concert with the ingestion of meals. Thus, the gastrointestinal system has evolved a large number of regulatory mechanisms that act both locally and to coordinate the function of the gut, and the organs that drain into it, over long distances. The lumen of the gastrointestinal tract is functionally contiguous with the outside of the body. The intestine also has a very substantial surface area, which is important for its absorptive function. Finally, the gut is an unusual organ in that it becomes colonized, almost from birth, with a large number of commensal bacteria (particularly in the colon, or large intestine). This 429
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relationship is mutually beneficial, because the bacteria perform several metabolic functions that cannot be accomplished with mammalian enzymes, and likely also provide some degree of protection against subsequent infection with pathogenic microorganisms that might cause disease. Nevertheless, the constant presence of bacterial and other stimuli, as well as the large sur-
face area that must be defended against potentially harmful substances, doubtlessly accounts for the fact that the intestine has a very well-developed local immune system that comprises both innate and adaptive immune effectors (see Chapter 3). Indeed, there are more lymphocytes in the wall of the intestine than there are circulating in the blood.
STRUCTURAL CONSIDERATIONS
is a layer of loose connective tissue known as the lamina propria, which in turn is surrounded by concentric layers of smooth muscle, oriented circumferentially and then longitudinally to the axis of the gut (the circular and longitudinal muscle layers, respectively). The intestine is also amply supplied with blood vessels, nerve endings, and lymphatics, which are all important in its function. The epithelium of the intestine is also further specialized in a way that maximizes the surface area available for nutrient absorption. Throughout the small intestine, it is folded up into fingerlike projections called villi (Figure 26–2). Between the villi are infoldings known as crypts. Stem cells that give rise to both crypt and villus epithelial cells reside toward the base of the crypts and are responsible for completely renewing the epithelium every few days or so. Indeed, the gastrointestinal epithelium is one of the most rapidly dividing tissues in the body. Daughter cells undergo several rounds of cell division in the crypts then migrate out onto the villi, where they are eventually shed and lost in the stool. The villus epithelial cells are also notable for the extensive microvilli that characterize their apical membranes. These microvilli are endowed with a dense glycocalyx (the brush border) that probably
The parts of the gastrointestinal tract that are encountered by the meal or its residues include, in order, the mouth, esophagus, stomach, duodenum, jejunum, ileum, cecum, colon, rectum, and anus. Throughout the length of the intestine, glandular structures deliver secretions into the lumen, particularly in the stomach and mouth. Also important in the process of digestion are secretions from the pancreas and the biliary system of the liver. The intestinal tract is also functionally divided into segments that restrict the flow of intestinal contents to optimize digestion and absorption. These sphincters include the upper and lower esophageal sphincters, the pylorus that retards emptying of the stomach, the ileocecal valve that retains colonic contents (including large numbers of bacteria) in the large intestine, and the inner and outer anal sphincters. After toilet training, the latter permit delaying the elimination of wastes until a time when it is socially convenient. The intestine is composed of functional layers (Figure 26–1). Immediately adjacent to nutrients in the lumen is a single layer of columnar epithelial cells. This represents the barrier that nutrients must traverse to enter the body. Below the epithelium
Lumen Epithelium Basement memdrane
Mucosa
Lamina propria Muscularis mucosa
Submucosa
Circular muscle Myenteric plexus
Muscularis propria
Longitudinal muscle
Mesothelium (Serosa)
FIGURE 26–1
Organization of the wall of the intestine into functional layers. (Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
Simple columnar epithelium
Lacteal Villus
Capillary network
Goblet cells
Intestinal crypt
Lymph vessel Arteriole Venule
FIGURE 26–2
The structure of intestinal villi and crypts.
(Reproduced with permission, from Fox SI: Human Physiology, 10th ed. McGraw-Hill, 2008.)
protects the cells to some extent from the effects of digestive enzymes. Some digestive enzymes are also actually part of the brush border, being membrane-bound proteins. These socalled “brush border hydrolases” perform the final steps of digestion for specific nutrients.
GASTROINTESTINAL SECRETIONS SALIVARY SECRETION The first secretion encountered when food is ingested is saliva. Saliva is produced by three pairs of salivary glands that drain into the oral cavity. It has a number of organic constituents that serve to initiate digestion (particularly of starch, mediated by amylase) and which also protect the oral cavity from bacteria (such as immunoglobulin A and lysozyme). Saliva also serves to lubricate the food bolus (aided by mucins). Saliva is also hypotonic compared with plasma and alkaline; the latter feature is important to neutralize any gastric secretions that reflux into the esophagus. The salivary glands consist of blind end pieces (acini) that produce the primary secretion containing the organic constituents dissolved in a fluid that is essentially identical in its
431
composition to plasma. The salivary glands are actually extremely active when maximally stimulated, secreting their own weight in saliva every minute. To accomplish this, they are richly endowed with surrounding blood vessels that dilate when salivary secretion is initiated. The composition of the saliva is then modified as it flows from the acini out into ducts that eventually coalesce and deliver the saliva into the mouth. Na+ and Cl– are extracted and K+ and bicarbonate are added. Because the ducts are relatively impermeable to water, the loss of NaCl renders the saliva hypotonic, particularly at low secretion rates. As the rate of secretion increases, there is less time for NaCl to be extracted and the tonicity of the saliva rises, but it always stays somewhat hypotonic with respect to plasma. Overall, the three pairs of salivary glands that drain into the mouth supply 1000 to 1500 mL of saliva per day. Salivary secretion is almost entirely controlled by neural influences, with the parasympathetic branch of the autonomic nervous system playing the most prominent role (Figure 26–3). Sympathetic input slightly modifies the composition of saliva (particularly by increasing proteinaceous content), but has little influence on volume. Secretion is triggered by reflexes that are stimulated by the physical act of chewing, but is actually initiated even before the meal is taken into the mouth as a result of central triggers that are prompted by thinking about, seeing, or smelling food. Indeed, salivary secretion can readily be conditioned, as in the classical experiments of Pavlov where dogs were conditioned to salivate in response to a ringing bell by associating this stimulus with a meal. Salivary secretion is also prompted by nausea, but inhibited by fear or during sleep. Saliva performs a number of important functions: it facilitates swallowing, keeps the mouth moist, serves as a solvent for the molecules that stimulate the taste buds, aids speech by facilitating movements of the lips and tongue, and keeps the mouth and teeth clean. The saliva also has some antibacterial action, and patients with deficient salivation (xerostomia) have a higher than normal incidence of dental caries. The buffers in saliva help maintain the oral pH at about 7.0. They also help neutralize gastric acid and relieve heartburn when gastric juice is regurgitated into the esophagus.
GASTRIC SECRETION Food is stored in the stomach; mixed with acid, mucus, and pepsin; and released at a controlled, steady rate into the duodenum (see Clinical Box 26–1).
ANATOMIC CONSIDERATIONS The gross anatomy of the stomach is shown in Figure 26–4. The gastric mucosa contains many deep glands. In the cardia and the pyloric region, the glands secrete mucus. In the body of the stomach, including the fundus, the glands also contain parietal (oxyntic) cells, which secrete hydrochloric acid and intrinsic factor, and chief (zymogen, peptic) cells, which secrete pepsinogens (Figure 26–5) These secretions mix with
432
SECTION V Gastrointestinal Physiology
Smell Taste Sound Sight
Higher centers Parotid gland
ACh
Otic ganglion
Pressure in mouth Parasympathetics
Submandibular gland
ACh Submandibular ganglion
Increased salivary secretion via effects on • Acinar secretion • Vasodilatation
FIGURE 26–3
Salivatory nucleus of medulla − Sleep Fatigue Fear
Regulation of salivary secretion by the parasympathetic nervous system. ACh, acetylcholine. (Adapted from Barrett KE: Gas-
trointestinal Physiology. McGraw-Hill, 2006.)
mucus secreted by the cells in the necks of the glands. Several of the glands open on a common chamber (gastric pit) that opens in turn on the surface of the mucosa. Mucus is also secreted along with HCO3– by mucus cells on the surface of the epithelium between glands. The stomach has a very rich blood and lymphatic supply. Its parasympathetic nerve supply comes from the vagi and its sympathetic supply from the celiac plexus.
ORIGIN & REGULATION OF GASTRIC SECRETION The stomach also adds a significant volume of digestive juices to the meal. Like salivary secretion, the stomach actually readies itself to receive the meal before it is actually taken in, during the so-called cephalic phase that can be influenced by food preferences. Subsequently, there is a gastric phase of secretion that is quantitatively the most significant, and finally an intestinal phase once the meal has left the stomach. Each phase is closely regulated by both local and distant triggers. The gastric secretions (Table 26–1) arise from glands in the wall of the stomach that drain into its lumen, and also from the surface cells that secrete primarily mucus and bicarbonate to protect the stomach from digesting itself, as well as substances known as trefoil peptides that stabilize the mucusbicarbonate layer. The glandular secretions of the stomach differ in different regions of the organ. The most characteristic secretions derive from the glands in the fundus or body of the stomach. These contain two distinctive cell types from
which the gastric secretions arise: the parietal cells, which secrete hydrochloric acid and intrinsic factor; and the chief cells, which produce pepsinogens and gastric lipase (Figure 26–5). The acid secreted by parietal cells serves to sterilize the meal and also to begin the hydrolysis of dietary macromolecules. Intrinsic factor is important for the later absorption of vitamin B12, or cobalamin (Figure 26–6). Pepsinogen is the precursor of pepsin, which initiates protein digestion. Lipase similarly begins the digestion of dietary fats. There are three primary stimuli of gastric secretion, each with a specific role to play in matching the rate of secretion to functional requirements (Figure 26–7). Gastrin is a hormone that is released by G cells in the antrum of the stomach both in response to a specific neurotransmitter released from enteric nerve endings, known as gastrin releasing peptide (GRP, or bombesin), and also in response to the presence of oligopeptides in the gastric lumen. Gastrin is then carried through the bloodstream to the fundic glands, where it binds to receptors not only on parietal (and likely, chief cells) to activate secretion, but also on so-called enterochromaffin-like cells (ECL cells) that are located in the gland, and release histamine. Histamine is also a trigger of parietal cell secretion, via binding to H2 histamine receptors. Finally, parietal and chief cells can also be stimulated by acetylcholine, released from enteric nerve endings in the fundus. During the cephalic phase of gastric secretion, secretion is predominantly activated by vagal input that originates from the brain region known as the dorsal vagal complex, which coordinates input from higher centers. Vagal outflow to the stomach then releases GRP and acetylcholine, thereby initiating secretory
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
433
Acid, intrinsic factor, pepsinogen
CLINICAL BOX 26–1 Peptic Ulcer Disease
Mucus layer
Gastric and duodenal ulceration in humans is related primarily to a breakdown of the barrier that normally prevents irritation and autodigestion of the mucosa by the gastric secretions. Infection with the bacterium Helicobacter pylori disrupts this barrier, as do aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit the production of prostaglandins and consequently decrease mucus and HCO3– secretion. The NSAIDs are widely used to combat pain and treat arthritis. An additional cause of ulceration is prolonged excess secretion of acid. An example of this is the ulcers that occur in the Zollinger–Ellison syndrome. This syndrome is seen in patients with gastrinomas. These tumors can occur in the stomach and duodenum, but most of them are found in the pancreas. The gastrin causes prolonged hypersecretion of acid, and severe ulcers are produced. Gastric and duodenal ulcers can be given a chance to heal by inhibition of acid secretion with drugs such as cimetidine that block the H2 histamine receptors on parietal cells or omeprazole and related drugs that inhibit H+–K+ ATPase. H. pylori can be eradicated with antibiotics, and NSAID-induced ulcers can be treated by stopping the NSAID or, when this is not advisable, by treatment with the prostaglandin agonist misoprostol. Gastrinomas can sometimes be removed surgically.
function. However, before the meal enters the stomach, there are few additional triggers and thus the amount of secretion is limited. Once the meal is swallowed, on the other hand, meal constituents trigger substantial release of gastrin and the physical
Surface mucous cells (mucus, trefoil peptide, bicarbonate secretion) Cell migration
Mucous neck cells (stem cell compartment)
Parietal cells (acid, intrinsic factor secretion)
ECL cell (histamine secretion)
Chief cells (pepsinogen secretion)
FIGURE 26–5
Structure of a gastric gland from the fundus and body of the stomach. These acid- and pepsinogen-producing glands are referred to as “oxyntic” glands in some sources. (Adapted
from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
Fundus Esophagus Lower esophageal sphincter
Body (secretes mucus, pepsinogen, and HCI) Duodenum
presence of the meal also distends the stomach and activates stretch receptors, which provoke a “vago-vagal” as well as local reflexes that further amplify secretion. The presence of the meal also buffers gastric acidity that would otherwise serve as a feedback inhibitory signal to shut off secretion secondary to the release of somatostatin, which inhibits both G and ECL cells as well as secretion by parietal cells themselves (Figure 26–7). This probably represents a key mechanism whereby gastric secretion
TABLE 26–1 Contents of normal gastric juice (fasting state). Cations: Na+, K+, Mg2+, H+ (pH approximately 1.0)
Pyloric sphincter
Antrum (secretes mucus, pepsinogen, and gastrin)
FIGURE 26–4
Anions: Cl–, HPO42–, SO42– Pepsins Lipase
Anatomy of the stomach. The principal secretions of the body and antrum are listed in parentheses. (Reproduced
Mucus
with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The
Intrinsic factor
Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
434
SECTION V Gastrointestinal Physiology
CH3
CH2CH2CONH2 C NH2COCH2
CH3 CH2CONH2
A N
H3C H 3C
CH2CH2CONH2
B N CN Co +
NH2COCH2
CH
N
C
CH3 CH2
C
CH3 CH2CH2CONH2
C CH3
CH2 CH3
CH3
N
D
O
CHCH2 NH O−
O
N
P O
O HO H
H
N
CH3 CH3
H O
HO CH2
FIGURE 26–6
H
Cyanocobalamin (vitamin B12).
is terminated after the meal moves from the stomach into the small intestine. Gastric parietal cells are highly specialized for their unusual task of secreting concentrated acid (Figure 26–8). The cells are packed with mitochondria that supply energy to drive the apical H,K-ATPase, or proton pump, that moves H+ ions out of the parietal cell against a concentration gradient of more than a
million-fold. At rest, the proton pumps are sequestered within the parietal cell in a series of membrane compartments known as tubulovesicles. When the parietal cell begins to secrete, on the other hand, these vesicles fuse with invaginations of the apical membrane known as canaliculi, thereby substantially amplifying the apical membrane area and positioning the proton pumps to begin acid secretion (Figure 26–9). The apical membrane also contains potassium channels, which supply the K+ ions to be exchanged for H+, and Cl– channels that supply the counterion for HCl secretion (Figure 26–10). The secretion of protons is also accompanied by the release of equivalent numbers of bicarbonate ions into the bloodstream, which as we will see, are later used to neutralize gastric acidity once its function is complete (Figure 26–10). The three agonists of the parietal cell—gastrin, histamine, and acetylcholine—each bind to distinct receptors on the basolateral membrane (Figure 26–9). Gastrin and acetylcholine promote secretion by elevating cytosolic free calcium concentrations, whereas histamine increases intracellular cyclic adenosine 3',5'-monophosphate (cAMP). The net effect of these second messengers are the transport and morphological changes described above. However, it is important to be aware that the two distinct pathways for activation are synergistic, with a greater than additive effect on secretion rates when histamine plus gastrin or acetylcholine, or all three, are present simultaneously. The physiologic significance of this synergism is that high rates of secretion can be stimulated with relatively small changes in availability of each of the stimuli. Synergism is also therapeutically significant because secretion can be markedly inhibited by blocking the action of only one of the triggers (most commonly that of histamine, via H2 histamine antagonists that are widely used therapies for adverse effects of excessive gastric secretion, such as reflux).
FUNDUS
ANTRUM Peptides/amino acids
GRP
H+
G cell
ACh
H+ −
Parietal cell
D cell
P SST
Gastrin Chief cell ACh
?
?
Histamine ACh Circulation
ECL cell
Nerve ending
FIGURE 26–7 Regulation of gastric acid and pepsin secretion by soluble mediators and neural input. Gastrin is released from G cells in the antrum and travels through the circulation to influence the activity of ECL cells and parietal cells. The specific agonists of the chief cell are not well understood. Gastrin release is negatively regulated by luminal acidity via the release of somatostatin from antral D cells. (Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
435
PANCREATIC SECRETION The pancreatic juice contains enzymes that are of major importance in digestion (see Table 26–2). Its secretion is controlled in part by a reflex mechanism and in part by the gastrointestinal hormones secretin and cholecystokinin (CCK).
IC MV
M
IC M TV
ANATOMIC CONSIDERATIONS G
M
IC IC
FIGURE 26–8
Composite diagram of a parietal cell, showing the resting state (lower left) and the active state (upper right). The resting cell has intracellular canaliculi (IC), which open on the apical membrane of the cell, and many tubulovesicular structures (TV) in the cytoplasm. When the cell is activated, the TVs fuse with the cell membrane and microvilli (MV) project into the canaliculi, so the area of cell membrane in contact with gastric lumen is greatly increased. M, mitochondrion; G, Golgi apparatus. (Adapted from Junqueira LC, Carneiro J:
The portion of the pancreas that secretes pancreatic juice is a compound alveolar gland resembling the salivary glands. Granules containing the digestive enzymes (zymogen granules) are formed in the cell and discharged by exocytosis (see Chapter 2) from the apexes of the cells into the lumens of the pancreatic ducts (Figure 26–11). The small duct radicles coalesce into a single duct (pancreatic duct of Wirsung), which usually joins the common bile duct to form the ampulla of Vater (Figure 26–12). The ampulla opens through the duodenal papilla, and its orifice is encircled by the sphincter of Oddi. Some individuals have an accessory pancreatic duct (duct of Santorini) that enters the duodenum more proximally.
Basic Histology: Text & Atlas, 10th ed. McGraw-Hill, 2003.)
COMPOSITION OF PANCREATIC JUICE Gastric secretion adds about 2.5 L per day to the intestinal contents. However, despite their substantial volume and fine control, gastric secretions are dispensable for the full digestion and absorption of a meal, with the exception of cobalamin absorption. This illustrates an important facet of gastrointestinal physiology, that digestive and absorptive capacity are markedly in excess of normal requirements. On the other hand, if gastric secretion is chronically reduced, individuals may display increased susceptibility to infections acquired via the oral route.
The pancreatic juice is alkaline (Table 26–3) and has a high HCO3– content (approximately 113 mEq/L vs. 24 mEq/L in plasma). About 1500 mL of pancreatic juice is secreted per day. Bile and intestinal juices are also neutral or alkaline, and these three secretions neutralize the gastric acid, raising the pH of the duodenal contents to 6.0 to 7.0. By the time the chyme reaches the jejunum, its pH is nearly neutral, but the intestinal contents are rarely alkaline.
Secreting
Resting Canaliculus
H+, K+ ATPase
Tubulovesicle
Ca++ M3 CCK−B
M3 H2
Ca++ cAMP
ACh
CCK−B Gastrin
H2 Histamine
FIGURE 26–9
Parietal cell receptors and schematic representation of the morphological changes depicted in Figure 26–7. Amplification of the apical surface area is accompanied by an increased density of H +, K+–ATPase molecules at this site. Note that acetylcholine (ACh) and gastrin signal via calcium, whereas histamine signals via cAMP. (Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
436
SECTION V Gastrointestinal Physiology
Lumen
Blood Stream
Na+, K+ ATPase +
2K
Potassium channel
3Na+ Na+ NHE-1
H2O + CO2 C.A.II
H+
HCO3−
K+ +
+
H , K ATPase
H+ + HCO3−
H+ Cl−
HCO3− Cl−
ClC Chloride channel
Apical
Cl−/HCO3− exchanger
Basolateral
FIGURE 26–10 Ion transport proteins of parietal cells. Protons are generated in the cytoplasm via the action of carbonic anhydrase II (C.A. II). Bicarbonate ions are exported from the basolateral pole of the cell either by vesicular fusion or via a chloride/bicarbonate exchanger. (Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
The potential danger of the release into the pancreas of a small amount of trypsin is apparent; the resulting chain reaction would produce active enzymes that could digest the pancreas. It is therefore not surprising that the pancreas normally contains a trypsin inhibitor. Another enzyme activated by trypsin is phospholipase A2. This enzyme splits a fatty acid off phosphatidylcholine (PC), forming lyso-PC. Lyso-PC damages cell membranes. It has been hypothesized that in acute pancreatitis, a severe and sometimes fatal disease, phospholipase A2 is activated in the pancreatic ducts, with the formation of lyso-PC from the PC that is a normal constituent of bile. This causes disruption of pancreatic tissue and necrosis of surrounding fat. Small amounts of pancreatic digestive enzymes normally leak into the circulation, but in acute pancreatitis, the circulating levels of the digestive enzymes rise markedly. Measurement of the plasma amylase or lipase concentration is therefore of value in diagnosing the disease.
REGULATION OF THE SECRETION OF PANCREATIC JUICE Secretion of pancreatic juice is primarily under hormonal control. Secretin acts on the pancreatic ducts to cause copious secretion of a very alkaline pancreatic juice that is rich in HCO3– and poor in enzymes. The effect on duct cells is due to an increase in intracellular cAMP. Secretin also stimulates bile secretion. CCK acts on the acinar cells to cause the release of zymogen granules and production of pancreatic juice rich in enzymes but low in volume. Its effect is mediated by phospholipase C (see Chapter 2).
The response to intravenous secretin is shown in Figure 26–13. Note that as the volume of pancreatic secretion increases, its Cl– concentration falls and its HCO3– concentration increases. Although HCO3– is secreted in the small ducts, it is reabsorbed in the large ducts in exchange for Cl– (Figure 26–14). The magnitude of the exchange is inversely proportionate to the rate of flow. Like CCK, acetylcholine acts on acinar cells via phospholipase C to cause discharge of zymogen granules, and stimulation of the vagi causes secretion of a small amount of pancreatic juice rich in enzymes. There is evidence for vagally mediated conditioned reflex secretion of pancreatic juice in response to the sight or smell of food.
BILIARY SECRETION An additional secretion important for gastrointestinal function, bile, arises from the liver. The bile acids contained therein are important in the digestion and absorption of fats. In addition, bile serves as a critical excretory fluid by which the body disposes of lipid soluble end products of metabolism as well as lipid soluble xenobiotics. Bile is also the only route by which the body can dispose of cholesterol—either in its native form, or following conversion to bile acids. In this chapter and the next, we will be concerned with the role of bile as a digestive fluid. In Chapter 29, a more general consideration of the transport and metabolic functions of the liver will be presented.
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
437
TABLE 26–2 Principal digestive enzymes.* Source
Enzyme
Activator
Substrate
Catalytic Function or Products
Salivary glands
Salivary α-amylase
Cl–
Starch
Hydrolyzes 1:4α linkages, producing α-limit dextrins, maltotriose, and maltose
Lingual glands
Lingual lipase
Triglycerides
Fatty acids plus 1,2-diacylglycerols
Stomach
Pepsins (pepsinogens)
Proteins and polypeptides
Cleave peptide bonds adjacent to aromatic amino acids
Triglycerides
Fatty acids and glycerol
HCl
Gastric lipase Exocrine pancreas
Trypsin (trypsinogen)
Enteropeptidase
Proteins and polypeptides
Cleave peptide bonds on carboxyl side of basic amino acids (arginine or lysine)
Chymotrypsins (chymotrypsinogens)
Trypsin
Proteins and polypeptides
Cleave peptide bonds on carboxyl side of aromatic amino acids
Elastase (proelastase)
Trypsin
Elastin, some other proteins
Cleaves bonds on carboxyl side of aliphatic amino acids
Carboxypeptidase A (procarboxypeptidase A)
Trypsin
Proteins and polypeptides
Cleave carboxyl terminal amino acids that have aromatic or branched aliphatic side chains
Carboxypeptidase B (procarboxypeptidase B)
Trypsin
Proteins and polypeptides
Cleave carboxyl terminal amino acids that have basic side chains
Colipase (procolipase)
Trypsin
Fat droplets
Facilitates exposure of active site of pancreaticlipase
Pancreatic lipase
...
Triglycerides
Monoglycerides and fatty acids
Cholesteryl esters
Cholesterol
Bile salt-acid lipase
Intestinal mucosa
Cytoplasm of mucosal cells
Cholesteryl ester hydrolase
...
Cholesteryl esters
Cholesterol
Pancreatic α-amylase
Cl–
Starch
Same as salivary α-amylase
Ribonuclease
...
RNA
Nucleotides
Deoxyribonuclease
...
DNA
Nucleotides
Phospholipase A2 (pro-phospholipase A2)
Trypsin
Phospholipids
Fatty acids, lysophospholipids
Enteropeptidase
...
Trypsinogen
Trypsin
Aminopeptidases
...
Polypeptides
Cleave amino terminal amino acid from peptide
Carboxypeptidases
...
Polypeptides
Cleave carboxyl terminal amino acid from peptide
Endopeptidases
...
Polypeptides
Cleave between residues in midportion of peptide
Dipeptidases
...
Dipeptides
Two amino acids
Maltase
...
Maltose, maltotriose, α-dextrins
Glucose
Lactase
...
Lactose
Galactose and glucose
Sucrasea
...
Sucrose; also maltotriose and maltose
Fructose and glucose
α-Dextrinasea
...
α-Dextrins, maltose maltotriose
Glucose
Trehalase
...
Trehalose
Glucose
Nuclease and related enzymes
...
Nucleic acids
Pentoses and purine and pyrimidine bases
Various peptidases
...
Di-, tri-, and tetrapeptides
Amino acids
*Corresponding proenzymes, where relevant, are shown in parentheses a
Sucrase and a-dextrinase are separate subunits of a single protein.
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SECTION V Gastrointestinal Physiology
TABLE 26–3 Composition of normal human Endocrine cells of pancreas
pancreatic juice. Cations: Na+, K+, Ca2+, Mg2+ (pH approximately 8.0)
Exocrine cells (secrete enzymes)
Anions: HCO3–, Cl–, SO42–, HPO42– Digestive enzymes (see Table 26–1; 95% of protein in juice) Other proteins Duct cells (secrete bicarbonate)
Gallbladder
Pancreas
Pancreatic duct
Common bile duct from gallbladder
Duodenum
FIGURE 26–11
Structure of the pancreas. (Reproduced with per-
mission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
BILE
sorbed in the intestine and then excreted again by the liver (enterohepatic circulation). The glucuronides of the bile pigments, bilirubin and biliverdin, are responsible for the golden yellow color of bile. The formation of these breakdown products of hemoglobin is discussed in detail in Chapter 29, and their excretion is discussed below. The bile acids secreted into the bile are conjugated to glycine or taurine, a derivative of cysteine. The bile acids are synthesized from cholesterol. The four major bile acids found in humans are listed in Figure 26–15. In common with vitamin D, cholesterol, a variety of steroid hormones, and the digitalis glycosides, the bile acids contain the steroid nucleus (see Chapter 22). The two principal (primary) bile acids formed in the liver are cholic acid and chenodeoxycholic acid. In the colon, bacteria convert cholic acid to deoxycholic acid and chenodeoxycholic acid to lithocholic acid. In addition, small quantities of ursodeoxycholic acid are formed from chenodeoxycholic acid. Ursodeoxycholic acid is a tautomer of chenodeoxycholic acid at the 7-position. Because they are formed
Bile is made up of the bile acids, bile pigments, and other substances dissolved in an alkaline electrolyte solution that resembles pancreatic juice (Table 26–4). About 500 mL is secreted per day. Some of the components of the bile are reab-
Cystic duct Gallbladder
150
Concentration of electrolytes (meq/L) and amylase (U/mL)
Right hepatic duct
Secretin 12.5 units/kg IV
Left hepatic duct
Common hepatic duct Bile duct
120
90
60
− (CI )
30
(Amylase) (K+)
Pancreas 0 Accessory pancreatic duct Ampulla of bile duct
Duodenum
Pancreatic duct
FIGURE 26–12
Connections of the ducts of the gallbladder, liver, and pancreas. (Adapted from Bell GH, Emslie-Smith D, Paterson CR: Textbook
of Physiology and Biochemistry, 9th ed. Churchill Livingstone, 1976.)
(HCO3−)
−20 −10
0 +10 +20 +30 +40 Time (min)
Volume of secretion (mL) 0.3 0.2 17.7 15.2 5.1 0.6
FIGURE 26–13 Effect of a single dose of secretin on the composition and volume of the pancreatic juice in humans.
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
439
Basolateral
Duct lumen CO2 + H2O
−
H+
C.A
−
NHE-1
−
HCO3 + H+
HCO3 −
Cl /HCO3 Exchanger
Na+ − 2HCO3
NBC
Na+ 3Na+ 2K+
Cl− +
Na+, K+ ATPase
K+ channel cAMP
CFTR
FIGURE 26–14 Ion transport pathways present in pancreatic duct cells. CA, carbonic anhydrase; NHE-1, sodium/hydrogen exchanger1; NBC, sodium-bicarbonate cotransporter. (Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
TABLE 26–4 Composition of human hepatic duct bile. 97.0%
Water Bile salts
0.7%
Bile pigments
0.2%
Cholesterol
0.06%
Inorganic salts
0.7%
Fatty acids
0.15%
Phosphatidylcholine
0.2%
Fat
0.1%
Alkaline phosphatase
by bacterial action, deoxycholic, lithocholic, and ursodeoxycholic acids are called secondary bile acids. The bile salts have a number of important actions: they reduce surface tension and, in conjunction with phospholipids and monoglycerides, are responsible for the emulsification of fat preparatory to its digestion and absorption in the small intestine (see Chapter 27). They are amphipathic, that is, they have both hydrophilic and hydrophobic domains; one surface of the molecule is hydrophilic because the polar peptide bond and the carboxyl and hydroxyl groups are on that surface, whereas the other surface is hydrophobic. Therefore, the bile salts tend to form cylindrical disks called micelles. A top view of micelles is shown in Figure 26–16 and a side view of one in
… BULK SOLUTION OF INTESTINAL CONTENTS
OH
CH3 COOH
12 CH3
Dietary triglyceride
3
7
HO
OH Cholic acid
Pancreatic lipase
Group at position
Cholic acid Chenodeoxycholic acid Deoxycholic acid Lithocholic acid
FIGURE 26–15
ion S rpt B so ce of b a n FA rese UNSTIRRED p in LAYER F in A ab ab so se rp nc tio eo n fB S
3
7
12
Percent in human bile
OH OH OH OH
OH OH H H
OH H OH H
50 30 15 5
Human bile acids. The numbers in the formula for cholic acid refer to the positions in the steroid ring.
Mucosa
FIGURE 26–16
Lipid digestion and passage to intestinal mucosa. Fatty acids (FA) are liberated by the action of pancreatic lipase on dietary triglycerides and, in the presence of bile salts (BS), form micelles (the circular structures), which diffuse through the unstirred layer to the mucosal surface. (Adapted from Thomson ABR: Intestinal absorption of lipids: Influence of the unstirred water layer and bile acid micelle. In: Disturbances in Lipid and Lipoprotein Metabolism. Dietschy JM, Gotto AM Jr, Ontko JA [editors]: American Physiological Society, 1978.)
440
SECTION V Gastrointestinal Physiology
Charged side chain
Hepatic synthesis Sphincter of Oddi
OH group
Simple micelle
Spillover from liver into systemic circulation
Gallbladder Active ileal uptake Return to liver
Bile acid monomers
Small intestine
Large intestine Spillover into colon
Passive uptake of deconjugated bile acids from colon Fecal loss ( = hepatic synthesis)
FIGURE 26–18 Mixed micelle
Quantitative aspects of the circulation of bile acids. The majority of the bile acid pool circulates between the small intestine and liver. A minority of the bile acid pool is in the systemic circulation (due to incomplete hepatocyte uptake from the portal blood) or spills over into the colon and is lost to the stool. Fecal loss must be equivalent to hepatic synthesis of bile acids at steady state.
(Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
Phosphatidylcholine Cholesterol
FIGURE 26–17
Physical forms adopted by bile acids in solution. Micelles are shown in cross-section, and are actually thought to be cylindrical in shape. Mixed micelles of bile acids present in hepatic bile also incorporate cholesterol and phosphatidylcholine. (Adapted from Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006.)
Figure 26–17. Their hydrophilic portions face out and their hydrophobic portions face in. Above a certain concentration, called the critical micelle concentration, all bile salts added to a solution form micelles. Lipids collect in the micelles, with cholesterol in the hydrophobic center and amphipathic phospholipids and monoglycerides lined up with their hydrophilic heads on the outside and their hydrophobic tails in the center. The micelles play an important role in keeping lipids in solution and transporting them to the brush border of the intestinal epithelial cells, where they are absorbed (see Chapter 27). Ninety to 95% of the bile salts are absorbed from the small intestine. Once they are deconjugated, they can be absorbed by nonionic diffusion, but most are absorbed in their conjugated forms from the terminal ileum (Figure 26–18) by an extremely efficient Na+–bile salt cotransport system powered by basolateral Na+–K+ ATPase. The remaining 5–10% of the bile salts enter the colon and are converted to the salts of deoxycholic acid and lithocholic acid. Lithocholate is relatively insoluble and is mostly excreted in the stools; only 1% is absorbed. However, deoxycholate is absorbed. The absorbed bile salts are transported back to the liver in the portal vein and reexcreted in the bile (enterohepatic circu-
lation) (Figure 26–18). Those lost in the stool are replaced by synthesis in the liver; the normal rate of bile salt synthesis is 0.2 to 0.4 g/d. The total bile salt pool of approximately 3.5 g recycles repeatedly via the enterohepatic circulation; it has been calculated that the entire pool recycles twice per meal and six to eight times per day. When bile is excluded from the intestine, up to 50% of ingested fat appears in the feces. A severe malabsorption of fat-soluble vitamins also results. When bile salt reabsorption is prevented by resection of the terminal ileum or by disease in this portion of the small intestine, the amount of fat in the stools is also increased because when the enterohepatic circulation is interrupted, the liver cannot increase the rate of bile salt production to a sufficient degree to compensate for the loss.
INTESTINAL FLUID & ELECTROLYTE TRANSPORT The intestine itself also supplies a fluid environment in which the processes of digestion and absorption can occur. Then, when the meal has been assimilated, fluid used during digestion and absorption is reclaimed by transport back across the epithelium to avoid dehydration. Water moves passively into and out of the gastrointestinal lumen, driven by electrochemical gradients established by the active transport of ions and other solutes. In the period after a meal, much of the fluid reuptake is driven by the coupled transport of nutrients, such as glucose, with sodium ions. In the period between meals, absorptive mechanisms center exclusively around electrolytes. In both cases, secretory fluxes of fluid are largely driven by the active transport of chloride ions into the lumen, although absorption still predominates overall.
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
441
TABLE 26–5 Daily water turnover (mL) in the gastrointestinal tract. 2K+ 2000
Ingested Endogenous secretions
NHE-3? NHE-2?
H+
1500
Stomach
2500 HCO3
K+
KCC1 ?
500
Pancreas
1500
Intestine
+1000
Cl−
FIGURE 26–19
Electroneutral NaCl absorption in the small intestine and colon. NaCl enters across the apical membrane via the coupled activity of a sodium/hydrogen exchanger and a chloride/bicarbonate exchanger.
7000 Total input
9000
Reabsorbed
8800
Jejunum
5500
Ileum
2000
Colon
+1300
EN
aC
Na+
8800 Balance in stool
Cl−
CLD
–
3Na+ Na+,K+ ATPase
7000
Salivary glands
Bile
Na+
K+
2K+
3Na+
200 Na+,K+ ATPase
Data from Moore EW: Physiology of Intestinal Water and Electrolyte Absorption. American Gastroenterological Society, 1976.
Cl−
Overall water balance in the gastrointestinal tract is summarized in Table 26–5. The intestines are presented each day with about 2000 mL of ingested fluid plus 7000 mL of secretions from the mucosa of the gastrointestinal tract and associated glands. Ninety-eight percent of this fluid is reabsorbed, with a daily fluid loss of only 200 mL in the stools. In the small intestine, secondary active transport of Na+ is important in bringing about absorption of glucose, some amino acids, and other substances such as bile acids (see above). Conversely, the presence of glucose in the intestinal lumen facilitates the reabsorption of Na+. In the period between meals, when nutrients are not present, sodium and chloride are absorbed together from the lumen by the coupled activity of a sodium/hydrogen exchanger (NHE) and chloride/bicarbonate exchanger in the apical membrane, in a so-called electroneutral mechanism (Figure 26–19). Water then follows to maintain an osmotic balance. In the colon, moreover, an additional electrogenic mechanism for sodium absorption is expressed, particularly in the distal colon. In this mechanism, sodium enters across the apical membrane via an ENaC (epithelial sodium) channel that is identical to that expressed in the distal tubule of the kidney (Figure 26–20). This underpins the ability of the colon to desiccate the stool and ensure that only a small portion of the fluid load used daily in the digestion and absorption of meals is lost from the body. Following a low-salt diet, increased expression of ENaC in response to aldosterone increases the ability to reclaim sodium from the stool.
FIGURE 26–20
Electrogenic sodium absorption in the colon. Sodium enters the epithelial cell via epithelial sodium channels (ENaC).
Despite the predominance of absorptive mechanisms, secretion also takes place continuously throughout the small intestine and colon to adjust the local fluidity of the intestinal contents as needed for mixing, diffusion, and movement of the meal and its residues along the length of the gastrointestinal tract. Cl– normally enters enterocytes from the interstitial fluid via Na+–K+–2Cl– cotransporters in their basolateral membranes (Figure 26–21), and the Cl– is then secreted into the intestinal lumen via channels that are regulated by various protein kinases. The cystic fibrosis transmembrane conductance regulator (CFTR) channel that is defective in the disease of cystic fibrosis is quantitatively most important, and is activated by protein kinase A and hence by cAMP (see Clinical Box 26–2). Water moves into or out of the intestine until the osmotic pressure of the intestinal contents equals that of the plasma. The osmolality of the duodenal contents may be hypertonic or hypotonic, depending on the meal ingested, but by the time the meal enters the jejunum, its osmolality is close to that of
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SECTION V Gastrointestinal Physiology
CLINICAL BOX 26–2
Na+
_
2CI TR CF
Cl−
K+
Cholera
NKCC1
Na+ 2K+
3Na+ Na+, K+ ATPase
K+
FIGURE 26–21
Chloride secretion in the small intestine and colon. Chloride uptake occurs via the sodium/potassium/2 chloride cotransporter, NKCC1. Chloride exit is via the cystic fibrosis transmembrane conductance regulator (CFTR) as well as perhaps via other chloride channels, not shown.
plasma. This osmolality is maintained throughout the rest of the small intestine; the osmotically active particles produced by digestion are removed by absorption, and water moves passively out of the gut along the osmotic gradient thus generated. In the colon, Na+ is pumped out and water moves passively with it, again along the osmotic gradient. Saline cathartics such as magnesium sulfate are poorly absorbed salts that retain their osmotic equivalent of water in the intestine, thus increasing intestinal volume and consequently exerting a laxative effect. Some K+ is secreted into the intestinal lumen, especially as a component of mucus. K+ channels are present in the luminal as well as the basolateral membrane of the enterocytes of the colon, so K+ is secreted into the colon. In addition, K+ moves passively down its electrochemical gradient. The accumulation of K+ in the colon is partially offset by H+–K+ ATPase in the luminal membrane of cells in the distal colon, with resulting active transport of K+ into the cells. Nevertheless, loss of ileal or colonic fluids in chronic diarrhea can lead to severe hypokalemia. When the dietary intake of K+ is high for a prolonged period, aldosterone secretion is increased and more K+ enters the colon. This is due in part to the appearance of more Na+– K+ ATPase pumps in the basolateral membranes of the cells, with a consequent increase in intracellular K+ and K+ diffusion across the luminal membranes of the cells.
GASTROINTESTINAL REGULATION The various functions of the gastrointestinal tract, including secretion, digestion, and absorption (Chapter 27) and motility (Chapter 28) must be regulated in an integrated way to ensure efficient assimilation of nutrients after a meal. There are three main modalities for gastrointestinal regulation that operate in a complementary fashion to ensure that function is appropriate. First, endocrine regulation is mediated by the release of
Cholera is a severe secretory diarrheal disease that often occurs in epidemics associated with natural disasters where normal sanitary practices break down. Along with other secretory diarrheal illnesses produced by bacteria and viruses, cholera causes a significant amount of morbidity and mortality, particularly among the young and in developing countries. The cAMP concentration in intestinal epithelial cells is increased in cholera. The cholera bacillus stays in the intestinal lumen, but it produces a toxin that binds to GM-1 ganglioside receptors on the apical membrane of intestinal epithelial cells, and this permits part of the A subunit (A1 peptide) of the toxin to enter the cell. The A1 peptide binds adenosine diphosphate ribose to the α subunit of Gs, inhibiting its GTPase activity (see Chapter 2). Therefore, the constitutively activated G protein produces prolonged stimulation of adenylyl cyclase and a marked increase in the intracellular cAMP concentration. In addition to increased Cl– secretion, the function of the mucosal NHE carrier for Na+ is reduced, thus reducing NaCl absorption. The resultant increase in electrolyte and water content of the intestinal contents causes the diarrhea. However, Na+–K+ ATPase and the Na+/ glucose cotransporter are unaffected, so coupled reabsorption of glucose and Na+ bypasses the defect. This is the physiologic basis for the treatment of Na+ and water loss in diarrhea by oral administration of solutions containing NaCl and glucose. Cereals containing carbohydrates are also useful in the treatment of diarrhea.
hormones by triggers associated with the meal. These hormones travel through the bloodstream to change the activity of a distant segment of the gastrointestinal tract, an organ draining into it (eg, the pancreas), or both. Second, some similar mediators are not sufficiently stable to persist in the bloodstream, but instead alter the function of cells in the local area where they are released, in a paracrine fashion. Finally, the intestinal system is endowed with extensive neural connections. These include connections to the central nervous system (extrinsic innervation), but also the activity of a largely autonomous enteric nervous system that comprises both sensory and secreto-motor neurons. The enteric nervous system integrates central input to the gut, but can also regulate gut function independently in response to changes in the luminal environment. In some cases, the same substance can mediate regulation by endocrine, paracrine, and neurocrine pathways (eg, cholecystokinin, see below).
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
HORMONES/PARACRINES Biologically active polypeptides that are secreted by nerve cells and gland cells in the mucosa act in a paracrine fashion, but they also enter the circulation. Measurement of their concentrations in blood after a meal has shed light on the roles these gastrointestinal hormones play in the regulation of gastrointestinal secretion and motility. When large doses of the hormones are given, their actions overlap. However, their physiologic effects appear to be relatively discrete. On the basis of structural similarity (Table 26–6) and, to a degree, similarity of function, the key hormones fall into one of two families: the gastrin family, the primary members of which are gastrin and CCK; and the secretin family, the primary members of which are secretin, glucagon, glicentin (GLI), vasoactive intestinal peptide (VIP; actually a neurotransmitter, or neurocrine), and gastric inhibitory polypeptide (also known as glucose-dependent insulinotropic peptide, or GIP). There are also other hormones that do not fall readily into these families.
ENTEROENDOCRINE CELLS More than 15 types of hormone-secreting enteroendocrine cells have been identified in the mucosa of the stomach, small intestine, and colon. Many of these secrete only one hormone and are identified by letters (G cells, S cells, etc). Others manufacture serotonin or histamine and are called enterochromaffin or enterochromaffin-like (ECL) cells, respectively.
GASTRIN Gastrin is produced by cells called G cells in the antral portion of the gastric mucosa (Figure 26–22). G cells are flask-shaped, with a broad base containing many gastrin granules and a narrow apex that reaches the mucosal surface. Microvilli project from the apical end into the lumen. Receptors mediating gastrin responses to changes in gastric contents are present on the microvilli. Other cells in the gastrointestinal tract that secrete hormones have a similar morphology. Gastrin is typical of a number of polypeptide hormones in that it shows both macroheterogeneity and microheterogeneity. Macroheterogeneity refers to the occurrence in tissues and body fluids of peptide chains of various lengths; microheterogeneity refers to differences in molecular structure due to derivatization of single amino acid residues. Preprogastrin is processed into fragments of various sizes. Three main fragments contain 34, 17, and 14 amino acid residues. All have the same carboxyl terminal configuration (Table 26–6). These forms are also known as G 34, G 17, and G 14 gastrins, respectively. Another form is the carboxyl terminal tetrapeptide, and there is also a large form that is extended at the amino terminal and contains more than 45 amino acid residues. One form of derivatization is sulfation of the tyrosine that is the sixth amino acid residue from the carboxyl terminal. Approximately equal amounts of nonsulfated and sulfated forms are present in
443
blood and tissues, and they are equally active. Another derivatization is amidation of the carboxyl terminal phenylalanine. What is the physiologic significance of this marked heterogeneity? Some differences in activity exist between the various components, and the proportions of the components also differ in the various tissues in which gastrin is found. This suggests that different forms are tailored for different actions. However, all that can be concluded at present is that G 17 is the principal form with respect to gastric acid secretion. The carboxyl terminal tetrapeptide has all the activities of gastrin but only 10% of the strength of G 17. G 14 and G 17 have half-lives of 2 to 3 min in the circulation, whereas G 34 has a half-life of 15 min. Gastrins are inactivated primarily in the kidney and small intestine. In large doses, gastrin has a variety of actions, but its principal physiologic actions are stimulation of gastric acid and pepsin secretion and stimulation of the growth of the mucosa of the stomach and small and large intestines (trophic action). Gastrin secretion is affected by the contents of the stomach, the rate of discharge of the vagus nerves, and bloodborne factors (Table 26–7). Atropine does not inhibit the gastrin response to a test meal in humans, because the transmitter secreted by the postganglionic vagal fibers that innervate the G cells is gastrin-releasing polypeptide (GRP; see below) rather than acetylcholine. Gastrin secretion is also increased by the presence of the products of protein digestion in the stomach, particularly amino acids, which act directly on the G cells. Phenylalanine and tryptophan are particularly effective. Acid in the antrum inhibits gastrin secretion, partly by a direct action on G cells and partly by release of somatostatin, a relatively potent inhibitor of gastrin secretion. The effect of acid is the basis of a negative feedback loop regulating gastrin secretion. Increased secretion of the hormone increases acid secretion, but the acid then feeds back to inhibit further gastrin secretion. In conditions such as pernicious anemia in which the acid-secreting cells of the stomach are damaged, gastrin secretion is chronically elevated.
CHOLECYSTOKININ Cholecystokinin (CCK) is secreted by cells in the mucosa of the upper small intestine. It has a plethora of actions in the gastrointestinal system, but the most important appear to be the stimulation of pancreatic enzyme secretion, the contraction of the gallbladder (the action for which it was named), and relaxation of the sphincter of Oddi, which allows both bile and pancreatic juice to flow into the intestinal lumen. Like gastrin, CCK shows both macroheterogeneity and microheterogeneity. Prepro-CCK is processed into many fragments. A large CCK contains 58 amino acid residues (CCK 58). In addition, there are CCK peptides that contain 39 amino acid residues (CCK 39) and 33 amino acid residues (CCK 33), several forms that contain 12 (CCK 12) or slightly more amino acid residues, and a form that contains 8 amino acid residues (CCK 8). All of these forms have the same 5 amino acids at the carboxyl terminal as gastrin (Table 26–6).
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SECTION V Gastrointestinal Physiology
TABLE 26–6. Structures of some of the hormonally active polypeptides secreted by cells in the human gastrointestinal tract.a Gastrin Family CCK 39
Tyr I le Gln Gln Ala Arg Lys → Ala Pro Ser Gly Arg Met Ser Ile Val Lys Asn Leu Gln Asn Leu Asp Pro Ser His Arg → Ile Ser Asp Arg → Asp Tys Met →Gly Trp Met Asp Phe-NH2
Gastrin 34
(pyro)Glu Leu Gly Pro Gln Gly Pro Pro His Leu Val Ala Asp Pro Ser Lys →Lys Gln Gly →Pro Trp Leu Glu Glu Glu Glu Glu Ala Tys →Gly Trp Met Asp Phe-NH2
GIP Secretin Family
Glucagon
Tyr Ala Glu Gly Thr Phe Ile Ser Asp Tyr Ser Ile Ala Met Asp Lys Ile His Gln Gln Asp Phe Val Asn Trp Leu Leu Ala Glu Lys Gly Lys Lys Asn Asp Trp Lys His Asn Ile Thr Gln
His Ser Gln Gly Thr Phe Thr Ser Asp Tyr Ser Lys Tyr Leu Asp Ser Arg Arg Ala Gln Asp Phe Val Gln Trp Leu Met Asn Thr
Secretin
His Ser Asp Gly Thr Phe Thr Ser Glu Leu Ser Arg Leu Arg Glu Gly Ala Arg Leu Gln Arg Leu Leu Gln Gly Leu Val-NH2
Other Polypeptides
VIP
His Ser Asp Ala Val Phe Thr Asp Asn Tyr Thr Arg Leu Arg Lys Gln Met Ala Val Lys Lys Tyr Leu Asn S er Ile Leu Asn-NH2
Motilin
Phe Val Pro Ile Phe Thr Tyr Gly Glu Leu Gln Arg Met Gln Glu Lys Glu Arg Asn Lys G ly Gln
Substance P
Arg Pro Lys Pro Gln Gln Phe Phe Gly Leu Met-NH2
GRP
Val Pro Leu Pro Ala Gly Gly Gly Thr Val Leu Thr Lys Met Tyr Pro Arg Gly Asn His Trp Ala Val Gly His Leu Met-NH2
Guanylin
Pro Asn Thr Cys Glu Ile Cys Ala Tyr Ala Ala Cys Thr Gly Cys
a Homologous amino acid residues are enclosed by the lines that generally cross from one polypeptide to another. Arrows indicate points of cleavage to form smaller variants. Tys, tyrosine sulfate. All gastrins occur in unsulfated (gastrin I) and sulfated (gastrin II) forms. Glicentin, an additional member of the secretin family, is a C-terminally extended relative of glucagon.
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
Gastrin
CCK
Secretin
GIP
445
Motilin
Fundus Antrum
Duodenum
Jejunum
Ileum
Colon
FIGURE 26–22 Sites of production of the five gastrointestinal hormones along the length of the gastrointestinal tract. The width of the bars reflects the relative abundance at each location.
TABLE 26–7 Stimuli that affect gastrin secretion. Stimuli that increase gastrin secretion Luminal Peptides and amino acids Distention Neural Increased vagal discharge via GRP Bloodborne Calcium Epinephrine Stimuli that inhibit gastrin secretion Luminal Acid Somatostatin Bloodborne Secretin, GIP, VIP, glucagon, calcitonin
The carboxyl terminal tetrapeptide (CCK 4) also exists in tissues. The carboxyl terminal is amidated, and the tyrosine that is the seventh amino acid residue from the carboxyl terminal is sulfated. Unlike gastrin, the nonsulfated form of CCK has not been found in tissues. However, derivatization of other amino acid residues in CCK can occur. The half-life of circulating CCK is about 5 minutes, but little is known about its metabolism. In addition to its secretion by I cells in the upper intestine, CCK is found in nerves in the distal ileum and colon. It is also found in neurons in the brain, especially the cerebral cortex, and in nerves in many parts of the body (see Chapter 7). In the brain, it may be involved in the regulation of food intake, and it appears to be related to the production of anxiety and analgesia. The CCK secreted in the duodenum and jejunum is probably mostly CCK 8 and CCK 12, although CCK 58 is also present in the intestine and circulating blood in some species. The enteric and pancreatic nerves contain primarily CCK 4. CCK 58 and CCK 8 are found in the brain. In addition to its primary actions, CCK augments the action of secretin in producing secretion of an alkaline pancreatic juice. It also inhibits gastric emptying, exerts a trophic effect on the pancreas, increases the synthesis of enterokinase, and may enhance the motility of the small intestine and colon. There is some evidence that, along with secretin, it augments the contraction of the pyloric sphincter, thus preventing the reflux of duodenal contents into the stomach. Gastrin and CCK stimulate glucagon secretion, and since the secretion of
446
SECTION V Gastrointestinal Physiology
both gastrointestinal hormones is increased by a protein meal, either or both may be the “gut factor” that stimulates glucagon secretion (see Chapter 21). Two CCK receptors have been identified. CCK-A receptors are primarily located in the periphery, whereas both CCK-A and CCK-B receptors are found in the brain. Both activate PLC, causing increased production of IP3 and DAG (see Chapter 2). The secretion of CCK is increased by contact of the intestinal mucosa with the products of digestion, particularly peptides and amino acids, and also by the presence in the duodenum of fatty acids containing more than 10 carbon atoms. There are also two protein releasing factors that activate CCK secretion, known as CCK-releasing peptide and monitor peptide, which derive from the intestinal mucosa and pancreas, respectively. Because the bile and pancreatic juice that enter the duodenum in response to CCK further the digestion of protein and fat, and the products of this digestion stimulate further CCK secretion, a sort of positive feedback operates in the control of the secretion of this hormone. However, the positive feedback is terminated when the products of digestion move on to the lower portions of the gastrointestinal tract, and also because CCK-releasing peptide and monitor peptide are degraded by proteolytic enzymes once these are no longer occupied in digesting dietary proteins.
SECRETIN Secretin occupies a unique position in the history of physiology. In 1902, Bayliss and Starling first demonstrated that the excitatory effect of duodenal stimulation on pancreatic secretion was due to a bloodborne factor. Their research led to the identification of the first hormone, secretin. They also suggested that many chemical agents might be secreted by cells in the body and pass in the circulation to affect organs some distance away. Starling introduced the term hormone to categorize such “chemical messengers.” Modern endocrinology is the proof of the correctness of this hypothesis. Secretin is secreted by S cells that are located deep in the glands of the mucosa of the upper portion of the small intestine. The structure of secretin (Table 26–6) is different from that of CCK and gastrin, but very similar to that of glucagon, GLI, VIP, and GIP. Only one form of secretin has been isolated, and the fragments of the molecule that have been tested to date are inactive. Its half-life is about 5 minutes, but little is known about its metabolism. Secretin increases the secretion of bicarbonate by the duct cells of the pancreas and biliary tract. It thus causes the secretion of a watery, alkaline pancreatic juice. Its action on pancreatic duct cells is mediated via cAMP. It also augments the action of CCK in producing pancreatic secretion of digestive enzymes. It decreases gastric acid secretion and may cause contraction of the pyloric sphincter. The secretion of secretin is increased by the products of protein digestion and by acid bathing the mucosa of the upper small intestine. The release of secretin by acid is another exam-
ple of feedback control: Secretin causes alkaline pancreatic juice to flood into the duodenum, neutralizing the acid from the stomach and thus inhibiting further secretion of the hormone.
GIP GIP contains 42 amino acid residues (Table 26–6) and is produced by K cells in the mucosa of the duodenum and jejunum. Its secretion is stimulated by glucose and fat in the duodenum, and because in large doses it inhibits gastric secretion and motility, it was named gastric inhibitory peptide. However, it now appears that it does not have significant gastric inhibiting activity when administered in smaller amounts comparable to those seen after a meal. In the meantime, it was found that GIP stimulates insulin secretion. Gastrin, CCK, secretin, and glucagon also have this effect, but GIP is the only one of these that stimulates insulin secretion when administered in doses that produce blood levels comparable to those produced by oral glucose. For this reason, it is often called glucose-dependent insulinotropic polypeptide. The glucagon derivative GLP-1 (7–36) (see Chapter 21) also stimulates insulin secretion and is said to be more potent in this regard than GIP. Therefore, it may also be a physiologic B cell-stimulating hormone of the gastrointestinal tract. The integrated action of gastrin, CCK, secretin, and GIP in facilitating digestion and utilization of absorbed nutrients is summarized in Figure 26–23.
Food in stomach Gastrin secretion
Increased acid secretion
Increased motility
Food and acid into duodenum
CCK and secretin secretion
Peptide YY?
GIP GLP-1 (7–26) secretion
Pancreatic and biliary secretion
Insulin secretion
Intestinal digestion of food
FIGURE 26–23
Integrated action of gastrointestinal hormones in regulating digestion and utilization of absorbed nutrients. The dashed arrows indicate inhibition. The exact identity of the hormonal factor or factors from the intestine that inhibit(s) gastric acid secretion and motility is unsettled, but it may be peptide YY.
CHAPTER 26 Overview of Gastrointestinal Function & Regulation
VIP VIP contains 28 amino acid residues (Table 26–6). It is found in nerves in the gastrointestinal tract and thus is not itself a hormone, despite its similarities to secretin. Prepro-VIP contains both VIP and a closely related polypeptide (PHM-27 in humans, PHI-27 in other species). VIP is also found in blood, in which it has a half-life of about 2 minutes. In the intestine, it markedly stimulates intestinal secretion of electrolytes and hence of water. Its other actions include relaxation of intestinal smooth muscle, including sphincters; dilation of peripheral blood vessels; and inhibition of gastric acid secretion. It is also found in the brain and many autonomic nerves (see Chapter 7), where it often occurs in the same neurons as acetylcholine. It potentiates the action of acetylcholine in salivary glands. However, VIP and acetylcholine do not coexist in neurons that innervate other parts of the gastrointestinal tract. VIP-secreting tumors (VIPomas) have been described in patients with severe diarrhea.
MOTILIN Motilin is a polypeptide containing 22 amino acid residues that is secreted by enterochromaffin cells and Mo cells in the stom-
Phase I -
447
ach, small intestine, and colon. It acts on G protein-coupled receptors on enteric neurons in the duodenum and colon and on injection produces contraction of smooth muscle in the stomach and intestines. Its circulating level increases at intervals of approximately 100 min in the interdigestive state, and it is a major regulator of the migrating motor complexes (MMCs) (Figure 26–24) that control gastrointestinal motility between meals. Conversely, when a meal is ingested, secretion of motilin is suppressed until digestion and absorption are complete. The antibiotic erythromycin binds to motilin receptors, and derivatives of this compound may be of value in treating patients in whom gastrointestinal motility is decreased.
SOMATOSTATIN Somatostatin, the growth-hormone-inhibiting hormone originally isolated from the hypothalamus, is secreted as a paracrine by D cells in the pancreatic islets (see Chapter 21) and by similar D cells in the gastrointestinal mucosa. It exists in tissues in two forms, somatostatin 14 and somatostatin 28, and both are secreted. Somatostatin inhibits the secretion of gastrin, VIP, GIP, secretin, and motilin. Its secretion is stimulated by acid in the lumen, and it probably acts in a paracrine fashion to mediate the inhibition of gastrin secretion produced by acid. It also inhibits
Phases of III MMC
No spike potentials, no contractions
Phase II - Irregular spike potentials and contractions II
Phase III - Regular spike potentials and contractions
I MEAL
Stomach Propagation rate (5 cm/m)
Distal ileum ~90 min
FIGURE 26–24
Resumption of MMCs
Migrating motor complexes (MMCs). Note that the complexes move down the gastrointestinal tract at a regular rate during fasting, that they are completely inhibited by a meal, and that they resume 90–120 minutes after the meal. (Reproduced with permission from
Chang EB, Sitrin MD, Black DD: Gastrointestinal, Hepatobiliary, and Nutritional Physiology. Lippincott-Raven, 1996.)
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SECTION V Gastrointestinal Physiology
pancreatic exocrine secretion; gastric acid secretion and motility; gallbladder contraction; and the absorption of glucose, amino acids, and triglycerides.
OTHER GASTROINTESTINAL PEPTIDES PEPTIDE YY The structure of peptide YY is discussed in Chapter 21. It also inhibits gastric acid secretion and motility and is a good candidate to be the gastric inhibitory peptide (Figure 26–23). Its release from the jejunum is stimulated by fat.
OTHERS Ghrelin is secreted primarily by the stomach and appears to play an important role in the central control of food intake. It also stimulates growth hormone secretion by acting directly on receptors in the pituitary (see Chapter 24). Substance P (Table 26–6) is found in endocrine and nerve cells in the gastrointestinal tract and may enter the circulation. It increases the motility of the small intestine. The neurotransmitter GRP contains 27 amino acid residues, and the 10 amino acid residues at its carboxyl terminal are almost identical to those of amphibian bombesin. It is present in the vagal nerve endings that terminate on G cells and is the neurotransmitter producing vagally mediated increases in gastrin secretion. Glucagon from the gastrointestinal tract may be responsible (at least in part) for the hyperglycemia seen after pancreatectomy. Guanylin is a gastrointestinal polypeptide that binds to guanylyl cyclase. It is made up of 15 amino acid residues (Table 26–6) and is secreted by cells of the intestinal mucosa. Stimulation of guanylyl cyclase increases the concentration of intracellular cyclic 3',5'-guanosine monophosphate (cGMP), and this in turn causes increased secretion of Cl– into the intestinal lumen. Guanylin appears to act predominantly in a paracrine fashion, and it is produced in cells from the pylorus to the rectum. In an interesting example of molecular mimicry, the heat-stable enterotoxin of certain diarrhea-producing strains of E. coli has a structure very similar to guanylin and activates guanylin receptors in the intestine. Guanylin receptors are also found in the kidneys, the liver, and the female reproductive tract, and guanylin may act in an endocrine fashion to regulate fluid movement in these tissues as well, and particularly to integrate the actions of the intestine and kidneys.
the middle circular layer and the mucosa (Figure 26–1). Collectively, these neurons constitute the enteric nervous system. The system contains about 100 million sensory neurons, interneurons, and motor neurons in humans—as many as are found in the whole spinal cord—and the system is probably best viewed as a displaced part of the central nervous system (CNS) that is concerned with the regulation of gastrointestinal function. It is sometimes referred to as the “little brain” for this reason. It is connected to the CNS by parasympathetic and sympathetic fibers but can function autonomously without these connections (see below). The myenteric plexus innervates the longitudinal and circular smooth muscle layers and is concerned primarily with motor control, whereas the submucous plexus innervates the glandular epithelium, intestinal endocrine cells, and submucosal blood vessels and is primarily involved in the control of intestinal secretion. The neurotransmitters in the system include acetylcholine, the amines norepinephrine and serotonin, the amino acid γ-aminobutyrate (GABA), the purine adenosine triphosphate (ATP), the gases NO and CO, and many different peptides and polypeptides (Table 26–8). Some of these peptides also act in a paracrine fashion, and some enter the bloodstream, becoming hormones. Not surprisingly, most of them are also found in the brain.
EXTRINSIC INNERVATION The intestine receives a dual extrinsic innervation from the autonomic nervous system, with parasympathetic cholinergic activity generally increasing the activity of intestinal smooth
TABLE 26–8 Principal peptides found in the enteric nervous system. CGRP CCK Endothelin-2 Enkephalins Galanin GRP Neuropeptide Y Neurotensin Peptide YY PACAP
THE ENTERIC NERVOUS SYSTEM
Somatostatin Substance P
Two major networks of nerve fibers are intrinsic to the gastrointestinal tract: the myenteric plexus (Auerbach’s plexus), between the outer longitudinal and middle circular muscle layers, and the submucous plexus (Meissner’s plexus), between
TRH VIP
CHAPTER 26 Overview of Gastrointestinal Function & Regulation muscle and sympathetic noradrenergic activity generally decreasing it while causing sphincters to contract. The preganglionic parasympathetic fibers consist of about 2000 vagal efferents and other efferents in the sacral nerves. They generally end on cholinergic nerve cells of the myenteric and submucous plexuses. The sympathetic fibers are postganglionic, but many of them end on postganglionic cholinergic neurons, where the norepinephrine they secrete inhibits acetylcholine secretion by activating α2 presynaptic receptors. Other sympathetic fibers appear to end directly on intestinal smooth muscle cells. The electrical properties of intestinal smooth muscle are discussed in Chapter 5. Still other fibers innervate blood vessels, where they produce vasoconstriction. It appears that the intestinal blood vessels have a dual innervation: They have an extrinsic noradrenergic innervation and an intrinsic innervation by fibers of the enteric nervous system. VIP and NO are among the mediators in the intrinsic innervation, which seems, among other things, to be responsible for the hyperemia that accompanies digestion of food. It is unsettled whether the blood vessels have an additional cholinergic innervation.
449
Heart Vena cava
1300 mL/min
500 mL/min a rt
ery *
Liver
700 mL/min
He
pa
t
ic
Hepatic veins
Spleen Stomach
Portal vein
Celiac artery
Aorta
Pancreas 700 mL/min Superior mesenteric artery
Small intestine Colon
400 mL/min
GASTROINTESTINAL (SPLANCHNIC) CIRCULATION A final general point that should be made about the gastrointestinal tract relates to its unusual circulatory features. The blood flow to the stomach, intestines, pancreas, and liver is arranged in a series of parallel circuits, with all the blood from the intestines and pancreas draining via the portal vein to the liver (Figure 26–25). The blood from the intestines, pancreas, and spleen drains via the hepatic portal vein to the liver and from the liver via the hepatic veins to the inferior vena cava. The viscera and the liver receive about 30% of the cardiac output via the celiac, superior mesenteric, and inferior mesenteric arteries. The liver receives about 1300 mL/min from the portal vein and 500 mL/min from the hepatic artery during fasting, and the portal supply increases still further after meals.
Inferior mesenteric artery Rest of body *Branches of the hepatic artery also supply the stomach, pancreas and small intestine
FIGURE 26–25
Schematic of the splanchnic circulation under fasting conditions. Note that even during fasting, the liver receives the majority of its blood supply via the portal vein.
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CHAPTER SUMMARY ■
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The gastrointestinal system evolved as a portal to permit controlled nutrient uptake in multicellular organisms. It is functionally continuous with the outside environment and is defended by a well-developed mucosal immune system. Nevertheless, the gut usually lives in harmony with an extensive commensal microflora, particularly in the colon. Digestive secretions serve to chemically alter the components of meals (particularly macromolecules) such that their constituents can be absorbed across the epithelium. Meal components are acted on sequentially by saliva, gastric juice, pancreatic juice, and bile, which contain enzymes, ions, water, and other specialized components. The intestine and the organs that drain into it secrete about 8 L of fluid per day, which are added to water consumed in food and
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■
beverages. Most of this fluid is reabsorbed, leaving only approximately 200 mL to be lost to the stool. Fluid secretion and absorption are both dependent on the active epithelial transport of ions, nutrients, or both. Gastrointestinal functions are regulated in an integrated fashion by endocrine, paracrine, and neurocrine mechanisms. Hormones and paracrine factors are released from enteroendocrine cells in response to signals coincident with the intake of meals. The enteric nervous system conveys information from the central nervous system to the gastrointestinal tract, but also often can activate programmed responses of secretion and motility in an autonomous fashion. The intestine has an unusual circulation, in that the majority of its venous outflow does not return directly to the heart, but rather is directed initially to the liver via the portal vein.
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MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. Water is absorbed in the jejunum, ileum, and colon and excreted in the feces. Arrange these in order of the amount of water absorbed or excreted from greatest to smallest. A) colon, jejunum, ileum, feces B) feces, colon, ileum, jejunum C) jejunum, ileum, colon, feces D) colon, ileum, jejunum, feces E) feces, jejunum, ileum, colon 2. Drugs and toxins that increase the cAMP content of the intestinal mucosa cause diarrhea because they A) increase Na+–K+ cotransport in the small intestine. B) increase K+ secretion into the colon. C) inhibit K+ absorption in the crypts of Lieberkühn. D) increase Na+ absorption in the small intestine. E) increase Cl– secretion into the intestinal lumen. 3. A patient with a tumor secreting abnormal amounts of gastrin (gastrinoma) would be most likely to exhibit which of the following? A) decreased chief cell exocytosis B) duodenal ulceration C) increased gastric pH in the period between meals D) a reduced incidence of gastroesophageal reflux disease E) protein malabsorption 4. Which of the following has the highest pH? A) gastric juice B) hepatic bile C) pancreatic juice D) saliva E) secretions of the intestinal glands 5. Which of the following would not be produced by total pancreatectomy? A) vitamin E deficiency B) hyperglycemia C) metabolic acidosis D) weight gain E) decreased absorption of amino acids
CHAPTER RESOURCES Baron TH, Morgan DE: Current concepts: Acute necrotizing pancreatitis. N Engl J Med 1999;340:1412. Barrett KE: Gastrointestinal Physiology. McGraw-Hill, 2006. Bengmark S: Econutrition and health maintenance—A new concept to prevent GI inflammation, ulceration, and sepsis. Clin Nutr 1996;15:1.
Chong L, Marx J (editors): Lipids in the limelight. Science 2001;294:1861. Go VLW, et al: The Pancreas: Biology, Pathobiology and Disease, 2nd ed. Raven Press, 1993. Hersey SJ, Sachs G: Gastric acid secretion. Physiol Rev 1995;75:155. Hofmann AF: Bile acids: The good, the bad, and the ugly. News Physiol Sci 1999;14:24. Hunt RH, Tytgat GN (editors): Helicobacter pylori: Basic Mechanisms to Clinical Cure. Kluwer Academic, 2000. Itoh Z: Motilin and clinical application. Peptides 1997;18:593. Johnston DE, Kaplan MM: Pathogenesis and treatment of gallstones. N Engl J Med 1993;328:412. Kunzelmann K, Mall M: Electrolyte transport in the mammalian colon: Mechanisms and implications for disease. Physiol Rev 2002;82:245. Lamberts SWJ, et al: Octreotide. N Engl J Med 1996;334:246. Lewis JH (editor): A Pharmacological Approach to Gastrointestinal Disorders. Williams & Wilkins, 1994. Meier PJ, Stieger B: Molecular mechanisms of bile formation. News Physiol Sci 2000;15:89. Montecucco C, Rappuoli R: Living dangerously: How Helicobacter pylori survives in the human stomach. Nat Rev Mol Cell Biol 2001;2:457. Nakazato M: Guanylin family: New intestinal peptides regulating electrolyte and water homeostasis. J Gastroenterol 2001;36:219. Rabon EC, Reuben MA: The mechanism and structure of the gastric H+, K+–ATPase. Annu Rev Physiol 1990;52:321. Sachs G, Zeng N, Prinz C: Pathophysiology of isolated gastric endocrine cells. Annu Rev Physiol 1997;59:234. Sellin JH: SCFAs: The enigma of weak electrolyte transport in the colon. News Physiol Sci 1999;14:58. Specian RD, Oliver MG: Functional biology of intestinal goblet cells. Am J Med 1991;260:C183. Topping DL, Clifton PM: Short-chain fatty acids and human colonic function: Select resistant starch and nonstarch polysaccharides. Physiol Rev 2001;81:1031. Trauner M, Meier PJ, Boyer JL: Molecular mechanisms of cholestasis. N Engl J Med 1998;339:1217. Walsh JH (editor): Gastrin. Raven Press, 1993. Williams JA, Blevins GT Jr: Cholecystokinin and regulation of pancreatic acinar cell function. Physiol Rev 1993;73:701. Wolfe MM, Lichtenstein DR, Singh G: Gastrointestinal toxicity of nonsteroidal anti-inflammatory drugs. N Engl J Med 1999;340:1888. Wright EM: The intestinal Na+/glucose cotransporter. Annu Rev Physiol 1993;55:575. Young JA, van Lennep EW: The Morphology of Salivary Glands. Academic Press, 1978. Zoetendal EG et al: Molecular ecological analysis of the gastrointestinal microbiota: A review. J Nutr 2004;134:465.
27 C
Digestion, Absorption, & Nutritional Principles
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■
■ ■ ■ ■ ■
Understand how nutrients are delivered to the body and the chemical processes needed to convert them to a form suitable for absorption. List the major dietary carbohydrates and define the luminal and brush border processes that produce absorbable monosaccharides as well as the transport mechanisms that provide for the uptake of these hydrophilic molecules. Understand the process of protein assimilation, and the ways in which it is comparable to, or converges from, that used for carbohydrates. Define the stepwise processes of lipid digestion and absorption, the role of bile acids in solubilizing the products of lipolysis, and the consequences of fat malabsorption. Identify the source and functions of short-chain fatty acids in the colon. Delineate the mechanisms of uptake for vitamins and minerals. Understand basic principles of energy metabolism and nutrition.
INTRODUCTION The gastrointestinal system is the portal through which nutritive substances, vitamins, minerals, and fluids enter the body. Proteins, fats, and complex carbohydrates are broken down into absorbable units (digested), principally in the small intestine. The products of digestion and the vitamins, minerals, and water cross the mucosa and enter the lymph or the blood (absorption). The digestive and absorptive processes are the subject of this chapter. Digestion of the major foodstuffs is an orderly process involving the action of a large number of digestive enzymes (Table 27–1). Enzymes from the salivary glands attack carbohydrates (and fats in some species); enzymes from the stomach attack proteins and fats; and enzymes from the exocrine
portion of the pancreas attack carbohydrates, proteins, lipids, DNA, and RNA. Other enzymes that complete the digestive process are found in the luminal membranes and the cytoplasm of the cells that line the small intestine. The action of the enzymes is aided by the hydrochloric acid secreted by the stomach and the bile secreted by the liver. Most substances pass from the intestinal lumen into the enterocytes and then out of the enterocytes to the interstitial fluid. The processes responsible for movement across the luminal cell membrane are often quite different from those responsible for movement across the basal and lateral cell membranes to the interstitial fluid.
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SECTION V Gastrointestinal Physiology
TABLE 27–1 Normal transport of substances by the intestine and location of maximum absorption or secretion.a Small Intestine Upperb
Mid
Lower
Colon
Sugars (glucose, galactose, etc)
++
+++
++
0
Amino acids
++
++
++
0
Water-soluble and fat-soluble vitamins except vitamin B12
+++
++
0
0
Betaine, dimethylglycine, sarcosine
+
++
++
?
Antibodies in newborns
+
++
+++
?
Pyrimidines (thymine and uracil)
+
+
?
?
+++
++
+
0
Bile acids
+
+
+++
Vitamin B12
Absorption of:
Long-chain fatty acid absorption and conversion to triglyceride
0
+
+++
0
Na+
+++
++
+++
+++
K+
+
+
+
Sec
Ca2+
+++
++
+
?
Fe2+
+++
+
+
?
Cl–
+++
++
+
+
SO42–
++
+
0
?
aAmount of absorption is graded + to +++. Sec, secreted when luminal K+ is low. bUpper small intestine refers primarily to jejunum, although the duodenum is similar in most cases studied (with the notable exception that the duodenum secretes HCO – 3 and shows little net absorption or secretion of NaCl).
DIGESTION & ABSORPTION: CARBOHYDRATES DIGESTION The principal dietary carbohydrates are polysaccharides, disaccharides, and monosaccharides. Starches (glucose polymers) and their derivatives are the only polysaccharides that are digested to any degree in the human gastrointestinal tract. Amylopectin, which constitutes 80–90% of dietary starch, is a branched molecule, whereas amylose is a straight chain with only 1:4α linkages (Figure 27-1). The disaccharides lactose (milk sugar) and sucrose (table sugar) are also ingested, along with the monosaccharides fructose and glucose. In the mouth, starch is attacked by salivary α-amylase. However, the optimal pH for this enzyme is 6.7, and its action is inhibited by the acidic gastric juice when food enters the stomach. In the small intestine, both the salivary and the pancreatic α-amylase also act on the ingested polysaccharides. Both the salivary and the pancreatic α-amylases hydrolyze 1:4α linkages but spare 1:6α linkages and terminal 1:4α linkages. Consequently, the end products of α-amylase digestion are oligosaccharides: the disaccharide maltose; the trisaccha-
ride maltotriose; and α-limit dextrins, polymers of glucose containing an average of about eight glucose molecules with 1:6α linkages (Figure 27–1). The oligosaccharidases responsible for the further digestion of the starch derivatives are located in the brush border of small intestinal epithelial cells (Figure 27–1). Some of these enzymes have more than one substrate. Isomaltase is mainly responsible for hydrolysis of 1:6α linkages. Along with maltase and sucrase, it also breaks down maltotriose and maltose. Sucrase and isomaltase are initially synthesized as a single glycoprotein chain which is inserted into the brush border membrane. It is then hydrolyzed by pancreatic proteases into sucrase and isomaltase subunits. Sucrase hydrolyzes sucrose into a molecule of glucose and a molecule of fructose. In addition, two disaccharidases are present in the brush border: lactase, which hydrolyzes lactose to glucose and galactose, and trehalase, which hydrolyzes trehalose, a 1:1α-linked dimer of glucose, into two glucose molecules. Deficiency of one or more of the brush border oligosaccharidases may cause diarrhea, bloating, and flatulence after ingestion of sugar (Clinical Box 27–1). The diarrhea is due to the increased number of osmotically active oligosaccharide molecules that remain in the intestinal lumen, causing the volume of the intestinal contents to increase. In the colon,
CHAPTER 27 Digestion, Absorption, & Nutritional Principles
Glucose
α1,4 bond
1
453
Maltose Maltotriose
Amylose Glucoamylase Sucrase Isomaltase Amylase α1,6 bond
Amylopectin
2 α-limit dextrin
Glucoamylase Maltose Maltotriose
+
Glucose oligomers Isomaltase + α-limit dextrin
Glucoamylase Sucrase Isomaltase
FIGURE 27–1
Left: Structure of amylose and amylopectin, which are polymers of glucose (indicated by circles). These molecules are partially digested by the enzyme amylase, yielding the products shown at the bottom of the figure. Right: Brush border hydrolases responsible for the sequential digestion of the products of luminal starch digestion (1, linear oligomers; 2, alpha-limit dextrins).
bacteria break down some of the oligosaccharides, further increasing the number of osmotically active particles. The bloating and flatulence are due to the production of gas (CO2 and H2) from disaccharide residues in the lower small intestine and colon.
ABSORPTION Hexoses are rapidly absorbed across the wall of the small intestine (Table 27–1). Essentially all the hexoses are removed before the remains of a meal reach the terminal part of the ileum. The sugar molecules pass from the mucosal cells to the blood in the capillaries draining into the portal vein. The transport of most hexoses is dependent on Na+ in the intestinal lumen; a high concentration of Na+ on the mucosal surface of the cells facilitates and a low concentration inhibits sugar influx into the epithelial cells. This is because glucose and Na+ share the same cotransporter, or symport, the sodium-dependent glucose transporter (SGLT, Na+ glucose cotransporter) (Figure 27–2). The members of this family of transporters, SGLT 1 and SGLT 2, resemble the glucose transporters responsible for facilitated diffusion (see Chapter 21) in that they cross the cell membrane 12 times and have their –COOH and –NH2 terminals on the cytoplasmic side of the
membrane. However, there is no homology to the glucose transporter (GLUT) series of transporters. SGLT-1 is responsible for uptake of dietary glucose from the gut. The related transporter, SGLT 2, is responsible for glucose transport out of the renal tubules (see Chapter 38). Because the intracellular Na+ concentration is low in intestinal cells as it is in other cells, Na+ moves into the cell along its concentration gradient. Glucose moves with the Na+ and is released in the cell (Figure 27–2). The Na+ is transported into the lateral intercellular spaces, and the glucose is transported by GLUT 2 into the interstitium and thence to the capillaries. Thus, glucose transport is an example of secondary active transport (see Chapter 2); the energy for glucose transport is provided indirectly, by the active transport of Na+ out of the cell. This maintains the concentration gradient across the luminal border of the cell, so that more Na+ and consequently more glucose enter. When the Na+/glucose cotransporter is congenitally defective, the resulting glucose/galactose malabsorption causes severe diarrhea that is often fatal if glucose and galactose are not promptly removed from the diet. The use of glucose and its polymers to retain Na+ in diarrheal disease was discussed in Chapter 26. SGLT-1 also transports galactose, but fructose utilizes a different mechanism. Its absorption is independent of Na+ or the
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SECTION V Gastrointestinal Physiology
CLINICAL BOX 27–1 Lactose Intolerance In most mammals and in many races of humans, intestinal lactase activity is high at birth, then declines to low levels during childhood and adulthood. The low lactase levels are associated with intolerance to milk (lactose intolerance). Most Europeans and their American descendants retain sufficient intestinal lactase activity in adulthood; the incidence of lactase deficiency in northern and western Europeans is only about 15%. However, the incidence in blacks, American Indians, Asians, and Mediterranean populations is 70–100%. When such individuals ingest dairy products, they are unable to digest lactose sufficiently, and so symptoms such as bloating, pain, gas, and diarrhea are produced by the unabsorbed osmoles that are subsequently digested by colonic bacteria. Milk intolerance can be ameliorated by administration of commercial lactase preparations, but this is expensive. Yogurt is better tolerated than milk in intolerant individuals because it contains its own bacterial lactase.
transport of glucose and galactose; it is transported instead by facilitated diffusion from the intestinal lumen into the enterocytes by GLUT 5 and out of the enterocytes into the interstitium by GLUT 2. Some fructose is converted to glucose in the mucosal cells. Insulin has little effect on intestinal transport of sugars. In this respect, intestinal absorption resembles glucose reabsorption in the proximal convoluted tubules of the kidneys (see Chapter 38); neither process requires phosphorylation, and both are essentially normal in diabetes but are depressed by the drug phlorizin. The maximal rate of glucose absorption from the intestine is about 120 g/h.
PROTEINS & NUCLEIC ACIDS PROTEIN DIGESTION Protein digestion begins in the stomach, where pepsins cleave some of the peptide linkages. Like many of the other enzymes concerned with protein digestion, pepsins are secreted in the form of inactive precursors (proenzymes) and activated in the
1
Sucrose
Sucrase
Isomaltase
Na+
Brush border membrane
SGLT-1
GLUT5
Cytosol Glucose Fructose
Lactose
2 Lactase
Na+ SGLT-1
Cytosol Glucose Galactose
FIGURE 27–2
Brush border digestion and assimilation of the disaccharides sucrose (panel 1) and lactose (panel 2). SGLT-1, sodiumglucose cotransporter-1.
CHAPTER 27 Digestion, Absorption, & Nutritional Principles gastrointestinal tract. The pepsin precursors are called pepsinogens and are activated by gastric acid. Human gastric mucosa contains a number of related pepsinogens, which can be divided into two immunohistochemically distinct groups, pepsinogen I and pepsinogen II. Pepsinogen I is found only in acid-secreting regions, whereas pepsinogen II is also found in the pyloric region. Maximal acid secretion correlates with pepsinogen I levels. Pepsins hydrolyze the bonds between aromatic amino acids such as phenylalanine or tyrosine and a second amino acid, so the products of peptic digestion are polypeptides of very diverse sizes. Because pepsins have a pH optimum of 1.6 to 3.2, their action is terminated when the gastric contents are mixed with the alkaline pancreatic juice in the duodenum and jejunum. The pH of the intestinal contents in the duodenal bulb is 2.0 to 4.0, but in the rest of the duodenum it is about 6.5. In the small intestine, the polypeptides formed by digestion in the stomach are further digested by the powerful proteolytic enzymes of the pancreas and intestinal mucosa. Trypsin, the chymotrypsins, and elastase act at interior peptide bonds in the peptide molecules and are called endopeptidases. The formation of the active endopeptidases from their inactive precursors occurs only when they have reached their site of action, secondary to the action of the brush border hydrolase, enterokinase (Figure 27–3). The powerful protein-splitting enzymes of the pancreatic juice are secreted as inactive proenzymes. Trypsinogen is converted to the active enzyme trypsin by enterokinase when the pancreatic juice enters the duodenum. Enterokinase
contains 41% polysaccharide, and this high polysaccharide content apparently prevents it from being digested itself before it can exert its effect. Trypsin converts chymotrypsinogens into chymotrypsins and other proenzymes into active enzymes (Figure 27–3). Trypsin can also activate trypsinogen; therefore, once some trypsin is formed, there is an auto-catalytic chain reaction. Enterokinase deficiency occurs as a congenital abnormality and leads to protein malnutrition. The carboxypeptidases of the pancreas are exopeptidases that hydrolyze the amino acids at the carboxyl ends of the polypeptides (Figure 27–4). Some free amino acids are liberated in the intestinal lumen, but others are liberated at the cell surface by the aminopeptidases, carboxypeptidases, endopeptidases, and dipeptidases in the brush border of the mucosal cells. Some di- and tripeptides are actively transported into the intestinal cells and hydrolyzed by intracellular peptidases, with the amino acids entering the bloodstream. Thus, the final digestion to amino acids occurs in three locations: the intestinal lumen, the brush border, and the cytoplasm of the mucosal cells.
ABSORPTION At least seven different transport systems transport amino acids into enterocytes. Five of these require Na+ and cotransport amino acids and Na+ in a fashion similar to the cotransport of Na+ and glucose (Figure 27–3). Two of these five also require Cl–. In two systems, transport is independent of Na+.
Pancreatic juice
Enterokinase
Trypsinogen Trypsin Trypsinogen
Trypsin
Chymotrypsinogen
Chymotrypsin
Proelastase
Elastase
Procarboxypeptidase A
Carboxypeptidase A
Procarboxypeptidase B
Carboxypeptidase B
Lumen
FIGURE 27–3
455
Epithelium
Mechanism to avoid activation of pancreatic proteases until they are in the duodenal lumen.
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SECTION V Gastrointestinal Physiology
Ser Chymotrypsin Elastase
Carboxypeptidase A
Peptide with C-terminal neutral AA
Large peptides
Arg Ser Short peptides free neutral and basic AA’s
Trypsin Carboxypeptidase B Arg Peptide with C-terminal basic AA
FIGURE 27–4
Luminal digestion of peptides by pancreatic endopeptidases and exopeptidases. Individual amino acids are shown as
squares.
The di- and tripeptides are transported into enterocytes by a system known as PepT1 (or peptide transporter 1) that requires H+ instead of Na+ (Figure 27–5). There is very little absorption of larger peptides. In the enterocytes, amino acids released from the peptides by intracellular hydrolysis plus the amino acids absorbed from the intestinal lumen and brush border are transported out of the enterocytes along their basolateral borders by at least five transport systems. From there, they enter the hepatic portal blood. Absorption of amino acids is rapid in the duodenum and jejunum but slow in the ileum. Approximately 50% of the digested protein comes from ingested food, 25% from proteins in digestive juices, and 25% from desquamated mucosal cells. Only 2–5% of the protein in the small intestine escapes digestion and absorption. Some of this is eventually digested by bac-
3Na+
Na+ NHE
2K+
H+ H+
Cytosolic digestion
PEPT1
Di-, tripeptides
Basolateral amino acid transporters
FIGURE 27–5 Disposition of short peptides in intestinal epithelial cells. Peptides are absorbed together with a proton supplied by an apical sodium/hydrogen exchanger (NHE) by the peptide transporter 1 (PepT1). Absorbed peptides are digested by cytosolic proteases, and any amino acids that are surplus to the needs of the epithelial cell are transported into the bloodstream by a series of basolateral transport proteins.
terial action in the colon. Almost all of the protein in the stools is not of dietary origin but comes from bacteria and cellular debris. Evidence suggests that the peptidase activities of the brush border and the mucosal cell cytoplasm are increased by resection of part of the ileum and that they are independently altered in starvation. Thus, these enzymes appear to be subject to homeostatic regulation. In humans, a congenital defect in the mechanism that transports neutral amino acids in the intestine and renal tubules causes Hartnup disease. A congenital defect in the transport of basic amino acids causes cystinuria. However, most patients do not experience nutritional deficiencies of these amino acids because peptide transport compensates. In infants, moderate amounts of undigested proteins are also absorbed. The protein antibodies in maternal colostrum are largely secretory immunoglobulins (IgAs), the production of which is increased in the breast in late pregnancy. They cross the mammary epithelium by transcytosis and enter the circulation of the infant from the intestine, providing passive immunity against infections. Absorption is by endocytosis and subsequent exocytosis. Protein absorption declines with age, but adults still absorb small quantities. Foreign proteins that enter the circulation provoke the formation of antibodies, and the antigen–antibody reaction occurring on subsequent entry of more of the same protein may cause allergic symptoms. Thus, absorption of proteins from the intestine may explain the occurrence of allergic symptoms after eating certain foods. The incidence of food allergy in children is said to be as high as 8%. Certain foods are more allergenic than others. Crustaceans, mollusks, and fish are common offenders, and allergic responses to legumes, cows’ milk, and egg white are also relatively frequent. Absorption of protein antigens, particularly bacterial and viral proteins, takes place in large microfold cells or M cells, specialized intestinal epithelial cells that overlie aggregates of lymphoid tissue (Peyer’s patches). These cells pass the antigens
CHAPTER 27 Digestion, Absorption, & Nutritional Principles to the lymphoid cells, and lymphocytes are activated. The activated lymphoblasts enter the circulation, but they later return to the intestinal mucosa and other epithelia, where they secrete IgA in response to subsequent exposures to the same antigen. This secretory immunity is an important defense mechanism (see Chapter 3).
NUCLEIC ACIDS Nucleic acids are split into nucleotides in the intestine by the pancreatic nucleases, and the nucleotides are split into the nucleosides and phosphoric acid by enzymes that appear to be located on the luminal surfaces of the mucosal cells. The nucleosides are then split into their constituent sugars and purine and pyrimidine bases. The bases are absorbed by active transport.
LIPIDS FAT DIGESTION A lingual lipase is secreted by Ebner’s glands on the dorsal surface of the tongue in some species, and the stomach also secretes a lipase (Table 27–1). They are of little quantitative significance for lipid digestion other than in the setting of pancreatic insufficiency, however. Most fat digestion therefore begins in the duodenum, pancreatic lipase being one of the most important enzymes involved. This enzyme hydrolyzes the 1- and 3-bonds of the triglycerides (triacylglycerols) with relative ease but acts on the 2bonds at a very low rate, so the principal products of its action are free fatty acids and 2-monoglycerides (2-monoacylglycerols). It acts on fats that have been emulsified (see below). Its activity is facilitated when an amphipathic helix that covers the active site like a lid is bent back. Colipase, a protein with a molecular weight of about 11,000, is also secreted in the pancreatic juice, and when this molecule binds to the –COOHterminal domain of the pancreatic lipase, opening of the lid is facilitated. Colipase is secreted in an inactive proform (Table 27–1) and is activated in the intestinal lumen by trypsin. Another pancreatic lipase that is activated by bile salts has been characterized. This 100,000-kDa cholesterol esterase represents about 4% of the total protein in pancreatic juice. In adults, pancreatic lipase is 10–60 times more active, but unlike pancreatic lipase, this bile salt-activated lipase catalyzes the hydrolysis of cholesterol esters, esters of fat-soluble vitamins, and phospholipids, as well as triglycerides. A very similar enzyme is found in human milk. Fats are relatively insoluble, which limits their ability to cross the unstirred layer and reach the surface of the mucosal cells. However, they are finely emulsified in the small intestine by the detergent action of bile salts, lecithin, and monoglycerides. When the concentration of bile salts in the intestine is high, as it is after contraction of the gallbladder, lipids and bile
457
salts interact spontaneously to form micelles (Figure 26–16). These cylindrical aggregates, which are discussed in more detail in Chapter 29, take up lipids, and although their lipid concentration varies, they generally contain fatty acids, monoglycerides, and cholesterol in their hydrophobic centers. Micellar formation further solubilizes the lipids and provides a mechanism for their transport to the enterocytes. Thus, the micelles move down their concentration gradient through the unstirred layer to the brush border of the mucosal cells. The lipids diffuse out of the micelles, and a saturated aqueous solution of the lipids is maintained in contact with the brush border of the mucosal cells (Figure 26–16).
STEATORRHEA Pancreatectomized animals and patients with diseases that destroy the exocrine portion of the pancreas have fatty, bulky, clay-colored stools (steatorrhea) because of the impaired digestion and absorption of fat. The steatorrhea is due mostly to lipase deficiency. However, acid inhibits the lipase, and the lack of alkaline secretion from the pancreas also contributes by lowering the pH of the intestine contents. In some cases, hypersecretion of gastric acid can cause steatorrhea. Another cause of steatorrhea is defective reabsorption of bile salts in the distal ileum (see Chapter 29).
FAT ABSORPTION Traditionally, lipids were thought to enter the enterocytes by passive diffusion, but some evidence now suggests that carriers are involved. Inside the cells, the lipids are rapidly esterified, maintaining a favorable concentration gradient from the lumen into the cells (Figure 27–6). There are also carriers that export certain lipids back into the lumen, thereby limiting their oral availability. This is the case for plant sterols as well as cholesterol. The fate of the fatty acids in enterocytes depends on their size. Fatty acids containing less than 10 to 12 carbon atoms are water-soluble enough that they pass through the enterocyte unmodified and are actively transported into the portal blood. They circulate as free (unesterified) fatty acids. The fatty acids containing more than 10 to 12 carbon atoms are too insoluble for this. They are reesterified to triglycerides in the enterocytes. In addition, some of the absorbed cholesterol is esterified. The triglycerides and cholesterol esters are then coated with a layer of protein, cholesterol, and phospholipid to form chylomicrons. These leave the cell and enter the lymphatics, because they are too large to pass through the junctions between capillary endothelial cells (Figure 27–6). In mucosal cells, most of the triglyceride is formed by the acylation of the absorbed 2-monoglycerides, primarily in the smooth endoplasmic reticulum. However, some of the triglyceride is formed from glycerophosphate, which in turn is a product of glucose catabolism. Glycerophosphate is also
458
SECTION V Gastrointestinal Physiology also promote the absorption of Na+, although the exact mechanism for coupled Na+–SCFA absorption is unsettled. Smooth ER
FA/MG TG
Synthesis of TG and phospholipids Synthesis of apolipoproteins Apolipoprotein glycosylation
Rough ER
Exocytosis
Golgi
Chylomicrons
FIGURE 27–6
Intracellular handling of the products of lipid digestion. Absorbed fatty acids (FA) and monoglycerides (MG) are reesterified to form triglyceride (TG) in the smooth endoplasmic reticulum. Apoproteins synthesized in the rough endoplasmic reticulum are coated around lipid cores, and the resulting chylomicrons are secreted from the basolateral pole of epithelial cells by exocytosis.
converted into glycerophospholipids that participate in chylomicron formation. The acylation of glycerophosphate and the formation of lipoproteins occur in the rough endoplasmic reticulum. Carbohydrate moieties are added to the proteins in the Golgi apparatus, and the finished chylomicrons are extruded by exocytosis from the basal or lateral aspects of the cell. Absorption of long-chain fatty acids is greatest in the upper parts of the small intestine, but appreciable amounts are also absorbed in the ileum. On a moderate fat intake, 95% or more of the ingested fat is absorbed. The processes involved in fat absorption are not fully mature at birth, and infants fail to absorb 10–15% of ingested fat. Thus, they are more susceptible to the ill effects of disease processes that reduce fat absorption.
SHORT-CHAIN FATTY ACIDS IN THE COLON Increasing attention is being focused on short-chain fatty acids (SCFAs) that are produced in the colon and absorbed from it. SCFAs are two- to five-carbon weak acids that have an average normal concentration of about 80 mmol/L in the lumen. About 60% of this total is acetate, 25% propionate, and 15% butyrate. They are formed by the action of colonic bacteria on complex carbohydrates, resistant starches, and other components of the dietary fiber, that is, the material that escapes digestion in the upper gastrointestinal tract and enters the colon. Absorbed SCFAs are metabolized and make a significant contribution to the total caloric intake. In addition, they exert a trophic effect on the colonic epithelial cells, combat inflammation, and are absorbed in part by exchange for H+, helping to maintain acid–base equilibrium. SCFAs are absorbed by specific transporters present in colonic epithelial cells. SCFAs
ABSORPTION OF VITAMINS & MINERALS VITAMINS Absorption of the fat-soluble vitamins A, D, E, and K is deficient if fat absorption is depressed because of lack of pancreatic enzymes or if bile is excluded from the intestine by obstruction of the bile duct. Most vitamins are absorbed in the upper small intestine, but vitamin B12 is absorbed in the ileum. This vitamin binds to intrinsic factor, a protein secreted by the stomach, and the complex is absorbed across the ileal mucosa (see Chapter 26). Vitamin B12 absorption and folate absorption are Na+-independent, but all seven of the remaining water-soluble vitamins—thiamin, riboflavin, niacin, pyridoxine, pantothenate, biotin, and ascorbic acid—are absorbed by carriers that are Na+ cotransporters.
CALCIUM A total of 30–80% of ingested calcium is absorbed. The absorptive process and its relation to 1,25-dihydroxycholecalciferol are discussed in Chapter 23. Through this vitamin D derivative, Ca2+ absorption is adjusted to body needs; absorption is increased in the presence of Ca2+ deficiency and decreased in the presence of Ca2+ excess. Ca2+ absorption is also facilitated by protein. It is inhibited by phosphates and oxalates because these anions form insoluble salts with Ca2+ in the intestine. Magnesium absorption is also facilitated by protein.
IRON In adults, the amount of iron lost from the body is relatively small. The losses are generally unregulated, and total body stores of iron are regulated by changes in the rate at which it is absorbed from the intestine. Men lose about 0.6 mg/d, largely in the stools. Women have a variable, larger loss averaging about twice this value because of the additional iron lost during menstruation. The average daily iron intake in the United States and Europe is about 20 mg, but the amount absorbed is equal only to the losses. Thus, the amount of iron absorbed is normally about 3–6% of the amount ingested. Various dietary factors affect the availability of iron for absorption; for example, the phytic acid found in cereals reacts with iron to form insoluble compounds in the intestine, as do phosphates and oxalates. Most of the iron in the diet is in the ferric (Fe3+) form, whereas it is the ferrous (Fe2+) form that is absorbed. Fe3+ reductase activity is associated with the iron transporter in the
CHAPTER 27 Digestion, Absorption, & Nutritional Principles
459
Brush border Intestinal lumen
Heme
Enterocyte
HT
Blood
Heme HO2
Fe3+ reductase Fe2+
Hp
Fe2+ DMT1
Fe2+
Fe2+
Fe2+
FP
Fe3+
Fe3+-ferritin Shed
Fe3+−TF
FIGURE 27–7
Absorption of iron. Fe3+ is converted to Fe2+ by ferric reductase, and Fe2+ is transported into the enterocyte by the apical membrane iron transporter DMT1. Heme is transported into the enterocyte by a separate heme transporter (HT), and HO2 releases Fe 2+ from the heme. Some of the intracellular Fe2+ is converted to Fe3+ and bound to ferritin. The rest binds to the basolateral Fe 2+ transporter ferroportin (FP) and is transported to the interstitial fluid. The transport is aided by hephaestin (Hp). In plasma, Fe 2+ is converted to Fe3+ and bound to the iron transport protein transferrin (TF).
brush borders of the enterocytes (Figure 27–7). Gastric secretions dissolve the iron and permit it to form soluble complexes with ascorbic acid and other substances that aid its reduction to the Fe2+ form. The importance of this function in humans is indicated by the fact that iron deficiency anemia is a troublesome and relatively frequent complication of partial gastrectomy. Almost all iron absorption occurs in the duodenum. Transport of Fe2+ into the enterocytes occurs via divalent metal transporter 1 (DMT1) (Figure 27–7). Some is stored in ferritin, and the remainder is transported out of the enterocytes by a basolateral transporter named ferroportin 1. A protein called hephaestin (Hp) is associated with ferroportin 1. It is not a transporter itself, but it facilitates basolateral transport. In the plasma, Fe2+ is converted to Fe3+ and bound to the iron transport protein transferrin. This protein has two iron-binding sites. Normally, transferrin is about 35% saturated with iron, and the normal plasma iron level is about 130 μg/dL (23 μmol/ L) in men and 110 μg/dL (19 μmol/L) in women. Heme (see Chapter 32) binds to an apical transport protein in enterocytes and is carried into the cytoplasm. In the cytoplasm, HO2, a subtype of heme oxygenase, removes Fe2+ from the porphyrin and adds it to the intracellular Fe2+ pool. Seventy percent of the iron in the body is in hemoglobin, 3% in myoglobin, and the rest in ferritin, which is present not only in enterocytes, but also in many other cells. Apoferritin is a globular protein made up of 24 subunits. Ferritin is readily visible under the electron microscope and has been used as a tracer in studies of phagocytosis and related phenomena. Ferritin molecules in lysosomal membranes may aggregate in deposits that contain as much as 50% iron. These deposits are called hemosiderin. Intestinal absorption of iron is regulated by three factors: recent dietary intake of iron, the state of the iron stores in the
body, and the state of erythropoiesis in the bone marrow. The normal operation of the factors that maintain iron balance is essential for health (Clinical Box 27–2).
NUTRITIONAL PRINCIPLES & ENERGY METABOLISM The animal organism oxidizes carbohydrates, proteins, and fats, producing principally CO2, H2O, and the energy necessary for life processes (Clinical Box 27–3). CO2, H2O, and energy are also produced when food is burned outside the body. However, in the body, oxidation is not a one-step, semiexplosive reaction but a complex, slow, stepwise process called catabolism, which liberates energy in small, usable amounts. Energy can be stored in the body in the form of special energy-rich phosphate compounds and in the form of proteins, fats, and complex carbohydrates synthesized from simpler molecules. Formation of these substances by processes that take up rather than liberate energy is called anabolism. This chapter consolidates consideration of endocrine function by providing a brief summary of the production and utilization of energy and the metabolism of carbohydrates, proteins, and fats.
METABOLIC RATE The amount of energy liberated by the catabolism of food in the body is the same as the amount liberated when food is burned outside the body. The energy liberated by catabolic processes in the body is used for maintaining body functions, digesting and metabolizing food, thermoregulation, and physical activity. It appears as external work, heat, and energy storage: Energy output = External work + Energy storage + Heat
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SECTION V Gastrointestinal Physiology
CLINICAL BOX 27–2
CLINICAL BOX 27–3
Disorders of Iron Uptake
Obesity
Iron deficiency causes anemia. Conversely, iron overload causes hemosiderin to accumulate in the tissues, producing hemosiderosis. Large amounts of hemosiderin can damage tissues, causing hemochromatosis. This syndrome is characterized by pigmentation of the skin, pancreatic damage with diabetes (“bronze diabetes"), cirrhosis of the liver, a high incidence of hepatic carcinoma, and gonadal atrophy. Hemochromatosis may be hereditary or acquired. The most common cause of the hereditary forms is a mutated HFE gene that is common in the Caucasian population. It is located on the short arm of chromosome 6 and is closely linked to the human leukocyte antigen-A (HLA-A) locus. It is still unknown precisely how mutations in HFE cause hemochromatosis, but individuals who are homogenous for HFE mutations absorb excess amounts of iron because HFE normally inhibits expression of the duodenal transporters that participate in iron uptake. If the abnormality is diagnosed before excessive amounts of iron accumulate in the tissues, life expectancy can be prolonged by repeated withdrawal of blood. Acquired hemochromatosis occurs when the iron-regulating system is overwhelmed by excess iron loads due to chronic destruction of red blood cells, liver disease, or repeated transfusions in diseases such as intractable anemia.
Obesity is the most common and most expensive nutritional problem in the United States. A convenient and reliable indicator of body fat is the body mass index (BMI), which is body weight (in kilograms) divided by the square of height (in meters). Values above 25 are abnormal. Individuals with values of 25–30 are overweight, and those with values > 30 are obese. In the United States, 55% of the population are overweight and 22% are obese. The incidence of obesity is also increasing in other countries. Indeed, the Worldwatch Institute has estimated that although starvation continues to be a problem in many parts of the world, the number of overweight people in the world is now as great as the number of underfed. Obesity is a problem because of its complications. It is associated with accelerated atherosclerosis and an increased incidence of gallbladder and other diseases. Its association with type 2 diabetes is especially striking. As weight increases, insulin resistance increases and frank diabetes appears. At least in some cases, glucose tolerance is restored when weight is lost. In addition, the mortality rates from many kinds of cancer are increased in obese individuals. The causes of the high incidence of obesity in the general population are probably multiple. Studies of twins raised apart show a definite genetic component. It has been pointed out that through much of human evolution, famines were common, and mechanisms that permitted increased energy storage as fat had survival value. Now, however, food is plentiful in many countries, and the ability to gain and retain fat has become a liability. As noted above, the fundamental cause of obesity is still an excess of energy intake in food over energy expenditure. If human volunteers are fed a fixed high-calorie diet, some gain weight more rapidly than others, but the slower weight gain is due to increased energy expenditure in the form of small, fidgety movements (nonexercise activity thermogenesis; NEAT). Body weight generally increases at a slow but steady rate throughout adult life. Decreased physical activity is undoubtedly a factor in this increase, but decreased sensitivity to leptin may also play a role.
The amount of energy liberated per unit of time is the metabolic rate. Isotonic muscle contractions perform work at a peak efficiency approximating 50%: Work done Efficiency = -----------------------------------------------------Total energy expended
Essentially all of the energy of isometric contractions appears as heat, because little or no external work (force multiplied by the distance that the force moves a mass) is done (see Chapter 5). Energy is stored by forming energy-rich compounds. The amount of energy storage varies, but in fasting individuals it is zero or negative. Therefore, in an adult individual who has not eaten recently and who is not moving (or growing, reproducing, or lactating), all of the energy output appears as heat.
CALORIES The standard unit of heat energy is the calorie (cal), defined as the amount of heat energy necessary to raise the temperature of 1 g of water 1 degree, from 15 °C to 16 °C. This unit is also called the gram calorie, small calorie, or standard calorie. The unit commonly used in physiology and medicine is the Calorie (kilocalorie; kcal), which equals 1000 cal.
CALORIMETRY The energy released by combustion of foodstuffs outside the body can be measured directly (direct calorimetry) by oxidizing the compounds in an apparatus such as a bomb calorimeter, a metal vessel surrounded by water inside an insulated container. The food is ignited by an electric spark. The change in the temperature of the water is a measure of the calories produced. Similar measurements of the energy released by combustion of compounds in living animals and humans are much more complex, but calorimeters have been constructed that can physically accommodate human beings. The heat
CHAPTER 27 Digestion, Absorption, & Nutritional Principles produced by their bodies is measured by the change in temperature of the water in the walls of the calorimeter. The caloric values of the common foodstuffs, as measured in a bomb calorimeter, are found to be 4.1 kcal/g of carbohydrate, 9.3 kcal/g of fat, and 5.3 kcal/g of protein. In the body, similar values are obtained for carbohydrate and fat, but the oxidation of protein is incomplete, the end products of protein catabolism being urea and related nitrogenous compounds in addition to CO2 and H2O (see below). Therefore, the caloric value of protein in the body is only 4.1 kcal/g.
INDIRECT CALORIMETRY Energy production can also be calculated by measuring the products of the energy-producing biologic oxidations; that is, CO2, H2O, and the end products of protein catabolism produced, but this is difficult. However, O2 is not stored, and except when an O2 debt is being incurred, the amount of O2 consumption per unit of time is proportionate to the energy liberated by metabolism. Consequently, measurement of O2 consumption (indirect calorimetry) is used to determine the metabolic rate.
RESPIRATORY QUOTIENT (RQ) The respiratory quotient (RQ) is the ratio in the steady state of the volume of CO2 produced to the volume of O2 consumed per unit of time. It should be distinguished from the respiratory exchange ratio (R), which is the ratio of CO2 to O2 at any given time whether or not equilibrium has been reached. R is affected by factors other than metabolism. RQ and R can be calculated for reactions outside the body, for individual organs and tissues, and for the whole body. The RQ of carbohydrate is 1.00, and that of fat is about 0.70. This is because H and O are present in carbohydrate in the same proportions as in water, whereas in the various fats, extra O2 is necessary for the formation of H2O. Carbohydrate: C6H12O6 + 6O2 → 6CO2 + 6H2O (glucose) RQ = 6/6 = 1.00 Fat: 2C51H98O6 + 145O2 → 102CO2 + 98H2O (tripalmitin) RQ = 102/145 = 0.703 Determining the RQ of protein in the body is a complex process, but an average value of 0.82 has been calculated. The approximate amounts of carbohydrate, protein, and fat being oxidized in the body at any given time can be calculated from the RQ and the urinary nitrogen excretion. RQ and R for the whole body differ in various conditions. For example, during hyperventilation, R rises because CO2 is being blown off. During strenuous exercise, R may reach 2.00 because CO2 is being
461
blown off and lactic acid from anaerobic glycolysis is being converted to CO2 (see below). After exercise, R may fall for a while to 0.50 or less. In metabolic acidosis, R rises because respiratory compensation for the acidosis causes the amount of CO2 expired to rise (see Chapter 39). In severe acidosis, R may be greater than 1.00. In metabolic alkalosis, R falls. The O2 consumption and CO2 production of an organ can be calculated at equilibrium by multiplying its blood flow per unit of time by the arteriovenous differences for O2 and CO2 across the organ, and the RQ can then be calculated. Data on the RQ of individual organs are of considerable interest in drawing inferences about the metabolic processes occurring in them. For example, the RQ of the brain is regularly 0.97– 0.99, indicating that its principal but not its only fuel is carbohydrate. During secretion of gastric juice, the stomach has a negative R because it takes up more CO2 from the arterial blood than it puts into the venous blood (see Chapter 26).
MEASURING THE METABOLIC RATE In determining the metabolic rate, O2 consumption is usually measured with some form of oxygen-filled spirometer and a CO2-absorbing system. Such a device is illustrated in Figure 27–8. The spirometer bell is connected to a pen that writes on a rotating drum as the bell moves up and down. The slope of a line joining the ends of each of the spirometer excursions is proportional to the O2 consumption. The amount of O2 (in milliliters) consumed per unit of time is corrected to standard temperature and pressure (see Chapter 35) and then converted to energy production by multiplying by 4.82 kcal/L of O2 consumed.
FACTORS AFFECTING THE METABOLIC RATE The metabolic rate is affected by many factors (Table 27–2). The most important is muscular exertion. O2 consumption is elevated not only during exertion but also for as long afterward as is necessary to repay the O2 debt (see Chapter 5). Recently ingested foods also increase the metabolic rate because of their specific dynamic action (SDA). The SDA of a food is the obligatory energy expenditure that occurs during its assimilation into the body. It takes 30 kcal to assimilate the amount of protein sufficient to raise the metabolic rate 100 kcal; 6 kcal to assimilate a similar amount of carbohydrate; and 5 kcal to assimilate a similar amount of fat. The cause of the SDA, which may last up to 6 h, is uncertain. Another factor that stimulates metabolism is the environmental temperature. The curve relating the metabolic rate to the environmental temperature is U-shaped. When the environmental temperature is lower than body temperature, heatproducing mechanisms such as shivering are activated and the metabolic rate rises. When the temperature is high enough to raise the body temperature, metabolic processes generally accelerate, and the metabolic rate rises about 14% for each degree Celsius of elevation.
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SECTION V Gastrointestinal Physiology
Pulleys A Oxygen bell
Pulley Rotating drum
Water seal
v
Inhalation tube
B
Volume
Breathing chamber
CO2 absorber
v Exhalation tube Mouthpiece
Time
FIGURE 27–8 Diagram of a modified Benedict apparatus, a recording spirometer used for measuring human O2 consumption, and the record obtained with it. The slope of the line AB is proportionate to the O 2 consumption. V: one-way check valve. The metabolic rate determined at rest in a room at a comfortable temperature in the thermoneutral zone 12 to 14 h after the last meal is called the basal metabolic rate (BMR). This value falls about 10% during sleep and up to 40% during prolonged starvation. The rate during normal daytime activities is, of course, higher than the BMR because of muscular activity and food intake. The maximum metabolic rate reached during exercise is often said to be 10 times the BMR, but trained athletes can increase their metabolic rate as much as 20-fold.
TABLE 27–2 Factors affecting the metabolic rate. Muscular exertion during or just before measurement Recent ingestion of food High or low environmental temperature Height, weight, and surface area Sex Age Growth Reproduction
The BMR of a man of average size is about 2000 kcal/d. Large animals have higher absolute BMRs, but the ratio of BMR to body weight in small animals is much greater. One variable that correlates well with the metabolic rate in different species is the body surface area. This would be expected, since heat exchange occurs at the body surface. The actual relation to body weight (W) would be BMR = 3.52W0.67 However, repeated measurements by numerous investigators have come up with a higher exponent, averaging 0.75: BMR = 3.52W0.75 Thus, the slope of the line relating metabolic rate to body weight is steeper than it would be if the relation were due solely to body area (Figure 27–9). The cause of the greater slope has been much debated but remains unsettled. For clinical use, the BMR is usually expressed as a percentage increase or decrease above or below a set of generally used standard normal values. Thus, a value of +65 means that the individual’s BMR is 65% above the standard for that age and sex. The decrease in metabolic rate is part of the explanation of why, when an individual is trying to lose weight, weight loss is initially rapid and then slows down.
Lactation Emotional state Body temperature Circulating levels of thyroid hormones Circulating epinephrine and norepinephrine levels
ENERGY BALANCE The first law of thermodynamics, the principle that states that energy is neither created nor destroyed when it is converted from one form to another, applies to living organisms as well as inanimate systems. One may therefore speak of an energy
CHAPTER 27 Digestion, Absorption, & Nutritional Principles
105
CLINICAL BOX 27–4
Heat production (kcal/d)
Elephant
104
The Malabsorption Syndrome
Cow Chimpanzee Goat Sheep Rabbits Macaque Cats
103 102 Rat
Steer
Guinea pig
101 Mouse
100 10−3
10−2 10−1 100 101 102 Body weight (kg)
463
103
104
FIGURE 27–9
Correlation between metabolic rate and body weight, plotted on logarithmic scales. The slope of the colored line is 0.75. The black line represents the way surface area increases with weight for geometrically similar shapes and has a slope of 0.67.
(Modified from Kleiber M and reproduced with permission from McMahon TA: Size and shape in biology. Science 1973;179:1201. Copyright © 1973 by the American Association for the Advancement of Science.)
balance between caloric intake and energy output. If the caloric content of the food ingested is less than the energy output, that is, if the balance is negative, endogenous stores are utilized. Glycogen, body protein, and fat are catabolized, and the individual loses weight. If the caloric value of the food intake exceeds energy loss due to heat and work and the food is properly digested and absorbed, that is, if the balance is positive, energy is stored, and the individual gains weight. To balance basal output so that the energy-consuming tasks essential for life can be performed, the average adult must take in about 2000 kcal/d. Caloric requirements above the basal level depend on the individual’s activity. The average sedentary student (or professor) needs another 500 kcal, whereas a lumberjack needs up to 3000 additional kcal per day.
NUTRITION The aim of the science of nutrition is the determination of the kinds and amounts of foods that promote health and well-being. This includes not only the problems of undernutrition but those of overnutrition, taste, and availability (Clinical Box 27–4). However, certain substances are essential constituents of any human diet. Many of these compounds have been mentioned in previous sections of this chapter, and a brief summary of the essential and desirable dietary components is presented below.
ESSENTIAL DIETARY COMPONENTS An optimal diet includes, in addition to sufficient water (see Chapter 38), adequate calories, protein, fat, minerals, and vitamins.
The digestive and absorptive functions of the small intestine are essential for life. However, the digestive and absorptive capacity of the intestine is larger than needed for normal function (the anatomic reserve). Removal of short segments of the jejunum or ileum generally does not cause severe symptoms, and compensatory hypertrophy and hyperplasia of the remaining mucosa occur. However, when more than 50% of the small intestine is resected or bypassed, the absorption of nutrients and vitamins is so compromised that it is very difficult to prevent malnutrition and wasting (malabsorption). Resection of the ileum also prevents the absorption of bile acids, and this leads in turn to deficient fat absorption. It also causes diarrhea because the unabsorbed bile salts enter the colon, where they activate chloride secretion (see Chapter 26). Other complications of intestinal resection or bypass include hypocalcemia, arthritis, hyperuricemia, and possibly fatty infiltration of the liver, followed by cirrhosis. Various disease processes can also impair absorption without a loss of intestinal length. The pattern of deficiencies that results is sometimes called the malabsorption syndrome. This pattern varies somewhat with the cause, but it can include deficient absorption of amino acids, with marked body wasting and, eventually, hypoproteinemia and edema. Carbohydrate and fat absorption are also depressed. Because of the defective fat absorption, the fat-soluble vitamins (vitamins A, D, E, and K) are not absorbed in adequate amounts. One of the most interesting conditions causing the malabsorption syndrome is the autoimmune disease celiac disease. This disease occurs in genetically predisposed individuals who have the major histocompatibility complex (MHC) class II antigen HLA-DQ2 or DQ8 (see Chapter 3). In these individuals gluten and closely related proteins cause intestinal T cells to mount an inappropriate immune response that damages the intestinal epithelial cells and results in a loss of villi and a flattening of the mucosa. The proteins are found in wheat, rye, barley, and to a lesser extent in oats—but not in rice or corn. When grains containing gluten are omitted from the diet, bowel function is generally restored to normal.
CALORIC INTAKE & DISTRIBUTION As noted above, the caloric value of the dietary intake must be approximately equal to the energy expended if body weight is to be maintained. In addition to the 2000 kcal/d necessary to meet basal needs, 500 to 2500 kcal/d (or more) are required to meet the energy demands of daily activities. The distribution of the calories among carbohydrate, protein, and fat is determined partly by physiologic factors and partly by taste and economic considerations. A daily protein
464
SECTION V Gastrointestinal Physiology
intake of 1 g/kg body weight to supply the eight nutritionally essential amino acids and other amino acids is desirable. The source of the protein is also important. Grade I proteins, the animal proteins of meat, fish, dairy products, and eggs, contain amino acids in approximately the proportions required for protein synthesis and other uses. Some of the plant proteins are also grade I, but most are grade II because they supply different proportions of amino acid and some lack one or more of the essential amino acids. Protein needs can be met with a mixture of grade II proteins, but the intake must be large because of the amino acid wastage. Fat is the most compact form of food, since it supplies 9.3 kcal/g. However, often it is also the most expensive. Indeed, internationally there is a reasonably good positive correlation between fat intake and standard of living. In the past, Western diets have contained large amounts (100 g/d or more). The evidence indicating that a high unsaturated/saturated fat ratio in the diet is of value in the prevention of atherosclerosis and the current interest in preventing obesity may change this. In Central and South American Indian communities where corn (carbohydrate) is the dietary staple, adults live without ill effects for years on a very low fat intake. Therefore, provided that the needs for essential fatty acids are met, a low-fat intake does not seem to be harmful, and a diet low in saturated fats is desirable. Carbohydrate is the cheapest source of calories and provides 50% or more of the calories in most diets. In the average middle-class American diet, approximately 50% of the calories come from carbohydrate, 15% from protein, and 35% from fat. When calculating dietary needs, it is usual to meet the protein requirement first and then split the remaining calories between fat and carbohydrate, depending on taste, income, and other factors. For example, a 65-kg man who is moderately active needs about 2800 kcal/d. He should eat at least 65 g of protein daily, supplying 267 (65 × 4.1) kcal. Some of this should be grade I protein. A reasonable figure for fat intake is 50 to 60 g. The rest of the caloric requirement can be met by supplying carbohydrate.
MINERAL REQUIREMENTS A number of minerals must be ingested daily for the maintenance of health. Besides those for which recommended daily dietary allowances have been set, a variety of different trace elements should be included. Trace elements are defined as elements found in tissues in minute amounts. Those believed to be essential for life, at least in experimental animals, are listed in Table 27–3. In humans, iron deficiency causes anemia. Cobalt is part of the vitamin B12 molecule, and vitamin B12 deficiency leads to megaloblastic anemia (see Chapter 32). Iodine deficiency causes thyroid disorders (see Chapter 20). Zinc deficiency causes skin ulcers, depressed immune responses, and hypogonadal dwarfism. Copper deficiency causes anemia and changes in ossification. Chromium deficiency causes insulin resistance. Fluorine deficiency increases the incidence of dental caries. Conversely, some minerals can be toxic when present in the body in excess. For example, severe iron overload causes hemo-
TABLE 27–3 Trace elements believed essential for life. Arsenic
Manganese
Chromium
Molybdenum
Cobalt
Nickel
Copper
Selenium
Fluorine
Silicon
Iodine
Vanadium
Iron
Zinc
chromatosis, copper excess causes brain damage (Wilson disease), and aluminum poisoning in patients with renal failure who are receiving dialysis treatment causes a rapidly progressive dementia that resembles Alzheimer disease (see Chapter 19). Sodium and potassium are also essential minerals, but listing them is academic, because it is very difficult to prepare a sodium-free or potassium-free diet. A low-salt diet is well tolerated for prolonged periods because of the compensatory mechanisms that conserve Na+.
VITAMINS Vitamins were discovered when it was observed that diets adequate in calories, essential amino acids, fats, and minerals failed to maintain health. The term vitamin has now come to refer to any organic dietary constituent necessary for life, health, and growth that does not function by supplying energy. Because there are minor differences in metabolism between mammalian species, some substances are vitamins in one species and not in another. The sources and functions of the major vitamins in humans are listed in Table 27–4. Most vitamins have important functions in intermediary metabolism or the special metabolism of the various organ systems. Those that are water-soluble (vitamin B complex, vitamin C) are easily absorbed, but the fat-soluble vitamins (vitamins A, D, E, and K) are poorly absorbed in the absence of bile or pancreatic lipase. Some dietary fat intake is necessary for their absorption, and in obstructive jaundice or disease of the exocrine pancreas, deficiencies of the fat-soluble vitamins can develop even if their intake is adequate. Vitamin A and vitamin D are bound to transfer proteins in the circulation. The α-tocopherol form of vitamin E is normally bound to chylomicrons. In the liver, it is transferred to very low density lipoprotein (VLDL) and distributed to tissues by an α-tocopherol transfer protein. When this protein is abnormal due to mutation of its gene in humans, there is cellular deficiency of vitamin E and the development of a condition resembling Friedreich ataxia. Two Na+-dependent L-ascorbic acid transporters have recently been isolated. One is found in the kidneys, intestines, and liver, and the other in the brain and eyes.
CHAPTER 27 Digestion, Absorption, & Nutritional Principles
TABLE 27–4
465
Vitamins essential or probably essential to human nutrition.a
Vitamin
Action
Deficiency Symptoms
Sources
Chemistry
A (A1, A2)
Constituents of visual pigments (see Chapter 12: Vision); necessary for fetal development and for cell development throughout life
Night blindness, dry skin
Yellow vegetables and fruit
H2C
Cofactor in decarboxylations
Beriberi, neuritis
H3 C
CH3
C
CH3
C (CH CH C CH)2 CH2OH
H2 C
C
CH3
C
Vitamin A1 alcohol (retinol).
H2
B complex Thiamin (vitamin B1)
Liver, unrefined cereal grains
NH2
CH3
Riboflavin (vitamin B2)
Constituent of flavoproteins
Glossitis, cheilosis
S
N
CH2
+
N
N
Liver, milk
CH3 CH2CH2OH
CH2(CHOH)3 CH2OH
H
N
C
H 3C
C
C
C
H3C
C
C
C N
C
Constituent of NAD+ and NADP+
Pellagra
Yeast, lean meat, liver
C O N H C O
H
Niacin
N
COOH
Can be synthesized in body from tryptophan.
N
Pyridoxine (vitamin B6)
Pantothenic acid
Biotin
Forms prosthetic group of certain decarboxylases and transaminases. Converted in body into pyridoxal phosphate and pyridoxamine phosphate
Convulsions, hyperirritability
Constituent of CoA
Dermatitis, enteritis, alopecia, adrenal insufficiency
Eggs, liver, yeast
Dermatitis, enteritis
Egg yolk, liver, tomatoes
Catalyzes CO2 “fixation” (in fatty acid synthesis, etc)
Yeast, wheat, corn, liver
CH2OH HO
CH2OH
H3C N
H HO
CH3 H O
C C H
C
H N
Cyanocobalamin (vitamin B12) C
Sprue, anemia. Neural tube defects in children born to folate-deficient women
Leafy green vegetables
N H
S
CH (CH2)4COOH
HOOC
N O
H
CH2 CHNH C
NH CH2
CH2
COOH
Coenzyme in amino acid metabolism. Stimulates erythropoiesis
Pernicious anemia (see Chapter 26: Overview of Gastrointestinal Function & Regulation)
Liver, meat, eggs, milk
Maintains prosthetic metal ions in their reduced form; scavenges free radicals
Scurvy
Citrus fruits, leafy green vegetables
H
C H
H 2C
Coenzymes for “1-carbon” transfer; involved in methylating reactions
C N CH2CH2COOH
O
H C
Folates (folic acid) and related compounds
C
CH3 OH
Folic acid
N NH2 N
N OH
Complex of four substituted pyrrole rings around a cobalt atom (see Chapter 26: Overview of Gastrointestinal Function & Regulation)
CH2OH HO
C H O
C H
CO
C
C
OH
OH
Ascorbic acid (synthesized in most mammals except guinea pigs and primates, including humans).
(continued )
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SECTION V Gastrointestinal Physiology
TABLE 27–4 Vitamin
Action
Deficiency Symptoms
Sources
Chemistry
D group
Increase intestinal absorption of calcium and phosphate (see Chapter 21: Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone)
Rickets
Fish liver
Family of sterols (see Chapter 21: Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone)
E group
Antioxidants; cofactors in electron transport in cytochrome chain?
Ataxia and other symptoms and signs of spinocerebellar dysfunction
Milk, eggs, meat, leafy vegetables
Hemorrhagic phenomena
Leafy green vegetables
K group
a
Vitamins essential or probably essential to human nutrition.a (Continued)
Catalyze γ carboxylation of glutamic acid residues on various proteins concerned with blood clotting
CH3
H2
HO
H2
H3 C
(CH2)3 CH3
CH3
CH3
CH3
CH (CH2)3 CH (CH2)3 CH
O CH3 CH3 α-Tocopherol; β- and γ-tocopherol also active.
O CH3
O
Vitamin K3; a large number of similar compounds have biological activity.
Choline is synthesized in the body in small amounts, but it has recently been added to the list of essential nutrients.
The diseases caused by deficiency of each of the vitamins are listed in Table 27–4. It is worth remembering, however, particularly in view of the advertising campaigns for vitamin pills and supplements, that very large doses of the fat-soluble vitamins are definitely toxic. Hypervitaminosis A is characterized by anorexia, headache, hepatosplenomegaly, irritability, scaly dermatitis, patchy loss of hair, bone pain, and hyperostosis. Acute vitamin A intoxication was first described by Arctic explorers, who developed headache, diarrhea, and dizziness after eating polar bear liver. The liver of this animal is particularly rich in vitamin A. Hypervitaminosis D is associated with weight loss, calcification of many soft tissues, and eventual renal failure. Hypervitaminosis K is characterized by gastrointestinal disturbances and anemia. Large doses of water-soluble vitamins have been thought to be less likely to cause problems because they can be rapidly cleared from the body. However, it has been demonstrated that ingestion of megadoses of pyridoxine (vitamin B6) can produce peripheral neuropathy.
CHAPTER SUMMARY ■
■
■
A typical mixed meal consists of carbohydrates, proteins, and lipids (the latter largely in the form of triglycerides). Each must be digested to allow its uptake into the body. Specific transporters carry the products of digestion into the body. In the process of carbohydrate assimilation, the epithelium can only transport monomers, whereas for proteins, short peptides can be absorbed in addition to amino acids. The protein assimilation machinery, which rests heavily on the proteases in pancreatic juice, is arranged such that these enzymes are not activated until they reach their substrates in the
■
■ ■
small intestinal lumen. This is accomplished by the restricted localization of an activating enzyme, enterokinase. Lipids face special challenges to assimilation given their hydrophobicity. Bile acids solubilize the products of lipolysis in micelles and accelerate their ability to diffuse to the epithelial surface. The assimilation of triglycerides is enhanced by this mechanism, whereas that of cholesterol and fat-soluble vitamins absolutely requires it. The catabolism of nutrients provides energy to the body in a controlled fashion, via stepwise oxidations and other reactions. A balanced diet is important for health, and certain substances obtained from the diet are essential to life. The caloric value of dietary intake must be approximately equal to energy expenditure for homeostasis.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. Maximum absorption of short-chain fatty acids produced by bacteria occurs in the A) stomach. B) duodenum. C) jejunum. D) ileum. E) colon. 2. Calcium absorption is increased by A) hypercalcemia. B) oxalates in the diet. C) iron overload. D) 1,25-dihydroxycholecalciferol. E) increased Na+ absorption.
CHAPTER 27 Digestion, Absorption, & Nutritional Principles 3. A decrease in which of the following would be expected in a child exhibiting a congenital absence of enterokinase? A) incidence of pancreatitis B) glucose absorption C) bile acid reabsorption D) gastric pH E) protein assimilation 4. In Hartnup disease (a defect in the transport of neutral amino acids), patients do not become deficient in these amino acids due to the activity of A) PepT1. B) brush border peptidases. C) Na+,K+ ATPase. D) cystic fibrosis transmembrane conductance regulator (CFTR). E) trypsin. 5. A newborn baby is brought to the pediatrician suffering from severe diarrhea that worsens with meals. The symptoms diminish when nutrients are delivered intravenously. The child most likely has a mutation in which of the following intestinal transporters? A) Na+,K+ ATPase B) NHE3 C) SGLT1 D) H+,K+ ATPase E) NKCC1
467
CHAPTER RESOURCES Andrews NC: Disorders of iron metabolism. N Engl J Med 1999;341:1986. Chong L, Marx J (editors): Lipids in the limelight. Science 2001;294:1861. Farrell RJ, Kelly CP: Celiac sprue. N Engl J Med 2002;346:180. Hofmann AF: Bile acids: The good, the bad, and the ugly. News Physiol Sci 1999;14:24. Levitt MD, Bond JH: Volume, composition and source of intestinal gas. Gastroenterology 1970;59:921. Mann NS, Mann SK: Enterokinase. Proc Soc Exp Biol Med 1994;206:114. Meier PJ, Stieger B: Molecular mechanisms of bile formation. News Physiol Sci 2000;15:89. Topping DL, Clifton PM: Short-chain fatty acids and human colonic function: Select resistant starch and nonstarch polysaccharides. Physiol Rev 2001;81:1031. Wright EM: The intestinal Na+/glucose cotransporter. Annu Rev Physiol 1993;55:575.
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28 C
Gastrointestinal Motility
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■
List the major forms of motility in the gastrointestinal tract and their roles in digestion and excretion. Distinguish between peristalsis and segmentation. Explain the electrical basis of gastrointestinal contractions and the role of basic electrical activity in governing motility patterns. Describe how gastrointestinal motility changes during fasting. Understand how food is swallowed and transferred to the stomach. Define the factors that govern gastric emptying and the abnormal response of vomiting. Define how the motility patterns of the colon subserve its function to desiccate and evacuate the stool.
INTRODUCTION The digestive and absorptive functions of the gastrointestinal system outlined in the previous chapter depend on a variety of mechanisms that soften the food, propel it through the length of the gastrointestinal tract (Table 28–1), and mix it with hepatic bile stored in the gallbladder and digestive enzymes secreted by the salivary glands and pancreas. Some of these
mechanisms depend on intrinsic properties of the intestinal smooth muscle. Others involve the operation of reflexes involving the neurons intrinsic to the gut, reflexes involving the central nervous system (CNS), paracrine effects of chemical messengers, and gastrointestinal hormones.
GENERAL PATTERNS OF MOTILITY
decreased by the autonomic input to the gut, but its occurrence is independent of the extrinsic innervation. Indeed, progression of the contents is not blocked by removal and resuture of a segment of intestine in its original position and is blocked only if the segment is reversed before it is sewn back into place. Peristalsis is an excellent example of the integrated activity of the enteric nervous system. It appears that local stretch releases serotonin, which activates sensory neurons that activate the myenteric plexus. Cholinergic neurons passing in a retrograde direction in this plexus activate neurons that release substance P and acetylcholine, causing smooth muscle contraction. At the same time, cholinergic neurons passing in an anterograde
PERISTALSIS Peristalsis is a reflex response that is initiated when the gut wall is stretched by the contents of the lumen, and it occurs in all parts of the gastrointestinal tract from the esophagus to the rectum. The stretch initiates a circular contraction behind the stimulus and an area of relaxation in front of it (Figure 28–1). The wave of contraction then moves in an oral-to-caudal direction, propelling the contents of the lumen forward at rates that vary from 2 to 25 cm/s. Peristaltic activity can be increased or
469
470
SECTION V Gastrointestinal Physiology
TABLE 28–1 Mean lengths of various segments of the gastrointestinal tract as measured by intubation in living humans. Segment
Length (cm)
Pharynx, esophagus, and stomach
65
Duodenum
25
Jejunum and ileum
260
Colon
110
Data from Hirsch JE, Ahrens EH Jr, Blankenhorn DH: Measurement of human intestinal length in vivo and some causes of variation. Gastroenterology 1956;31:274.
direction activate neurons that secrete NO, vasoactive intestinal polypeptide (VIP), and adenosine triphosphate (ATP), producing the relaxation ahead of the stimulus.
SEGMENTATION & MIXING When the meal is present, the enteric nervous system promotes a motility pattern that is related to peristalsis, but is designed to retard the movement of the intestinal contents along the length of the intestinal tract to provide time for digestion and absorption (Figure 28–1). This motility pattern is known as segmentation, and it provides for ample mixing of the intestinal contents (known as chyme) with the digestive juices. A segment of bowel contracts at both ends, and then a second contraction occurs in the center of the segment to force the chyme both backward and forward. Unlike peristalsis, therefore, retrograde movement of the chyme occurs routinely in
Isolated contraction
Segmentation
the setting of segmentation. This mixing pattern persists for as long as nutrients remain in the lumen to be absorbed. It presumably reflects programmed activity of the bowel dictated by the enteric nervous system, and can occur independent of central input, although the latter can modulate it.
BASIC ELECTRICAL ACTIVITY & REGULATION OF MOTILITY Except in the esophagus and the proximal portion of the stomach, the smooth muscle of the gastrointestinal tract has spontaneous rhythmic fluctuations in membrane potential between about –65 and –45 mV. This basic electrical rhythm (BER) is initiated by the interstitial cells of Cajal, stellate mesenchymal pacemaker cells with smooth muscle-like features that send long multiply branched processes into the intestinal smooth muscle. In the stomach and the small intestine, these cells are located in the outer circular muscle layer near the myenteric plexus; in the colon, they are at the submucosal border of the circular muscle layer. In the stomach and small intestine, there is a descending gradient in pacemaker frequency, and as in the heart, the pacemaker with the highest frequency usually dominates. The BER itself rarely causes muscle contraction, but spike potentials superimposed on the most depolarizing portions of the BER waves do increase muscle tension (Figure 28–2). The depolarizing portion of each spike is due to Ca2+ influx, and the repolarizing portion is due to K+ efflux. Many polypeptides and neurotransmitters affect the BER. For example, acetylcholine increases the number of spikes and the tension of the smooth muscle, whereas epinephrine decreases the number of spikes and the tension. The rate of the BER is about 4/min in the stomach. It is about 12/min in the duodenum and falls to about 8/min in the distal ileum. In the colon, the BER rate rises from about 2/min at the cecum to about 6/min at the sigmoid. The function of the BER is to coordinate peristaltic and other motor activity; contractions occur only during the depolarizing part of the waves. After vagotomy or transection of the stomach wall, for example, peristalsis in the stomach becomes irregular and chaotic.
MIGRATING MOTOR COMPLEX Peristalsis
Contraction Relaxation
FIGURE 28–1 Patterns of gastrointestinal motility and propulsion. An isolated contraction moves contents orally and aborally. Segmentation mixes contents over a short stretch of intestine, as indicated by the time sequence from left to right. In the diagram on the left, the vertical arrows indicate the sites of subsequent contraction. Peristalsis involves both contraction and relaxation, and moves contents aborally.
During fasting between periods of digestion, the pattern of electrical and motor activity in gastrointestinal smooth muscle becomes modified so that cycles of motor activity migrate from the stomach to the distal ileum. Each cycle, or migrating motor complex (MMC), starts with a quiescent period (phase I), continues with a period of irregular electrical and mechanical activity (phase II), and ends with a burst of regular activity (phase III). The MMCs are initiated by motilin, migrate aborally at a rate of about 5 cm/min, and occur at intervals of approximately 90 min. Gastric secretion, bile flow, and pancreatic secretion increase during each MMC. They likely serve to clear the stomach and small intestine of luminal contents in preparation for the next meal. They are immediately stopped by ingestion of food
CHAPTER 28 Gastrointestinal Motility
471
95 fL are called macrocytes; cells with MCVs < 80 fL are called
microcytes; cells with MCHs < 25 g/dL are called hypochromic.
Red blood cells, like other cells, shrink in solutions with an osmotic pressure greater than that of normal plasma. In solutions with a lower osmotic pressure they swell, become spherical rather than disk-shaped, and eventually lose their hemoglobin (hemolysis). The hemoglobin of hemolyzed red cells dissolves in the plasma, coloring it red. A 0.9% sodium chloride solution is isotonic with plasma. When osmotic fragility is normal, red cells begin to hemolyze when suspended in 0.5% saline; 50% lysis occurs in 0.40–0.42% saline, and lysis is complete in 0.35% saline. In hereditary spherocytosis (congenital hemolytic icterus), the cells are spherocytic in normal plasma and hemolyze more readily than normal cells in hypotonic sodium chloride solutions. Abnormal spherocytes are also trapped and destroyed in the spleen, meaning that hereditary spherocytosis is one of the most common causes of hereditary hemolytic anemia. The spherocytosis is caused by mutations in proteins that make up the membrane skeleton of the erythrocyte, which normally maintain the shape and flexibility of the red cell membrane, including spectrin, the transmembrane protein band 3, and the linker protein, ankyrin. The condition can be cured by splenectomy, but this is not without other risks. Red cells can also be lysed by drugs and infections. The susceptibility of red cells to hemolysis by these agents is increased by deficiency of the enzyme glucose 6phosphate dehydrogenase (G6PD), which catalyzes the initial step in the oxidation of glucose via the hexose monophosphate pathway (see Chapter 1). This pathway generates dihydronicotinamide adenine dinucleotide phosphate (NADPH), which is needed for the maintenance of normal red cell fragility. Severe G6PD deficiency also inhibits the killing of bacteria by granulocytes and predisposes to severe infections.
HEMOGLOBIN IN THE FETUS The blood of the human fetus normally contains fetal hemoglobin (hemoglobin F). Its structure is similar to that of hemoglobin A except that the β chains are replaced by γ chains; that is, hemoglobin F is α2γ2. The γ chains also contain 146 amino acid residues but have 37 that differ from those in the β chain. Fetal hemoglobin is normally replaced by adult hemoglobin soon after birth (Figure 32–8). In certain individuals, it fails to disappear and persists throughout life. In the body, its O2 content at a given PO2 is greater than that of adult hemoglobin because it binds 2,3-BPG less avidly. Hemoglobin F is critical to facilitate movement of O2 from the maternal to the fetal circulation, particularly at later stages of gestation where oxygen demand increases (see Chapter 34). In young embryos there are, in addition, ζ and ε chains, forming Gower 1
526
SECTION VI Cardiovascular Physiology
Circulation 3 × 1013 red blood cells 900 g hemoglobin
+
NH3
β
COO
−
β
1 × 1010 RBC 0.3 g hemoglobin per hour
1 × 1010 RBC 0.3 g hemoglobin per hour
Bone marrow
Tissue macrophage system
Iron Diet
Bile pigments in stool, urine
Amino acids
FIGURE 32–5
+
NH3
Small amount of iron COO
α
Red cell formation and destruction. RBC, red
+
blood cells.
hemoglobin (ζ2ε2) and Gower 2 hemoglobin (α2ε2). There are two copies of the α globin gene on human chromosome 16. In addition, there are five globin genes in tandem on chromosome 11 that encode β, γ, and δ globin chains and the two chains normally found only during fetal life. Switching from one form of hemoglobin to another during development seems to be regulated largely by oxygen availability, with relative hypoxia favoring the production of hemoglobin F both via direct effects on globin gene expression, as well as up-regulated production of erythropoietin.
SYNTHESIS OF HEMOGLOBIN The average normal hemoglobin content of blood is 16 g/dL in men and 14 g/dL in women, all of it in red cells. In the body of a 70-kg man, there are about 900 g of hemoglobin, and 0.3 g of hemoglobin is destroyed and 0.3 g synthesized every hour
HC
(Figure 32–5). The heme portion of the hemoglobin molecule is synthesized from glycine and succinyl-CoA (see Clinical Box 32–2).
CATABOLISM OF HEMOGLOBIN When old red blood cells are destroyed by tissue macrophages, the globin portion of the hemoglobin molecule is split off, and the heme is converted to biliverdin. The enzyme involved is a subtype of heme oxygenase (see Figure 29–4), and CO is formed in the process. CO may be an intercellular messenger, like NO (see Chapters 2 and 3).
M
Fe
V
HC
N
N
NH3
al: Physiologische Chemie. Springer, 1975.)
CH
CH3
α
1 nm
FIGURE 32–6 Diagrammatic representation of a molecule of hemoglobin A, showing the four subunits. There are two α and two β polypeptide chains, each containing a heme moiety. These moieties are represented by the disks. (Reproduced with permission from Harper HA et
CH = CH2
CH3
−
CH
M
CH3
+ O2
N
M
N
N
Fe
N
Heme CH = CH2
N
H 2C
HC
CH
CH2
N
P
V
HC
CH
COOH CH2 N (imidazole)
FIGURE 32–7
CH3
CH2
P N
COOH
M
N
O2
(imidazole)
Polypeptide chain
Polypeptide chain
Deoxygenated hemoglobin
Oxyhemoglobin
Reaction of heme with O2. The abbreviations M, V, and P stand for the groups shown on the molecule on the left.
CLINICAL BOX 32–2 Abnormalities of Hemoglobin Production There are two major types of inherited disorders of hemoglobin in humans: the hemoglobinopathies, in which abnormal globin polypeptide chains are produced, and the thalassemias and related disorders, in which the chains are normal in structure but produced in decreased amounts or absent because of defects in the regulatory portion of the globin genes. Mutant genes that cause the production of abnormal hemoglobins are widespread, and over 1000 abnormal hemoglobins have been described in humans. In one of the most common examples, hemoglobin S, the α chains are normal but the β chains have a single substitution of a valine residue for one glutamic acid, leading to sickle cell anemia (Table 32–3). When an abnormal gene inherited from one parent dictates formation of an abnormal hemoglobin (ie, when the individual is heterozygous), half the circulating hemoglobin is abnormal and half is normal. When identical abnormal genes are inherited from both parents, the individual is homozygous and all the hemoglobin is abnormal. It is theoretically possible to inherit two different abnormal hemoglobins, one from the father and one from the mother. Studies of the inheritance and geographic distribution of abnormal hemoglobins have made it possible in some cases to decide where the mutant gene originated and approximately how long ago the mutation occurred. In general, harmful mutations tend to die out, but mutant genes that confer traits with survival value persist and spread in the population. Many of the abnormal hemoglobins are harmless; however, some have abnormal O2 equilibriums, while others cause anemia. For example, hemoglobin S polymerizes at low O2 tensions, and this causes the red cells to become sickle-shaped, hemolyze, and form aggregates that block blood vessels. The sickle cell gene is an example of a gene that has persisted and spread in the population due to its beneficial effect when present in heterozygous form. It originated in Africa, and confers resistance to one type of malaria. In some parts of Africa, 40% of the population is heterozygous for hemoglobin S. There is a corresponding prevalence of 10% among African Americans in the United States. Hemoglobin F decreases the polymerization of deoxygenated hemoglobin S, and hydroxyurea stimulates production of hemoglobin F in children and adults. It has proved to be a very valuable agent for the treatment of sickle cell disease. In patients with severe sickle cell disease, bone marrow transplantation has also been shown to have some benefit.
In humans, most of the biliverdin is converted to bilirubin (Figure 32–9) and excreted in the bile (see Chapter 29). The iron from the heme is reused for hemoglobin synthesis. Exposure of the skin to white light converts bilirubin to lumirubin, which has a shorter half-life than bilirubin. Photo-
Globin chain synthesis (% of total)
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
50
α chain
40
γ chain (fetal) β chain (adult)
30 20
527
∋ and ζ chains (embryonic)
10 δ chain 0 3 6 Gestation (months)
FIGURE 32–8
Birth
3 6 Age (months)
Development of human hemoglobin chains.
therapy (exposure to light) is of value in treating infants with jaundice due to hemolysis. Iron is essential for hemoglobin synthesis; if blood is lost from the body and the iron deficiency is not corrected, iron deficiency anemia results. The metabolism of iron is discussed in Chapter 27.
BLOOD TYPES The membranes of human red cells contain a variety of blood group antigens, which are also called agglutinogens. The most important and best known of these are the A and B antigens, but there are many more.
THE ABO SYSTEM The A and B antigens are inherited as mendelian dominants, and individuals are divided into four major blood types on this basis. Type A individuals have the A antigen, type B have the B, type AB have both, and type O have neither. The A and B antigens are complex oligosaccharides that differ in their terminal sugar. An H gene codes for a fucose transferase that adds a terminal fucose, forming the H antigen that is usually present in individuals of all blood types (Figure 32–10). Individuals who are type A also express a second transferase that catalyzes placement of a terminal N-acetylgalactosamine on the H antigen, whereas individuals who are type B express a transferase that places a terminal galactose. Individuals who are type AB have both transferases. Individuals who are type O have neither, so the H antigen persists. Antibodies against red cell agglutinogens are called agglutinins. Antigens very similar to A and B are common in intestinal bacteria and possibly in foods to which newborn individuals are exposed. Therefore, infants rapidly develop antibodies against the antigens not present in their own cells. Thus, type A individuals develop anti-B antibodies, type B individuals develop anti-A antibodies, type O individuals develop both, and type AB individuals develop neither (Table 32–4). When the plasma of a type A individual is mixed with type B red cells, the anti-B antibodies cause the type B red
528
SECTION VI Cardiovascular Physiology
TABLE 32–3 Partial amino acid composition of normal human β chain, and some hemoglobins with abnormal β chains.a
Positions on Polypeptide Chain of Hemoglobin Hemoglobin
123
67
26
63
67
121
146
A (normal)
Val-His-Leu
Glu-Glu
Glu
His
Val
Glu
His
S (sickle cell)
Val
C
Lys
GSan Jose
Gly
E
Lys
MSaskatoon
Tyr
MMilwaukee
Glu
OArabia
Lys
a
Other hemoglobins have abnormal α chains. Abnormal hemoglobins that are very similar electrophoretically but differ slightly in composition are indicated by the same letter and a subscript indicating the geographic location where they were first discovered; hence, MSaskatoon and MMilwaukee.
M
cells to clump (agglutinate), as shown in Figure 32–11. The other agglutination reactions produced by mismatched plasma and red cells are summarized in Table 32–4. Blood typing is performed by mixing an individual’s red blood cells with antisera containing the various agglutinins on a slide and seeing whether agglutination occurs.
V
HC
OH N
M
OH
M
N
NH H N
P
TRANSFUSION REACTIONS
V
H 2C
CH
P
M
FIGURE 32–9
Bilirubin. The abbreviations M, V, and P stand for the groups shown on the molecule on the left in Figure 32–7.
G
G F
F
G G
G
H antigen
F
G
A antigen
G G
G
G
G
G
G
C
C
C
G
B antigen
F
= fucose
G
= galactose
G
= N-acetylgalactosamine
C
= ceramide
G
= glucose
FIGURE 32–10 of red blood cells.
= lipid bilayer
Antigens of the ABO system on the surface
Dangerous hemolytic transfusion reactions occur when blood is transfused into an individual with an incompatible blood type; that is, an individual who has agglutinins against the red cells in the transfusion. The plasma in the transfusion is usually so diluted in the recipient that it rarely causes agglutination even when the titer of agglutinins against the recipient’s cells is high. However, when the recipient’s plasma has agglutinins against the donor’s red cells, the cells agglutinate and hemolyze. Free hemoglobin is liberated into the plasma. The severity of the resulting transfusion reaction may vary from an asymptomatic minor rise in the plasma bilirubin level to severe jaundice and renal tubular damage leading to anuria and death. Incompatibilities in the ABO blood group system are summarized in Table 32–4. Persons with type AB blood are “universal recipients” because they have no circulating agglutinins and can be given blood of any type without developing a transfusion reaction due to ABO incompatibility. Type O individuals are “universal donors” because they lack A and B antigens, and type O blood can be given to anyone without producing a transfusion reaction due to ABO incompatibility. This does not mean, however, that blood should ever be transfused without being cross-matched except in the most extreme emergencies, since the possibility of reactions or sensitization due to incompatibilities in systems other than ABO systems always
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
Anti-B
529
Anti-A
Type A
Type B
Type AB
FIGURE 32–11
Red cell agglutination in incompatible plasma.
exists. In cross-matching, donor red cells are mixed with recipient plasma on a slide and checked for agglutination. It is advisable to check the action of the donor’s plasma on the recipient cells in addition, even though, as noted above, this is rarely a source of trouble. A procedure that has recently become popular is to withdraw the patient’s own blood in advance of elective surgery and then infuse this blood back (autologous transfusion) if a transfusion is needed during the surgery. With iron treatment, 1000 to 1500 mL can be withdrawn over a 3-wk period. The popularity of banking one’s own blood is due primarily to fear of transmission of infectious diseases by heterologous transfu-
TABLE 32–4 Summary of ABO system. Blood Type
Agglutinins in Plasma
Frequency in United States (%)
Plasma Agglutinates Red Cells of Type:
O
Anti-A, anti-B
45
A, B, AB
A
Anti-B
41
B, AB
B
Anti-A
10
A, AB
AB
None
4
None
sions, but of course another advantage is elimination of the risk of transfusion reactions.
INHERITANCE OF A & B ANTIGENS The A and B antigens are inherited as mendelian allelomorphs, A and B being dominants. For example, an individual with type B blood may have inherited a B antigen from each parent or a B antigen from one parent and an O from the other; thus, an individual whose phenotype is B may have the genotype BB (homozygous) or BO (heterozygous). When the blood types of the parents are known, the possible genotypes of their children can be stated. When both parents are type B, they could have children with genotype BB (B antigen from both parents), BO (B antigen from one parent, O from the other heterozygous parent), or OO (O antigen from both parents, both being heterozygous). When the blood types of a mother and her child are known, typing can prove that a man cannot be the father, although it cannot prove that he is the father. The predictive value is increased if the blood typing of the parties concerned includes identification of antigens other than the ABO agglutinogens. With the use of DNA fingerprinting (see Chapter 1), the exclusion rate for paternity rises to close to 100%.
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SECTION VI Cardiovascular Physiology
OTHER AGGLUTINOGENS In addition to the ABO system of antigens in human red cells, there are systems such as the Rh, MNSs, Lutheran, Kell, Kidd, and many others. There are over 500 billion possible known blood group phenotypes, and because undiscovered antigens undoubtedly exist, it has been calculated that the number of phenotypes is actually in the trillions. The number of blood groups in animals is as large as it is in humans. An interesting question is why this degree of polymorphism developed and has persisted through evolution. Certain diseases are more common in individuals with one blood type or another, but the differences are not great. One, the Duffy antigen, is a chemokine receptor. Many of the others seem to be cell recognition molecules, but the significance of a recognition code of this complexity is unknown.
THE RH GROUP Aside from the antigens of the ABO system, those of the Rh system are of the greatest clinical importance. The Rh factor, named for the rhesus monkey because it was first studied using the blood of this animal, is a system composed primarily of the C, D, and E antigens, although it actually contains many more. Unlike the ABO antigens, the system has not been detected in tissues other than red cells. D is by far the most antigenic component, and the term Rh-positive as it is generally used means that the individual has agglutinogen D. The D protein is not glycosylated, and its function is unknown. The Rh-negative individual has no D antigen and forms the antiD agglutinin when injected with D-positive cells. The Rh typing serum used in routine blood typing is anti-D serum. Eighty-five percent of Caucasians are D-positive and 15% are D-negative; over 99% of Asians are D-positive. Unlike the antibodies of the ABO system, anti-D antibodies do not develop without exposure of a D-negative individual to D-positive red cells by transfusion or entrance of fetal blood into the maternal circulation. However, D-negative individuals who have received a transfusion of D-positive blood (even years previously) can have appreciable anti-D titers and thus may develop transfusion reactions when transfused again with Dpositive blood.
HEMOLYTIC DISEASE OF THE NEWBORN Another complication due to Rh incompatibility arises when an Rh-negative mother carries an Rh-positive fetus. Small amounts of fetal blood leak into the maternal circulation at the time of delivery, and some mothers develop significant titers of anti-Rh agglutinins during the postpartum period. During the next pregnancy, the mother’s agglutinins cross the placenta to the fetus. In addition, there are some cases of fetal–maternal hemorrhage during pregnancy, and sensitization can occur during pregnancy. In any case, when anti-Rh agglutinins cross the placenta to an Rh-positive fetus, they can cause
hemolysis and various forms of hemolytic disease of the newborn (erythroblastosis fetalis). If hemolysis in the fetus is severe, the infant may die in utero or may develop anemia, severe jaundice, and edema (hydrops fetalis). Kernicterus, a neurologic syndrome in which unconjugated bilirubin is deposited in the basal ganglia, may also develop, especially if birth is complicated by a period of hypoxia. Bilirubin rarely penetrates the brain in adults, but it does in infants with erythroblastosis, possibly in part because the blood–brain barrier is more permeable in infancy. However, the main reasons that the concentration of unconjugated bilirubin is very high in this condition are that production is increased and the bilirubin-conjugating system is not yet mature. About 50% of Rh-negative individuals are sensitized (develop an anti-Rh titer) by transfusion of Rh-positive blood. Because sensitization of Rh-negative mothers by carrying an Rh-positive fetus generally occurs at birth, the first child is usually normal. However, hemolytic disease occurs in about 17% of the Rh-positive fetuses born to Rh-negative mothers who have previously been pregnant one or more times with Rh-positive fetuses. Fortunately, it is usually possible to prevent sensitization from occurring the first time by administering a single dose of anti-Rh antibodies in the form of Rh immune globulin during the postpartum period. Such passive immunization does not harm the mother and has been demonstrated to prevent active antibody formation by the mother. In obstetric clinics, the institution of such treatment on a routine basis to unsensitized Rh-negative women who have delivered an Rhpositive baby has reduced the overall incidence of hemolytic disease by more than 90%. In addition, fetal Rh typing with material obtained by amniocentesis or chorionic villus sampling is now possible, and treatment with a small dose of Rh immune serum will prevent sensitization during pregnancy.
PLASMA The fluid portion of the blood, the plasma, is a remarkable solution containing an immense number of ions, inorganic molecules, and organic molecules that are in transit to various parts of the body or aid in the transport of other substances. Normal plasma volume is about 5% of body weight, or roughly 3500 mL in a 70-kg man. Plasma clots on standing, remaining fluid only if an anticoagulant is added. If whole blood is allowed to clot and the clot is removed, the remaining fluid is called serum. Serum has essentially the same composition as plasma, except that its fibrinogen and clotting factors II, V, and VIII (Table 32–5) have been removed and it has a higher serotonin content because of the breakdown of platelets during clotting.
PLASMA PROTEINS The plasma proteins consist of albumin, globulin, and fibrinogen fractions. Most capillary walls are relatively impermeable to the proteins in plasma, and the proteins therefore exert
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
TABLE 32–5 System for naming blood-clotting factors.
a
Factora
Names
I
Fibrinogen
II
Prothrombin
III
Thromboplastin
IV
Calcium
V
Proaccelerin, labile factor, accelerator globulin
VII
Proconvertin, SPCA, stable factor
VIII
Antihemophilic factor (AHF), antihemophilic factor A, antihemophilic globulin (AHG)
IX
Plasma thromboplastic component (PTC), Christmas factor, antihemophilic factor B
X
Stuart–Prower factor
XI
Plasma thromboplastin antecedent (PTA), antihemophilic factor C
XII
Hageman factor, glass factor
XIII
Fibrin-stabilizing factor, Laki–Lorand factor
HMW-K
High-molecular-weight kininogen, Fitzgerald factor
Pre-Ka
Prekallikrein, Fletcher factor
Ka
Kallikrein
PL
Platelet phospholipid
531
much of the rest of it is in the skin. Between 6% and 10% of the exchangeable pool is degraded per day, and the degraded albumin is replaced by hepatic synthesis of 200 to 400 mg/kg/ d. The albumin is probably transported to the extravascular areas by vesicular transport across the walls of the capillaries (see Chapter 2). Albumin synthesis is carefully regulated. It is decreased during fasting and increased in conditions such as nephrosis in which there is excessive albumin loss.
HYPOPROTEINEMIA Plasma protein levels are maintained during starvation until body protein stores are markedly depleted. However, in prolonged starvation and in malabsorption syndromes due to intestinal diseases, plasma protein levels are low (hypoproteinemia). They are also low in liver disease, because hepatic protein synthesis is depressed, and in nephrosis, because large amounts of albumin are lost in the urine. Because of the decrease in the plasma oncotic pressure, edema tends to develop. Rarely, there is congenital absence of one or another plasma protein. An example of congenital protein deficiency is the congenital form of afibrinogenemia, characterized by defective blood clotting.
HEMOSTASIS Hemostasis is the process of forming clots in the walls of damaged blood vessels and preventing blood loss while maintaining blood in a fluid state within the vascular system. A collection of complex interrelated systemic mechanisms operates to maintain a balance between coagulation and anticoagulation.
Factor VI is not a separate entity and has been dropped.
RESPONSE TO INJURY an osmotic force of about 25 mm Hg across the capillary wall (oncotic pressure; see Chapter 1) that pulls water into the blood. The plasma proteins are also responsible for 15% of the buffering capacity of the blood (see Chapter 39) because of the weak ionization of their substituent COOH and NH2 groups. At the normal plasma pH of 7.40, the proteins are mostly in the anionic form (see Chapter 1). Plasma proteins may have specific functions (eg, antibodies and the proteins concerned with blood clotting), whereas others function as carriers for various hormones, other solutes, and drugs.
ORIGIN OF PLASMA PROTEINS Circulating antibodies are manufactured by lymphocytes. Most of the other plasma proteins are synthesized in the liver. These proteins and their principal functions are listed in Table 32–6. Data on the turnover of albumin show that its synthesis plays an important role in the maintenance of normal levels. In normal adult humans, the plasma albumin level is 3.5 to 5.0 g/dL, and the total exchangeable albumin pool is 4.0 to 5.0 g/ kg body weight; 38–45% of this albumin is intravascular, and
When a small blood vessel is transected or damaged, the injury initiates a series of events (Figure 32–12) that lead to the formation of a clot. This seals off the damaged region and prevents further blood loss. The initial event is constriction of the vessel and formation of a temporary hemostatic plug of platelets that is triggered when platelets bind to collagen and aggregate. This is followed by conversion of the plug into the definitive clot. The constriction of an injured arteriole or small artery may be so marked that its lumen is obliterated, at least temporarily. The vasoconstriction is due to serotonin and other vasoconstrictors liberated from platelets that adhere to the walls of the damaged vessels.
THE CLOTTING MECHANISM The loose aggregation of platelets in the temporary plug is bound together and converted into the definitive clot by fibrin. Fibrin formation involves a cascade of enzymatic reactions and a series of numbered clotting factors (Table 32–5). The fundamental reaction is conversion of the soluble plasma protein fibrinogen to insoluble fibrin (Figure 32–13). The process
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SECTION VI Cardiovascular Physiology
TABLE 32–6 Some of the proteins synthesized by the liver: Physiologic functions and properties.
a
Serum or Plasma Concentration
Name
Principal Function
Binding Characteristics
Albumin
Binding and carrier protein; osmotic regulator
Hormones, amino acids, steroids, vitamins, fatty acids
Orosomucoid
Uncertain; may have a role in inflammation
α1-Antiprotease
Trypsin and general protease inhibitor
Proteases in serum and tissue secretions
1.3–1.4 mg/dL
α-Fetoprotein
Osmotic regulation; binding and carrier proteina
Hormones, amino acids
Found normally in fetal blood
α2-Macroglobulin
Inhibitor of serum endoproteases
Proteases
150–420 mg/dL
Antithrombin-III
Protease inhibitor of intrinsic coagulation system
1:1 binding to proteases
17–30 mg/dL
Ceruloplasmin
Transport of copper
Six atoms copper/mol
15–60 mg/dL
C-reactive protein
Uncertain; has role in tissue inflammation
Complement C1q
< 1 mg/dL; rises in inflammation
Fibrinogen
Precursor to fibrin in hemostasis
Haptoglobin
Binding, transport of cell-free hemoglobin
Hemoglobin 1:1 binding
40–180 mg/dL
Hemopexin
Binds to porphyrins, particularly heme for heme recycling
1:1 with heme
50–100 mg/dL
Transferrin
Transport of iron
Two atoms iron/mol
3.0–6.5 mg/dL
Apolipoprotein B
Assembly of lipoprotein particles
Lipid carrier
Angiotensinogen
Precursor to pressor peptide angiotensin II
Proteins, coagulation factors II, VII, IX, X
Blood clotting
Antithrombin C, protein C
Inhibition of blood clotting
Insulinlike growth factor I
Mediator of anabolic effects of growth hormone
IGF-I receptor
Steroid hormone-binding globulin
Carrier protein for steroids in bloodstream
Steroid hormones
3.3 mg/dL
Thyroxine-binding globulin
Carrier protein for thyroid hormone in bloodstream
Thyroid hormones
1.5 mg/dL
Transthyretin (thyroidbinding prealbumin)
Carrier protein for thyroid hormone in bloodstream
Thyroid hormones
25 mg/dL
4500–5000 mg/dL
Trace; rises in inflammation
200–450 mg/dL
20 mg/dL
The function of alpha-fetoprotein is uncertain, but because of its structural homology to albumin it is often assigned these functions.
involves the release of two pairs of polypeptides from each fibrinogen molecule. The remaining portion, fibrin monomer, then polymerizes with other monomer molecules to form fibrin. The fibrin is initially a loose mesh of interlacing strands. It is converted by the formation of covalent cross-linkages to a dense, tight aggregate (stabilization). This latter reaction is catalyzed by activated factor XIII and requires Ca2+. The conversion of fibrinogen to fibrin is catalyzed by thrombin. Thrombin is a serine protease that is formed from its circulating precursor, prothrombin, by the action of activated factor X. It has additional actions, including activation
of platelets, endothelial cells, and leukocytes via so-called proteinase activated receptors, which are G protein-coupled. Factor X can be activated by either of two systems, known as intrinsic and extrinsic (Figure 32–13). The initial reaction in the intrinsic system is conversion of inactive factor XII to active factor XII (XIIa). This activation, which is catalyzed by high-molecular-weight kininogen and kallikrein (see Chapter 33), can be brought about in vitro by exposing the blood to glass, or in vivo by collagen fibers underlying the endothelium. Active factor XII then activates factor XI, and active factor XI activates factor IX. Activated factor IX forms a complex with
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
533
INTRINSIC SYSTEM
Injury to wall of blood vessel
HMW kininogen Kallikrein Contraction
Collagen
Tissue thromboplastin
XII
XIIa HMW kininogen
Platelet reactions
Activation of coagulation
Loose platelet aggregation
Thrombin
EXTRINSIC SYSTEM
XIa
XI
TPL TFI IX
Temporary hemostatic plug
Definitive hemostatic plug
VIII
Limiting reactions
X
FIGURE 32–12
Thrombogenesis, N Engl J Med 1967;267:622.)
ANTICLOTTING MECHANISMS The tendency of blood to clot is balanced in vivo by reactions that prevent clotting inside the blood vessels, break down any clots that do form, or both. These reactions include the interaction between the platelet-aggregating effect of thromboxane A2 and the antiaggregating effect of prostacyclin, which causes clots to form at the site when a blood vessel is injured but keeps the vessel lumen free of clot (see Chapter 33 and Clinical Box 32–3). Antithrombin III is a circulating protease inhibitor that binds to serine proteases in the coagulation system, blocking their activity as clotting factors. This binding is facilitated by heparin, a naturally occurring anticoagulant that is a mixture of sulfated polysaccharides with molecular weights averaging 15,000–18,000. The clotting factors that are inhibited are the active forms of factors IX, X, XI, and XII. The endothelium of the blood vessels also plays an active role in preventing the extension of clots. All endothelial cells except those in the cerebral microcirculation produce thrombomodulin, a thrombin-binding protein, on their surfaces. In
VIIA Ca2+ PL TPL
Prothrombin
Thrombin
Fibrin
Fibrinogen
XIII
XIIIa
VII
Xa PL Ca2+ Va
V
Summary of reactions involved in hemostasis. The dashed arrow indicates inhibition. (Modified from Deykin D:
active factor VIII, which is activated when it is separated from von Willebrand factor. The complex of IXa and VIIIa activate factor X. Phospholipids from aggregated platelets (PL) and Ca2+ are necessary for full activation of factor X. The extrinsic system is triggered by the release of tissue thromboplastin, a protein–phospholipid mixture that activates factor VII. Tissue thromboplastin and factor VII activate factors IX and X. In the presence of PL, Ca2+, and factor V, activated factor X catalyzes the conversion of prothrombin to thrombin. The extrinsic pathway is inhibited by a tissue factor pathway inhibitor that forms a quaternary structure with tissue thromboplastin (TPL), factor VIIa, and factor Xa.
IXa PL Ca2+ VIIIa
Sta
biliz
atio
n
FIGURE 32–13
The clotting mechanism. a, active form of clotting factor. TPL, tissue thromboplastin; TFI, tissue factor pathway inhibitor. For other abbreviations, see Table 32–5.
circulating blood, thrombin is a procoagulant that activates factors V and VIII, but when it binds to thrombomodulin, it becomes an anticoagulant in that the thrombomodulin– thrombin complex activates protein C (Figure 32–14). Activated protein C (APC), along with its cofactor protein S, inactivates factors V and VIII and inactivates an inhibitor of tissue plasminogen activator, increasing the formation of plasmin. Plasmin (fibrinolysin) is the active component of the plasminogen (fibrinolytic) system (Figure 32–14). This enzyme lyses fibrin and fibrinogen, with the production of fibrinogen degradation products (FDP) that inhibit thrombin. Plasmin is formed from its inactive precursor, plasminogen, by the action of thrombin and tissue-type plasminogen activator (t-PA). It is also activated by urokinase-type plasminogen activator (u-PA). If the t-PA gene or the u-PA gene is knocked out in mice, some fibrin deposition occurs and clot lysis is slowed. However, when both are knocked out, spontaneous fibrin deposition is extensive. Human plasminogen consists of a 560-amino-acid heavy chain and a 241-amino-acid light chain. The heavy chain, with glutamate at its amino terminal, is folded into five loop structures, each held together by three disulfide bonds (Figure 32–15). These loops are called kringles because of their
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SECTION VI Cardiovascular Physiology
Endothelial cell
CLINICAL BOX 32–3
Thrombomodulin Thrombin
Abnormalities of Hemostasis In addition to clotting abnormalities due to platelet disorders, hemorrhagic diseases can be produced by selective deficiencies of most of the clotting factors (Table 32–7). Hemophilia A, which is caused by factor VIII deficiency, is relatively common. The disease has been treated with factor VIII-rich preparations made from plasma, or, more recently, factor VIII produced by recombinant DNA techniques. von Willebrand factor deficiency likewise causes a bleeding disorder (von Willebrand disease) by reducing platelet adhesion and by lowering plasma factor VIII. The condition can be congenital or acquired. The large von Willebrand molecule is subject to cleavage and resulting inactivation by the plasma metalloprotease ADAM 13 in vascular areas where fluid shear stress is elevated. Finally, when absorption of vitamin K is depressed along with absorption of other fat-soluble vitamins (see Chapter 27), the resulting clotting factor deficiencies may cause the development of a significant bleeding tendency. Formation of clots inside blood vessels is called thrombosis to distinguish it from the normal extravascular clotting of blood. Thromboses are a major medical problem. They are particularly prone to occur where blood flow is sluggish because the slow flow permits activated clotting factors to accumulate instead of being washed away. They also occur in vessels when the intima is damaged by atherosclerotic plaques, and over areas of damage to the endocardium. They frequently occlude the arterial supply to the organs in which they form, and bits of thrombus (emboli) sometimes break off and travel in the bloodstream to distant sites, damaging other organs. An example is obstruction of the pulmonary artery or its branches by thrombi from the leg veins (pulmonary embolism). Congenital absence of protein C leads to uncontrolled intravascular coagulation and, in general, death in infancy. If this condition is diagnosed and treatment is instituted, the coagulation defect disappears. Resistance to activated protein C is another cause of thrombosis, and this condition is common. It is due to a point mutation in the gene for factor V, which prevents activated protein C from inactivating the factor. Mutations in protein S and antithrombin III may less commonly increase the incidence of thrombosis. Disseminated intravascular coagulation is another serious complication of septicemia, extensive tissue injury, and other diseases in which fibrin is deposited in the vascular system and many small- and medium-sized vessels are thrombosed. The increased consumption of platelets and coagulation factors causes bleeding to occur at the same time. The cause of the condition appears to be increased generation of thrombin due to increased TPL activity without adequate tissue factor inhibitory pathway activity.
Protein C
Activated protein C (APC) + Protein S
VIlIa
Inactive VIIIa
Va
Inactive Va
Inactivates inhibitor of tissue plasminogen activator (t-PA) Plasminogen
Plasmin Thrombin t-PA, u-PA Lyses fibrin
FIGURE 32–14
The fibrinolytic system and its regulation by
protein C.
resemblance to a Danish pastry of the same name. The kringles are lysine-binding sites by which the molecule attaches to fibrin and other clot proteins, and they are also found in prothrombin. Plasminogen is converted to active plasmin when t-PA hydrolyzes the bond between Arg 560 and Val 561. Plasminogen receptors are located on the surfaces of many different types of cells and are plentiful on endothelial cells. When plasminogen binds to its receptors, it becomes activated, so intact blood vessel walls are provided with a mechanism that discourages clot formation. Human t-PA is now produced by recombinant DNA techniques for clinical use in myocardial infarction and stroke. Streptokinase, a bacterial enzyme, is also fibrinolytic and is also used in the treatment of early myocardial infarction (see Chapter 34).
t-PA
Asn
Ser Glu
FIGURE 32–15
Structure of human plasminogen. Note the Glu at the amino terminal, the Asn at the carboxyl terminal, and five uniquely shaped loop structures (kringles). Hydrolysis by t-PA at the arrow separates the carboxyl terminal light chain from the amino terminal heavy chain but leaves the disulfide bonds intact. This activates the molecule. (Modified and reproduced with permission from Bachman F, in: Thrombosis and
Hemostasis. Verstraete M et al [editors]. Leuven University Press, 1987.)
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
TABLE 32–7 Examples of diseases due to deficiency of clotting factors. Deficiency of Factor:
Clinical Syndrome
Cause
I
Afibrinogenemia
Depletion during pregnancy with premature separation of placenta; also congenital (rare)
II
Hypoprothrombinemia (hemorrhagic tendency in liver disease)
Decreased hepatic synthesis, usually secondary to vitamin K deficiency
V
Parahemophilia
Congenital
VII
Hypoconvertinemia
Congenital
VIII
Hemophilia A (classic hemophilia)
Congenital defect due to various abnormalities of the gene on X chromosome that codes for factor VIII; disease is therefore inherited as sex-linked characteristic
IX
Hemophilia B (Christmas disease)
Congenital
X
Stuart–Prower factor deficiency
Congenital
XI
PTA deficiency
Congenital
XII
Hageman trait
Congenital
ANTICOAGULANTS As noted above, heparin is a naturally occurring anticoagulant that facilitates the action of antithrombin III. Low-molecularweight fragments with an average molecular weight of 5000 have been produced from unfractionated heparin, and these low-molecular-weight heparins are seeing increased clinical use because they have a longer half-life and produce a more predictable anticoagulant response than unfractionated heparin. The highly basic protein protamine forms an irreversible complex with heparin and is used clinically to neutralize heparin. In vivo, a plasma Ca2+ level low enough to interfere with blood clotting is incompatible with life, but clotting can be prevented in vitro if Ca2+ is removed from the blood by the addition of substances such as oxalates, which form insoluble salts with Ca2+, or chelating agents, which bind Ca2+. Coumarin derivatives such as dicumarol and warfarin are also effective anticoagulants. They inhibit the action of vitamin K, which is a necessary cofactor for the enzyme that catalyzes the conversion of glutamic acid residues to γ-carboxyglutamic acid residues. Six of the proteins involved in clotting require conversion of a number of glutamic acid residues to γ-carboxyglutamic acid residues before being released into the circulation, and hence all six are vitamin K-dependent. These proteins are factors II (prothrombin), VII, IX, and X, protein C, and protein S (see above).
535
LYMPH Lymph is tissue fluid that enters the lymphatic vessels. It drains into the venous blood via the thoracic and right lymphatic ducts. It contains clotting factors and clots on standing in vitro. In most locations, it also contains proteins that traverse capillary walls and return to the blood via the lymph. Its protein content is generally lower than that of plasma, which contains about 7 g/dL, but lymph protein content varies with the region from which the lymph drains (Table 32–8). Water-insoluble fats are absorbed from the intestine into the lymphatics, and the lymph in the thoracic duct after a meal is milky because of its high fat content (see Chapter 27). Lymphocytes enter the circulation principally through the lymphatics, and there are appreciable numbers of lymphocytes in thoracic duct lymph.
STRUCTURAL FEATURES OF THE CIRCULATION Here, we will first describe the two major cell types that make up the blood vessels and then how they are arranged into the various vessel types that subserve the needs of the circulation.
ENDOTHELIUM Located between the circulating blood and the media and adventitia of the blood vessels, the endothelial cells constitute a large and important organ. They respond to flow changes, stretch, a variety of circulating substances, and inflammatory mediators. They secrete growth regulators and vasoactive substances (see below and Chapter 33).
TABLE 32–8 Probable approximate protein content of lymph in humans. Source of Lymph
Protein Content (g/dL)
Choroid plexus
0
Ciliary body
0
Skeletal muscle
2
Skin
2
Lung
4
Gastrointestinal tract
4.1
Heart
4.4
Liver
6.2
Data largely from JN Diana.
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SECTION VI Cardiovascular Physiology
VASCULAR SMOOTH MUSCLE
The increased K+ efflux increases the membrane potential, shutting off voltage-gated Ca2+ channels and producing relaxation. The site of action of the Ca2+ sparks is the β1-subunit of the BK channel, and mice in which this subunit is knocked out develop increased vascular tone and blood pressure. Obviously, therefore, the sensitivity of the β1 subunit to Ca2+ sparks plays an important role in the control of vascular tone.
The smooth muscle in blood vessel walls has been one of the most-studied forms of visceral smooth muscle because of its importance in the regulation of blood pressure and hypertension. The membranes of the muscle cells contain various types of K+, Ca2+, and Cl– channels. Contraction is produced primarily by the myosin light chain mechanism described in Chapter 5. However, vascular smooth muscle also undergoes prolonged contractions that determine vascular tone. These may be due in part to the latch-bridge mechanism (see Chapter 5), but other factors also play a role. Some of the molecular mechanisms that appear to be involved in contraction and relaxation are shown in Figure 32–16. Vascular smooth muscle cells provide an interesting example of the way high and low cytosolic Ca2+ can have different and even opposite effects (see Chapter 2). In these cells, influx of Ca2+ via voltage-gated Ca2+ channels produces a diffuse increase in cytosolic Ca2+ that initiates contraction. However, the Ca2+ influx also initiates Ca2+ release from the sarcoplasmic reticulum via ryanodine receptors (see Chapter 5), and the high local Ca2+ concentration produced by these Ca2+ sparks increases the activity of Ca2+-activated K+ channels in the cell membrane. These are also known as big K or BK channels because K+ flows through them at a high rate.
ARTERIES & ARTERIOLES The characteristics of the various types of blood vessels are listed in Table 32–9. The walls of all arteries are made up of an outer layer of connective tissue, the adventitia; a middle layer of smooth muscle, the media; and an inner layer, the intima, made up of the endothelium and underlying connective tissue (Figure 32–17). The walls of the aorta and other arteries of large diameter contain a relatively large amount of elastic tissue, primarily located in the inner and external elastic laminas. They are stretched during systole and recoil on the blood during diastole. The walls of the arterioles contain less elastic tissue but much more smooth muscle. The muscle is innervated by noradrenergic nerve fibers, which function as constrictors, and in some instances by cholinergic fibers, which dilate the
K+ Interstitial fluid
A
VGCC
PC
R AA
PS
PLD
DAG
BK G Ca2+ Ca2+ sparks
CaM
PKC Raf
Cytoplasm
GTP GDP Choline PKC RR
MLCK active
SR
MLCK inactive
MEK
ADP ATP
CaP- P Rho-kinase
P -MLC
MLC
Actin-CaP Actin Actin-CaD
CPI-17
PI
MLC phosphatase
CPI-17- P Actin
Myosin
MAPK CaD- P
Contraction
FIGURE 32–16
Some of the established and postulated mechanisms involved in the contraction and relaxation of vascular smooth muscle. A, agonist; AA, arachidonic acid; BK, Ca+-activated K+ channel; G, heterotrimeric G protein; MLC, myosin light chain; MLCK, myosin light chain kinase; PLD, phospholipase D; R, receptor; SF, sarcoplasmic reticulum; VGCC, voltage-gated Ca 2+ channel; RR, ryanodine receptors. For other abbreviations, see Chapter 2 and Appendix. (Modified from Khahl R: Mechanisms of vascular smooth muscle contraction. Council for High Blood Pressure Newsletter, Spring 2001.)
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
TABLE 32–9 Characteristics of various types of blood vessels in humans. All Vessels of Each Type
Vessel
Lumen Diameter
Wall Thickness
Approximate Total CrossSectional Area (cm2)
Percentage of Blood Volume Containeda
Aorta
2.5 cm
2 mm
4.5
2
Artery
0.4 cm
1 mm
20
8
Arteriole
30 μm
20 μm
400
1
Capillary
5 μm
1 μm
4500
5
20 μm
2 μm
4000
Venule Vein
0.5 cm
0.5 mm
40
Vena cava
3 cm
1.5 mm
18
54
a
In systemic vessels; there is an additional 12% in the heart and 18% in the pulmonary circulation.
vessels. The arterioles are the major site of the resistance to blood flow, and small changes in their caliber cause large changes in the total peripheral resistance.
CAPILLARIES The arterioles divide into smaller muscle-walled vessels, sometimes called metarterioles, and these in turn feed into capillaries (Figure 32–18). The openings of the capillaries are surrounded on the upstream side by minute smooth muscle precapillary sphincters. It is unsettled whether the metarterioles are innervated, and it appears that the precapillary sphincters are not. However, they can of course respond to local or circulating vasoconstrictor substances. The capillaries are about 5 μm in diameter at the arterial end and 9 μm in diameter at the venous end. When the sphincters are dilated, the
Endothelium Internal elastic lamina
Intima Media
External elastic lamina
FIGURE 32–17
Adventitia
Structure of normal muscle artery. (Reproduced
with permission from Ross R, Glomset JA: The pathogenesis of atherosclerosis. N Engl J Med 1976;295:369.)
Artery > 50 μm Precapillary sphincter
Venous end of capillary 9 μm Arterial end of capillary 5 μm
Arteriole 20–50 μm
Metarteriole 10–15 μm
Collecting venule
537
Small venule 20 μm
True capillary
FIGURE 32–18
The microcirculation. Arterioles give rise to metarterioles, which give rise to capillaries. The capillaries drain via short collecting venules to the venules. The walls of the arteries, arterioles, and small venules contain relatively large amounts of smooth muscle. There are scattered smooth muscle cells in the walls of the metarterioles, and the openings of the capillaries are guarded by muscular precapillary sphincters. The diameters of the various vessels are also shown. (Courtesy of JN Diana.)
diameter of the capillaries is just sufficient to permit red blood cells to squeeze through in “single file.” As they pass through the capillaries, the red cells become thimble- or parachuteshaped, with the flow pushing the center ahead of the edges. This configuration appears to be due simply to the pressure in the center of the vessel whether or not the edges of the red blood cell are in contact with the capillary walls. The total area of all the capillary walls in the body exceeds 6300 m2 in the adult. The walls, which are about 1 μm thick, are made up of a single layer of endothelial cells. The structure of the walls varies from organ to organ. In many beds, including those in skeletal, cardiac, and smooth muscle, the junctions between the endothelial cells (Figure 32–19) permit the passage of molecules up to 10 nm in diameter. It also appears that plasma and its dissolved proteins are taken up by endocytosis, transported across the endothelial cells, and discharged by exocytosis (vesicular transport; see Chapter 2). However, this process can account for only a small portion of the transport across the endothelium. In the brain, the capillaries resemble the capillaries in muscle, but the junctions between endothelial cells are tighter, and transport across them is largely limited to small molecules. In most endocrine glands, the intestinal villi, and parts of the kidneys, the cytoplasm of the endothelial cells is attenuated to form gaps called fenestrations. These fenestrations are 20 to 100 nm in diameter and may permit the passage of larger molecules, although they appear to be closed by a thin membrane. An exception to this, however, is found in the liver, where the sinusoidal capillaries are extremely porous, the endothelium is discontinuous, and gaps occur between endothelial cells that are not closed by membranes (see Figure 29–2). Some of the gaps are 600 nm in diameter, and others may be as large
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SECTION VI Cardiovascular Physiology
Pericyte
Vesicles
Fenestrations or pores
Interdigitated junction
Basal lamina Pericyte
FIGURE 32–19
Cross-sections of capillaries. Left: Type of capillary found in muscle. Right: Fenestrated type of capillary. (Reproduced with
permission from Fawcett DW: Bloom and Fawcett, Textbook of Histology, 11th ed. Saunders, 1986.)
as 3000 nm. They therefore permit the passage of large molecules, including plasma proteins, which is important for hepatic function (see Chapter 29). The permeabilities of capillaries in various parts of the body, expressed in terms of their hydraulic conductivity, are summarized in Table 32–10. Capillaries and postcapillary venules have pericytes around their endothelial cells (Figure 32–19). These cells have long processes that wrap around the vessels. They are contractile and release a wide variety of vasoactive agents. They also synthesize and release constituents of the basement membrane and extracellular matrix. One of their physiologic functions appears to be regulation of flow through the junctions between endothelial cells, particularly in the presence of inflammation. They are closely related to the mesangial cells in the renal glomeruli (see Chapter 38).
LYMPHATICS The lymphatics serve to collect plasma and its constituents that have exuded from the capillaries into the interstitial space. They drain from the body tissues via a system of vessels that coalesce and eventually enter the right and left subclavian veins at their junctions with the respective internal jugular veins. The lymph vessels contain valves and regularly traverse lymph nodes along their course. The ultrastructure of the small lymph vessels differs from that of the capillaries in several details: No fenestrations are visible in the lymphatic endothelium; very little if any basal lamina is present under the endothelium; and the junctions between endothelial cells are open, with no tight intercellular connections.
ARTERIOVENOUS ANASTOMOSES TABLE 32–10 Hydraulic conductivity of capillaries in various parts of the body. Conductivitya
Organ Brain (excluding circumventricular organs)
3
Skin
100
Skeletal muscle
250
Lung
340
Heart
860
Gastrointestinal tract (intestinal mucosa)
Type of Endothelium
Continuous
13,000
Fenestrated Glomerulus in kidney a
15,000 –
3 –1
Units of conductivity are 10 13 cm s
Data courtesy of JN Diana.
–1
dyne .
In the fingers, palms, and ear lobes, short channels connect arterioles to venules, bypassing the capillaries. These arteriovenous (A-V) anastomoses, or shunts, have thick, muscular walls and are abundantly innervated, presumably by vasoconstrictor nerve fibers.
VENULES & VEINS The walls of the venules are only slightly thicker than those of the capillaries. The walls of the veins are also thin and easily distended. They contain relatively little smooth muscle, but considerable venoconstriction is produced by activity in the noradrenergic nerves to the veins and by circulating vasoconstrictors such as endothelins. Variations in venous tone are important in circulatory adjustments. The intima of the limb veins is folded at intervals to form venous valves that prevent retrograde flow. The way these valves function was first demonstrated by William Harvey in the 17th century. No valves are present in the very small veins, the great veins, or the veins from the brain and viscera.
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
ANGIOGENESIS When tissues grow, blood vessels must proliferate if the tissue is to maintain a normal blood supply. Therefore, angiogenesis, the formation of new blood vessels, is important during fetal life and growth to adulthood. It is also important in adulthood for processes such as wound healing, formation of the corpus luteum after ovulation, and formation of new endometrium after menstruation. Abnormally, it is important in tumor growth; if tumors do not develop a blood supply, they do not grow. During embryonic development, a network of leaky capillaries is formed in tissues from angioblasts: this process is sometimes called vasculogenesis. Vessels then branch off from nearby vessels, hook up with the capillaries, and provide them with smooth muscle, which brings about their maturation. Angiogenesis in adults is presumably similar, but consists of new vessel formation by branching from pre-existing vessels rather than from angioblasts. Many factors are involved in angiogenesis. A key compound is the protein growth factor vascular endothelial growth factor (VEGF). This factor exists in multiple isoforms, and there are three VEGF receptors that are tyrosine kinases, which also cooperate with nonkinase co-receptors known as neuropilins in some cell types. VEGF appears to be primarily responsible for vasculogenesis, whereas the budding of vessels that connect to the immature capillary network is regulated by other as yet unidentified factors. Some of the VEGF isoforms and receptors may play a more prominent role in the formation of lymphatic vessels (lymphangiogenesis) than that of blood vessels. The actions of VEGF and related factors have received considerable attention in recent years because of the requirement for angiogenesis in the development of tumors. VEGF antagonists and other angiogenesis inhibitors have now entered clinical practice as adjunctive therapies for many malignancies and are being tested as first line therapies as well.
BIOPHYSICAL CONSIDERATIONS FOR CIRCULATORY PHYSIOLOGY FLOW, PRESSURE, & RESISTANCE Blood always flows, of course, from areas of high pressure to areas of low pressure, except in certain situations when momentum transiently sustains flow (see Figure 31–3). The relationship between mean flow, mean pressure, and resistance in the blood vessels is analogous in a general way to the relationship between the current, electromotive force, and resistance in an electrical circuit expressed in Ohm’s law: force (E) Current (I) = Electromotive -------------------------------------------------------Resistance (R) Pressure (P) Flow (F) = --------------------------------Resistance (R)
539
Flow in any portion of the vascular system is equal to the effective perfusion pressure in that portion divided by the resistance. The effective perfusion pressure is the mean intraluminal pressure at the arterial end minus the mean pressure at the venous end. The units of resistance (pressure divided by flow) are dyne·s/cm5. To avoid dealing with such complex units, resistance in the cardiovascular system is sometimes expressed in R units, which are obtained by dividing pressure in mm Hg by flow in mL/s (see also Table 34–1). Thus, for example, when the mean aortic pressure is 90 mm Hg and the left ventricular output is 90 mL/s, the total peripheral resistance is 90 mm Hg ------------------------- = 1 R unit 90 mL/s
METHODS FOR MEASURING BLOOD FLOW Blood flow can be measured by cannulating a blood vessel, but this has obvious limitations. Various noninvasive devices have therefore been developed to measure flow. Most commonly, blood velocity can be measured with Doppler flow meters. Ultrasonic waves are sent into a vessel diagonally, and the waves reflected from the red and white blood cells are picked up by a downstream sensor. The frequency of the reflected waves is higher by an amount that is proportionate to the rate of flow toward the sensor because of the Doppler effect. Indirect methods for measuring the blood flow of various organs in humans include adaptations of the Fick and indicator dilution techniques described in Chapter 31. One example is the use of the Kety N2O method for measuring cerebral blood flow (see Chapter 34). Another is determination of the renal blood flow by measuring the clearance of para-aminohippuric acid (see Chapter 38). A considerable amount of data on blood flow in the extremities has been obtained by plethysmography (Figure 32–20). The forearm, for example, is sealed in a watertight chamber (plethysmograph). Changes in the volume of the forearm, reflecting changes in the amount of blood and interstitial fluid it contains, displace the water, and this displacement is measured with a volume recorder. When the venous drainage of the forearm is occluded, the rate of increase in the volume of the forearm is a function of the arterial blood flow (venous occlusion plethysmography).
APPLICABILITY OF PHYSICAL PRINCIPLES TO FLOW IN BLOOD VESSELS Physical principles and equations that describe the behavior of perfect fluids in rigid tubes have often been used indiscriminately to explain the behavior of blood in blood vessels. Blood vessels are not rigid tubes, and the blood is not a perfect fluid but a two-phase system of liquid and cells. Therefore, the behavior of the circulation deviates, sometimes markedly, from that predicted by these principles. However, the physical
540
SECTION VI Cardiovascular Physiology can be expressed by the ratio of inertial to viscous forces as follows:
Volume recorder
•
ρ DV Re = ------------η
Water
Rubber sleeve
FIGURE 32–20
Plethysmography.
principles are of value when used as an aid to understanding what goes on in the body.
LAMINAR FLOW The flow of blood in straight blood vessels, like the flow of liquids in narrow rigid tubes, is normally laminar. Within the blood vessels, an infinitely thin layer of blood in contact with the wall of the vessel does not move. The next layer within the vessel has a low velocity, the next a higher velocity, and so forth, velocity being greatest in the center of the stream (Figure 32–21). Laminar flow occurs at velocities up to a certain critical velocity. At or above this velocity, flow is turbulent. Laminar flow is silent, but turbulent flow creates sounds. The probability of turbulence is also related to the diameter of the vessel and the viscosity of the blood. This probability
where Re is the Reynolds number, named for the man who described the relationship; ρ is the density of the fluid; D is the diameter of the tube under consideration; V is the velocity of the flow; and η is the viscosity of the fluid. The higher the value of Re, the greater the probability of turbulence. When D is in cm, V is in cm/s–1, and η is in poise; flow is usually not turbulent if Re is less than 2000. When Re is more than 3000, turbulence is almost always present. Laminar flow can be disturbed at the branching points of arteries, and the resulting turbulence may increase the likelihood that atherosclerotic plaques will be deposited. Constriction of an artery likewise increases the velocity of blood flow through the constriction, producing turbulence and sound beyond the constriction (Figure 32–22). Examples are bruits heard over arteries constricted by atherosclerotic plaques and the sounds of Korotkoff heard when measuring blood pressure (see below). In humans, the critical velocity is sometimes exceeded in the ascending aorta at the peak of systolic ejection, but it is usually exceeded only when an artery is constricted. Turbulence occurs more frequently in anemia because the viscosity of the blood is lower. This may be the explanation of the systolic murmurs that are common in anemia.
SHEAR STRESS & GENE ACTIVATION Flowing blood creates a force on the endothelium that is parallel to the long axis of the vessel. This shear stress (γ) is proportionate to viscosity (η) times the shear rate (dy/dr), which
C
Upstream Vessel wall
Velocity
Laminar
High velocity
Turbulent Laminar
A 0 R
FIGURE 32–22 Flow
FIGURE 32–21
Diagram of the velocities of concentric laminas of a viscous fluid flowing in a tube, illustrating the parabolic distribution of velocities.
Top: Effect of constriction (C) on the profile of velocities in a blood vessel. The arrows indicate direction of velocity components, and their length is proportionate to their magnitude. Bottom: Range of velocities at each point along the vessel. In the area of turbulence, there are many different anterograde (A) and some retrograde (R) velocities. (Modified and reproduced with permission from Richards KE: Doppler echocardiography in diagnosis and quantification of vascular disease. Mod Concepts Cardiovasc Dis 1987;56:43. By permission of the American Heart Association.)
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TABLE 32–11 Genes in human, bovine, and rabbit endothelial cells that are affected by shear stress, and transcription factors involved.a
a
Gene
Transcription Factors
Endothelin-1
AP-1
VCAM-1
AP-1, NF-κB
ACE
SSRE, AP-1, Egr-1
Tissue factor
SP1, Egr-1
TM
AP-1
PDGF-α
SSRE, Egr-1
PDGF-β
SSRE
ICAM-1
SSRE, AP-1, NF-κB
TGF-β
SSRE, AP-1, NF-κB
Egr-1
SREs
c-fos
SSRE
c-jun
SSRE, AP-1
NOS 3
SSRE, AP-1, NF-κB
MCP-1
SSRE, AP-1, NF-κB
Site of end point (tongue)
Site of injection (antecubital vein)
FIGURE 32–23
Pathway traversed by the injected material when the arm-to-tongue circulation time is measured.
Acronyms are expanded in the Appendix.
Modified from Braddock M et al: Fluid shear stress modulation of gene expression in endothelial cells. News Physiol Sci 1998;13:241.
is the rate at which the axial velocity increases from the vessel wall toward the lumen. γ = η (dy/dr) Change in shear stress and other physical variables, such as cyclic strain and stretch, produce marked changes in the expression of genes by endothelial cells. The genes that are activated include those that produce growth factors, integrins, and related molecules (Table 32–11).
AVERAGE VELOCITY When considering flow in a system of tubes, it is important to distinguish between velocity, which is displacement per unit time (eg, cm/s), and flow, which is volume per unit time (eg, cm3/s). Velocity (V) is proportional to flow (Q) divided by the area of the conduit (A): • V = Q ---A
Therefore, Q = A × V, and if flow stays constant, velocity increases in direct proportion to any decrease in A (Figure 32–22). The average velocity of fluid movement at any point in a system of tubes in parallel is inversely proportional to the total
cross-sectional area at that point. Therefore, the average velocity of the blood is high in the aorta, declines steadily in the smaller vessels, and is lowest in the capillaries, which have 1000 times the total cross-sectional area of the aorta (Table 32–9). The average velocity of blood flow increases again as the blood enters the veins and is relatively high in the vena cava, although not so high as in the aorta. Clinically, the velocity of the circulation can be measured by injecting a bile salt preparation into an arm vein and timing the first appearance of the bitter taste it produces (Figure 32–23). The average normal arm-to-tongue circulation time is 15 s.
POISEUILLE–HAGEN FORMULA The relationship between the flow in a long narrow tube, the viscosity of the fluid, and the radius of the tube is expressed mathematically in the Poiseuille–Hagen formula: ⎞ × ⎛ 1 ⎞ × ⎛ r-4⎞ F = ( PA – PB ) × ⎛ π ⎝ --8-⎠ ⎝ --η-⎠ ⎝ --L⎠
where F = flow PA – PB = pressure difference between two ends of the tube η = viscosity r = radius of tube L = length of tube Because flow is equal to pressure difference divided by resistance (R), 8ηL R = ---------πr 4
Because flow varies directly and resistance inversely with the fourth power of the radius, blood flow and resistance in
SECTION VI Cardiovascular Physiology
vivo are markedly affected by small changes in the caliber of the vessels. Thus, for example, flow through a vessel is doubled by an increase of only 19% in its radius; and when the radius is doubled, resistance is reduced to 6% of its previous value. This is why organ blood flow is so effectively regulated by small changes in the caliber of the arterioles and why variations in arteriolar diameter have such a pronounced effect on systemic arterial pressure.
VISCOSITY & RESISTANCE
FIGURE 32–25
14
Relative viscosity
Critical closing pressure
Pressure
The resistance to blood flow is determined not only by the radius of the blood vessels (vascular hindrance) but also by the viscosity of the blood. Plasma is about 1.8 times as viscous as water, whereas whole blood is 3 to 4 times as viscous as water. Thus, viscosity depends for the most part on the hematocrit, that is, the percentage of the volume of blood occupied by red blood cells. The effect of viscosity in vivo deviates from that predicted by the Poiseuille–Hagen formula. In large vessels, increases in hematocrit cause appreciable increases in viscosity. However, in vessels smaller than 100 μm in diameter—that is, in arterioles, capillaries, and venules—the viscosity change per unit change in hematocrit is much less than it is in largebore vessels. This is due to a difference in the nature of flow through the small vessels. Therefore, the net change in viscosity per unit change in hematocrit is considerably smaller in the body than it is in vitro (Figure 32–24). This is why hematocrit changes have relatively little effect on the peripheral resistance except when the changes are large. In severe polycythemia, the increase in resistance does increase the work of the heart. Con-
Glass viscometer
12
Blood vessels
Flow
542
10 8 Hind limb
6 4
Relation of pressure to flow in thin-walled
blood vessels.
versely, in marked anemia, peripheral resistance is decreased, in part because of the decline in viscosity. Of course, the decrease in hemoglobin decreases the O2-carrying ability of the blood, but the improved blood flow due to the decrease in viscosity partially compensates for this. Viscosity is also affected by the composition of the plasma and the resistance of the cells to deformation. Clinically significant increases in viscosity are seen in diseases in which plasma proteins such as the immunoglobulins are markedly elevated as well as when red blood cells are abnormally rigid (hereditary spherocytosis).
CRITICAL CLOSING PRESSURE In rigid tubes, the relationship between pressure and flow of homogeneous fluids is linear, but in thin-walled blood vessels in vivo it is not. When the pressure in a small blood vessel is reduced, a point is reached at which no blood flows, even though the pressure is not zero (Figure 32–25). This is because the vessels are surrounded by tissues that exert a small but definite pressure on them, and when the intraluminal pressure falls below the tissue pressure, they collapse. In inactive tissues, for example, the pressure in many capillaries is low because the precapillary sphincters and metarterioles are constricted, and many of these capillaries are collapsed. The pressure at which flow ceases is called the critical closing pressure.
LAW OF LAPLACE
2 20
40 Hematocrit
60
80%
FIGURE 32–24
Effect of changes in hematocrit on the relative viscosity of blood measured in a glass viscometer and in the hind leg of a dog. In each case, the middle line represents the mean and the upper and lower lines the standard deviation.
(Reproduced with permission from Whittaker SRF, Winton FR: The apparent viscosity of blood flowing in the isolated hind limb of the dog, and its variation with corpuscular concentration. J Physiol [Lond] 1933;78:338.)
The relationship between distending pressure and tension is shown diagrammatically in Figure 32–26. It is perhaps surprising that structures as thin-walled and delicate as the capillaries are not more prone to rupture. The principal reason for their relative invulnerability is their small diameter. The protective effect of small size in this case is an example of the operation of the law of Laplace, an important physical principle with several other applications in physiology. This law states that tension in the wall of a cylinder (T) is equal to the product of the transmural pressure (P) and the radius (r) divided by the wall thickness (w): T = Pr/w
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
543
RESISTANCE & CAPACITANCE VESSELS
T
P
T
FIGURE 32–26
Relationship between distending pressure (P) and wall tension (T) in a hollow viscus.
The transmural pressure is the pressure inside the cylinder minus the pressure outside the cylinder, but because tissue pressure in the body is low, it can generally be ignored and P equated to the pressure inside the viscus. In a thin-walled viscus, w is very small and it too can be ignored, but it becomes a significant factor in vessels such as arteries. Therefore, in a thin-walled viscus, P = T divided by the two principal radii of curvature of the viscus: 1 1⎞ P = T ⎛ --+ ⎝ r - r----⎠ 1
In vivo, the veins are an important blood reservoir. Normally, they are partially collapsed and oval in cross-section. A large amount of blood can be added to the venous system before the veins become distended to the point where further increments in volume produce a large rise in venous pressure. The veins are therefore called capacitance vessels. The small arteries and arterioles are referred to as resistance vessels because they are the principal site of the peripheral resistance (see below). At rest, at least 50% of the circulating blood volume is in the systemic veins, 12% is in the heart cavities, and 18% is in the low-pressure pulmonary circulation. Only 2% is in the aorta, 8% in the arteries, 1% in the arterioles, and 5% in the capillaries (Table 32–9). When extra blood is administered by transfusion, less than 1% of it is distributed in the arterial system (the “high-pressure system”), and all the rest is found in the systemic veins, pulmonary circulation, and heart chambers other than the left ventricle (the “low-pressure system”).
ARTERIAL & ARTERIOLAR CIRCULATION The pressure and velocities of the blood in the various parts of the systemic circulation are summarized in Figure 32–27. The general relationships in the pulmonary circulation are similar, but the pressure in the pulmonary artery is 25/10 mm Hg or less.
2
In a sphere, r1 = r2, so P = 2T -----r
Systolic
80 Diastolic TA
40
Vena cava
Veins
Venules
Capillaries
Arterioles
Arteries
0
Aorta
Consequently, the smaller the radius of a blood vessel, the lower the tension in the wall necessary to balance the distending pressure. In the human aorta, for example, the tension at normal pressures is about 170,000 dynes/cm, and in the vena cava it is about 21,000 dynes/cm; but in the capillaries, it is approximately 16 dynes/cm. The law of Laplace also makes clear a disadvantage faced by dilated hearts. When the radius of a cardiac chamber is increased, a greater tension must be developed in the myocardium to produce any given pressure; consequently, a dilated heart must do more work than a nondilated heart. In the lungs, the radii of curvature of the alveoli become smaller during expiration, and these structures would tend to collapse because of the pull of surface tension if the tension were not reduced by the surface-tension-lowering agent, surfactant (see Chapter 35). Another example of the operation of this law is seen in the urinary bladder (see Chapter 38).
Pressure (mm Hg)
T P = ---r
Mean velocity (cm/s)
In a cylinder such as a blood vessel, one radius is infinite, so
120
Velocity RR 0
FIGURE 32–27
Diagram of the changes in pressure and velocity as blood flows through the systemic circulation. TA, total cross-sectional area of the vessels, which increases from 4.5 cm 2 in the aorta to 4500 cm2 in the capillaries (Table 32–9). RR, relative resistance, which is highest in the arterioles.
SECTION VI Cardiovascular Physiology
VELOCITY & FLOW OF BLOOD Although the mean velocity of the blood in the proximal portion of the aorta is 40 cm/s, the flow is phasic, and velocity ranges from 120 cm/s during systole to a negative value at the time of the transient backflow before the aortic valve closes in diastole. In the distal portions of the aorta and in the large arteries, velocity is also much greater in systole than it is in diastole. However, the vessels are elastic, and forward flow is continuous because of the recoil during diastole of the vessel walls that have been stretched during systole (Figure 32–28). Pulsatile flow appears to maintain optimal function of the tissues, apparently via distinct effects on gene transcription. If an organ is perfused with a pump that delivers a nonpulsatile flow, inflammatory markers are produced, there is a gradual rise in vascular resistance, and ultimately tissue perfusion fails.
ARTERIAL PRESSURE The pressure in the aorta and in the brachial and other large arteries in a young adult human rises to a peak value (systolic pressure) of about 120 mm Hg during each heart cycle and falls to a minimum (diastolic pressure) of about 70 mm Hg. The arterial pressure is conventionally written as systolic pressure over diastolic pressure, for example, 120/70 mm Hg. One millimeter of mercury equals 0.133 kPa, so in SI units (see Appendix) this value is 16.0/9.3 kPa. The pulse pressure, the difference between the systolic and diastolic pressures, is normally about 50
Ao
100
Mean pressure Diastolic pressure
70
0
1
2 3 Time (s)
4
FIGURE 32–29
Brachial artery pressure curve of a normal young human, showing the relation of systolic and diastolic pressure to mean pressure. The shaded area above the mean pressure line is equal to the shaded area below it.
mm Hg. The mean pressure is the average pressure throughout the cardiac cycle. Because systole is shorter than diastole, the mean pressure is slightly less than the value halfway between systolic and diastolic pressure. It can actually be determined only by integrating the area of the pressure curve (Figure 32–29); however, as an approximation, mean pressure equals the diastolic pressure plus one-third of the pulse pressure. The pressure falls very slightly in the large- and mediumsized arteries because their resistance to flow is small, but it falls rapidly in the small arteries and arterioles, which are the main sites of the peripheral resistance against which the heart pumps. The mean pressure at the end of the arterioles is 30 to 38 mm Hg. Pulse pressure also declines rapidly to about 5 mm Hg at the ends of the arterioles (Figure 32–26). The magnitude of the pressure drop along the arterioles varies considerably depending on whether they are constricted or dilated.
The pressures in Figure 32–28 are those in blood vessels at heart level. The pressure in any vessel below heart level is increased and that in any vessel above heart level is decreased by the effect of gravity. The magnitude of the gravitational effect is 0.77 mm Hg/cm of vertical distance above or below the heart at the density of normal blood. Thus, in an adult human in the upright position, when the mean arterial pressure at heart level is 100 mm Hg, the mean pressure in a large artery in the head (50 cm above the heart) is 62 mm Hg (100 – [0.77 × 50]) and the pressure in a large artery in the foot (105 cm below the heart) is 180 mm Hg (100 + [0.77 × 105]). The effect of gravity on venous pressure is similar (Figure 32–30).
0 Flow (mL/s)
Systolic pressure
120
EFFECT OF GRAVITY PA
10 PV 0 50
IVC
0 5
RA
0 0.2
Pressure (mm Hg)
544
0.4
0.6
METHODS OF MEASURING BLOOD PRESSURE
Time (s)
FIGURE 32–28
Changes in blood flow during the cardiac cycle in the dog. Diastole is followed by systole starting at 0.1 and again at 0.5 s. Flow patterns in humans are similar. Ao, aorta; PA, pulmonary artery; PV, pulmonary vein; IVC, inferior vena cava; RA, renal artery. (Reproduced with permission from Milnor WR: Pulsatile blood flow. N Engl J
Med 1972;287:27.)
If a cannula is inserted into an artery, the arterial pressure can be measured directly with a mercury manometer or a suitably calibrated strain gauge. When an artery is tied off beyond the point at which the cannula is inserted, an end pressure is recorded, flow in the artery is interrupted, and all the kinetic energy of flow is converted into pressure energy. If, alternatively,
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
545
−80 −60 −40
Increment in venous pressure due to gravity (mm Hg)
A B C
−20
0
0
20
20
40
40
60
60
80
80
Increment or decrement in mean arterial pressure (mm Hg)
eral pressure at the constriction is decreased and the narrowing tends to maintain itself.
FIGURE 32–30
Effects of gravity on arterial and venous pressure. The scale on the right indicates the increment (or decrement) in mean pressure in a large artery at each level. The mean pressure in all large arteries is approximately 100 mm Hg when they are at the level of the left ventricle. The scale on the left indicates the increment in venous pressure at each level due to gravity. The manometers on the left of the figure indicate the height to which a column of blood in a tube would rise if connected to an ankle vein (A), the femoral vein (B), or the right atrium (C), with the subject in the standing position. The approximate pressures in these locations in the recumbent position; that is, when the ankle, thigh, and right atrium are at the same level, are A, 10 mm Hg; B, 7.5 mm Hg; and C, 4.6 mm Hg.
a T tube is inserted into a vessel and the pressure is measured in the side arm of the tube, the recorded side pressure, under conditions where pressure drop due to resistance is negligible, is lower than the end pressure by the kinetic energy of flow. This is because in a tube or a blood vessel the total energy—the sum of the kinetic energy of flow and the potential energy—is constant (Bernoulli’s principle). It is worth noting that the pressure drop in any segment of the arterial system is due both to resistance and to conversion of potential into kinetic energy. The pressure drop due to energy lost in overcoming resistance is irreversible, since the energy is dissipated as heat; but the pressure drop due to conversion of potential to kinetic energy as a vessel narrows is reversed when the vessel widens out again (Figure 32–31). Bernoulli’s principle also has a significant application in pathophysiology. According to the principle, the greater the velocity of flow in a vessel, the lower the lateral pressure distending its walls. When a vessel is narrowed, the velocity of flow in the narrowed portion increases and the distending pressure decreases. Therefore, when a vessel is narrowed by a pathologic process such as an atherosclerotic plaque, the lat-
AUSCULTATORY METHOD The arterial blood pressure in humans is routinely measured by the auscultatory method. An inflatable cuff (Riva–Rocci cuff) attached to a mercury manometer (sphygmomanometer) is wrapped around the arm and a stethoscope is placed over the brachial artery at the elbow. The cuff is rapidly inflated until the pressure is well above the expected systolic pressure in the brachial artery. The artery is occluded by the cuff, and no sound is heard with the stethoscope. The pressure in the cuff is then lowered slowly. At the point at which systolic pressure in the artery just exceeds the cuff pressure, a spurt of blood passes through with each heartbeat and, synchronously with each beat, a tapping sound is heard below the cuff. The cuff pressure at which the sounds are first heard is the systolic pressure. As the cuff pressure is lowered further, the sounds become louder, then dull and muffled. These are the sounds of Korotkoff. Finally, in most individuals, they disappear. When direct and indirect blood pressure measurements are made simultaneously, the diastolic pressure in resting adults correlates best with the pressure at which the sound disappears. However, in adults after exercise and in children, the diastolic pressure correlates best with the pressure at which the sounds become muffled. This is also true in diseases such as hyperthyroidism and aortic insufficiency. The sounds of Korotkoff are produced by turbulent flow in the brachial artery. When the artery is narrowed by the cuff, the velocity of flow through the constriction exceeds the critical velocity and turbulent flow results (Figure 32–22). At cuff pressures just below the systolic pressure, flow through the artery occurs only at the peak of systole, and the intermittent turbulence produces a tapping sound. As long as the pressure in the cuff is above the diastolic pressure in the artery, flow is
P
Flow
FIGURE 32–31
Bernoulli’s principle. When fluid flows through the narrow portion of the tube, the kinetic energy of flow is increased as the velocity increases, and the potential energy is reduced. Consequently, the measured pressure (P) is lower than it would have been at that point if the tube had not been narrowed. The dashed line indicates what the pressure drop due to frictional forces would have been if the tube had been of uniform diameter.
SECTION VI Cardiovascular Physiology
The blood pressure in the brachial artery in young adults in the sitting position at rest is approximately 120/70 mm Hg. Because the arterial pressure is the product of the cardiac output and the peripheral resistance, it is affected by conditions that affect either or both of these factors. Emotion increases the cardiac output and peripheral resistance, and about 20% of hypertensive patients have blood pressures that are higher in the doctor’s office than at home, going about their regular daily activities (“white coat hypertension”). Blood pressure normally falls up to 20 mm Hg during sleep. This fall is reduced or absent in hypertension. There is general agreement that blood pressure rises with advancing age, but the magnitude of this rise is uncertain because hypertension is a common disease and its incidence increases with advancing age (see Clinical Box 32–4). Individuals who have systolic blood pressures < 120 mm Hg at age 50 to 60 and never develop clinical hypertension still have systolic pressures that rise throughout life (Figure 32–32). This rise may be the closest approximation to the rise in normal individuals. Individuals with mild hypertension that is untreated show a significantly more rapid rise in systolic pressure. In both groups, diastolic pressure also rises, but then starts to fall in middle age as the stiffness of arteries increases. Consequently, pulse pressure rises with advancing age. It is interesting that systolic and diastolic blood pressures are lower in young women than in young men until age 55 to 65, after which they become comparable. Because there is a positive correlation between blood pressure and the incidence of heart attacks and strokes (see below), the lower blood pressure before menopause in women may be one reason that, on average, they live longer than men.
CAPILLARY CIRCULATION At any one time, only 5% of the circulating blood is in the capillaries, but this 5% is in a sense the most important part of the blood volume because it is the only pool from which O2 and nutrients can enter the interstitial fluid and into which CO2 and waste products can enter the bloodstream. Exchange across the capillary walls is essential to the survival of the tissues.
METHODS OF STUDY It is difficult to obtain accurate measurements of capillary pressures and flows. Capillary pressure has been estimated by determining the amount of external pressure necessary to
Systolic pressure (mm Hg)
NORMAL ARTERIAL BLOOD PRESSURE
Systolic 175 165
4
155 145 135
4 1
125 115
1
105
Diastolic 95 Diastolic pressure (mm Hg)
interrupted at least during part of diastole, and the intermittent sounds have a staccato quality. When the cuff pressure is near the arterial diastolic pressure, the vessel is still constricted, but the turbulent flow is continuous. Continuous sounds have a muffled rather than a staccato quality.
90 85
4 4
80 75
1
70
1
65
Pulse 95 Pulse pressure (mm Hg)
546
85
4
75 65 1
55 45 35
Age (y)
FIGURE 32–32
Effects of age and sex on arterial pressure components in humans. Data are from a large group of individuals who were studied every 2 y throughout their adult lives. Group 1: Individuals who had systolic blood pressures < 120 mm Hg at age 50 to 60. Group 4: Individuals who had systolic blood pressure ≥ 160 mm Hg at age 50 to 60, that is, individuals with mild, untreated hypertension. The red line shows the values for women, and the blue line shows the values for men. (Modified and reproduced with permission from Franklin SS et al:
Hemodynamic patterns of age-related changes in blood pressure: The Framingham Heart Study. Circulation 1997;96:308.)
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CLINICAL BOX 32–4 Hypertension Hypertension is a sustained elevation of the systemic arterial pressure. It is most commonly due to increased peripheral resistance and is a very common abnormality in humans. It can be produced by many diseases (Table 32–12) and causes a number of serious disorders. When the resistance against which the left ventricle must pump (afterload) is elevated for a long period, the cardiac muscle hypertrophies. The initial response is activation of immediate-early genes in the ventricular muscle, followed by activation of a series of genes involved in growth during fetal life. Left ventricular hypertrophy is associated with a poor prognosis. The total O2 consumption of the heart, already increased by the work of expelling blood against a raised pressure (see Chapter 31), is increased further because there is more muscle. Therefore, any decrease in coronary blood flow has more serious consequences in hypertensive patients than it does in normal individuals, and degrees of coronary vessel narrowing that do not produce symptoms when the size of the heart is normal may produce myocardial infarction when the heart is enlarged. The incidence of atherosclerosis increases in hypertension, and myocardial infarcts are common even when the heart is not enlarged. Eventually, the ability to compensate for the high peripheral resistance is exceeded, and the heart fails. Hypertensive individuals are also predisposed to thromboses of cerebral vessels and cerebral hemorrhage. An additional complication is renal failure. However, the incidence of heart failure, strokes, and renal failure can be markedly reduced by active treatment of hypertension, even when the hypertension is relatively mild. In about 88% of patients with elevated blood pressure, the cause of the hypertension is unknown, and they are said to have essential hypertension. At present, essential hypertension is treatable but not curable. Effective lowering of the blood pressure can be produced by drugs that block α-adrenergic receptors, either in the periphery or in the central nervous system; drugs that block β-adrenergic receptors; drugs that inhibit the activity of angiotensin-converting enzyme; and calcium channel blockers that relax vascular smooth muscle. Essential hypertension is probably polygenic in origin, and environmental factors are also involved.
occlude the capillaries or the amount of pressure necessary to make saline start to flow through a micropipette inserted so that its tip faces the arteriolar end of the capillary.
CAPILLARY PRESSURE & FLOW Capillary pressures vary considerably, but typical values in human nail bed capillaries are 32 mm Hg at the arteriolar end and 15 mm Hg at the venous end. The pulse pressure is ap-
In other, less common forms of hypertension, the cause is known. A review of these is helpful because it emphasizes ways disordered physiology can lead to disease. Pathology that compromises the renal blood supply leads to renal hypertension, as does narrowing (coarctation) of the thoracic aorta, which both increases renin secretion and increases peripheral resistance. Pheochromocytomas, adrenal medullary tumors that secrete norepinephrine and epinephrine, can cause sporadic or sustained hypertension (see Chapter 22). Estrogens increase angiotensinogen secretion, and contraceptive pills containing large amounts of estrogen cause hypertension (pill hypertension) on this basis (see Chapter 25). Increased secretion of aldosterone or other mineralocorticoids causes renal Na+ retention, which leads to hypertension. A primary increase in plasma mineralocorticoids inhibits renin secretion. For unknown reasons, plasma renin is also low in 10–15% of patients with essential hypertension and normal circulating mineralocortical levels (low renin hypertension). Mutations in a number of single genes are also known to cause hypertension. These cases of monogenic hypertension are rare but informative. One of these is glucocorticoid-remediable aldosteronism (GRA), in which a hybrid gene encodes an adrenocorticotropic hormone (ACTH)-sensitive aldosterone synthase, with resulting hyperaldosteronism (see Chapter 22). 11-β hydroxylase deficiency also causes hypertension by increasing the secretion of deoxycorticosterone (see Chapter 22). Normal blood pressure is restored when ACTH secretion is inhibited by administering a glucocorticoid. Mutations that decrease 11-β hydroxysteroid dehydrogenase cause loss of specificity of the mineralocorticoid receptors (see Chapter 22) with stimulation of them by cortisol and, in pregnancy, by the elevated circulating levels of progesterone. Finally, mutations of the genes for ENaCs that reduce degradation of the β or γ subunits increase ENaC activity and lead to excess renal Na+ retention and hypertension (Liddle syndrome; see Chapter 38).
proximately 5 mm Hg at the arteriolar end and zero at the venous end. The capillaries are short, but blood moves slowly (about 0.07 cm/s) because the total cross-sectional area of the capillary bed is large. Transit time from the arteriolar to the venular end of an average-sized capillary is 1 to 2 s.
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TABLE 32–12 Estimated frequency of various forms of hypertension in the general hypertensive population. Percentage of Population Essential hypertension
88
Renal hypertension Renovascular
2
Parenchymal
3
Endocrine hypertension Primary aldosteronism
5
Cushing syndrome
0.1
Pheochromocytoma
0.1
Other adrenal forms
0.2
Estrogen treatment (“pill hypertension”)
1
Miscellaneous (Liddle syndrome, coarctation of the aorta, etc)
0.6
Reproduced with permission from McPhee SJ, Lingappa V, Ganong WF: Pathophysiology of Disease, 4th ed. McGraw-Hill, 2003.
EQUILIBRATION WITH INTERSTITIAL FLUID As noted above, the capillary wall is a thin membrane made up of endothelial cells. Substances pass through the junctions between endothelial cells and through fenestrations when they are present. Some also pass through the cells by vesicular transport. The factors other than vesicular transport that are responsible for transport across the capillary wall are diffusion and filtration (see Chapter 1). Diffusion is quantitatively much more important. O2 and glucose are in higher concentration in the bloodstream than in the interstitial fluid and diffuse into the interstitial fluid, whereas CO2 diffuses in the opposite direction. The rate of filtration at any point along a capillary depends on a balance of forces sometimes called the Starling forces, after the physiologist who first described their operation in detail. One of these forces is the hydrostatic pressure gradient (the hydrostatic pressure in the capillary minus the hydrostatic pressure of the interstitial fluid) at that point. The interstitial fluid pressure varies from one organ to another, and there is considerable evidence that it is subatmospheric (about –2 mm Hg) in subcutaneous tissue. It is, however, positive in the liver and kidneys and as high as 6 mm Hg in the brain. The other force is the osmotic pressure gradient across the capillary wall (colloid osmotic pressure of plasma minus
colloid osmotic pressure of interstitial fluid). This component is directed inward. Thus: Fluid movement = k[(Pc – Pi) – (πc – πi)] where k = capillary filtration coefficient Pc = capillary hydrostatic pressure Pi = interstitial hydrostatic pressure πc = capillary colloid osmotic pressure πi = interstitial colloid osmotic pressure πi is usually negligible, so the osmotic pressure gradient (πc – πi) usually equals the oncotic pressure. The capillary filtration coefficient takes into account, and is proportional to, the permeability of the capillary wall and the area available for filtration. The magnitude of the Starling forces along a typical muscle capillary is shown in Figure 32–33. Fluid moves into the interstitial space at the arteriolar end of the capillary and into the capillary at the venular end. In other capillaries, the balance of Starling forces may be different. For example, fluid moves out of almost the entire length of the capillaries in the renal glomeruli. On the other hand, fluid moves into the capillaries through almost their entire length in the intestines. About 24 L of fluid is filtered through the capillaries per day. This is about 0.3% of the cardiac output. About 85% of the filtered fluid is reabsorbed into the capillaries, and the remainder returns to the circulation via the lymphatics. It is worth noting that small molecules often equilibrate with the tissues near the arteriolar end of each capillary. In this situation, total diffusion can be increased by increasing blood flow; that is, exchange is flow-limited (Figure 32–34). Conversely, transfer of substances that do not reach equilibrium with the tissues during their passage through the capillaries is said to be diffusion-limited.
Arteriole
Venule Interstitial space
37
17 Oncotic P = 25 Interstitial P = 1
FIGURE 32–33
Schematic representation of pressure gradients across the wall of a muscle capillary. The numbers at the arteriolar and venular ends of the capillary are the hydrostatic pressures in mm Hg at these locations. The arrows indicate the approximate magnitude and direction of fluid movement. In this example, the pressure differential at the arteriolar end of the capillary is 11 mm Hg ([37 – 1] – 25) outward; at the opposite end, it is 9 mm Hg (25 – [17 – 1]) inward.
Concentration in capillary blood
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
Y X
A
Distance along capillary
V
FIGURE 32–34
Flow-limited and diffusion-limited exchange across capillary walls. A and V indicate the arteriolar and venular ends of the capillary. Substance X equilibrates with the tissues (movement into the tissues equals movement out) well before the blood leaves the capillary, whereas substance Y does not equilibrate. If other factors stay constant, the amount of X entering the tissues can be increased only by increasing blood flow; that is, it is flow-limited. The movement of Y is diffusion-limited.
ACTIVE & INACTIVE CAPILLARIES In resting tissues, most of the capillaries are collapsed. In active tissues, the metarterioles and the precapillary sphincters dilate. The intracapillary pressure rises, overcoming the critical closing pressure of the vessels, and blood flows through all of the capillaries. Relaxation of the smooth muscle of the metarterioles and precapillary sphincters is due to the action of vasodilator metabolites formed in active tissue (see Chapter 33). After noxious stimulation, substance P released by the axon reflex (see Chapter 34) increases capillary permeability. Bradykinin and histamine also increase capillary permeability. When capillaries are stimulated mechanically, they empty (white reaction; see Chapter 34), probably due to contraction of the precapillary sphincters.
VENOUS CIRCULATION Blood flows through the blood vessels, including the veins, primarily because of the pumping action of the heart. However, venous flow is aided by the heartbeat, the increase in the negative intrathoracic pressure during each inspiration, and contractions of skeletal muscles that compress the veins (muscle pump).
VENOUS PRESSURE & FLOW The pressure in the venules is 12 to 18 mm Hg. It falls steadily in the larger veins to about 5.5 mm Hg in the great veins outside the thorax. The pressure in the great veins at their entrance into the right atrium (central venous pressure) averages 4.6 mm Hg, but fluctuates with respiration and heart action.
549
Peripheral venous pressure, like arterial pressure, is affected by gravity. It is increased by 0.77 mm Hg for each centimeter below the right atrium and decreased by a like amount for each centimeter above the right atrium the pressure is measured (Figure 32–30). Thus, on a proportional basis, gravity has a greater effect on venous than on arterial pressures. When blood flows from the venules to the large veins, its average velocity increases as the total cross-sectional area of the vessels decreases. In the great veins, the velocity of blood is about one fourth that in the aorta, averaging about 10 cm/s.
THORACIC PUMP During inspiration, the intrapleural pressure falls from –2.5 to –6 mm Hg. This negative pressure is transmitted to the great veins and, to a lesser extent, the atria, so that central venous pressure fluctuates from about 6 mm Hg during expiration to approximately 2 mm Hg during quiet inspiration. The drop in venous pressure during inspiration aids venous return. When the diaphragm descends during inspiration, intra-abdominal pressure rises, and this also squeezes blood toward the heart because backflow into the leg veins is prevented by the venous valves.
EFFECTS OF HEARTBEAT The variations in atrial pressure are transmitted to the great veins, producing the a, c, and v waves of the venous pressurepulse curve (see Chapter 31). Atrial pressure drops sharply during the ejection phase of ventricular systole because the atrioventricular valves are pulled downward, increasing the capacity of the atria. This action sucks blood into the atria from the great veins. The sucking of the blood into the atria during systole contributes appreciably to the venous return, especially at rapid heart rates. Close to the heart, venous flow becomes pulsatile. When the heart rate is slow, two periods of peak flow are detectable, one during ventricular systole, due to pulling down of the atrioventricular valves, and one in early diastole, during the period of rapid ventricular filling (Figure 32–28).
MUSCLE PUMP In the limbs, the veins are surrounded by skeletal muscles, and contraction of these muscles during activity compresses the veins. Pulsations of nearby arteries may also compress veins. Because the venous valves prevent reverse flow, the blood moves toward the heart. During quiet standing, when the full effect of gravity is manifest, venous pressure at the ankle is 85– 90 mm Hg (Figure 32–30). Pooling of blood in the leg veins reduces venous return, with the result that cardiac output is reduced, sometimes to the point where fainting occurs. Rhythmic contractions of the leg muscles while the person is standing
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serve to lower the venous pressure in the legs to less than 30 mm Hg by propelling blood toward the heart. This heartward movement of the blood is decreased in patients with varicose veins because their valves are incompetent. These patients may develop stasis and ankle edema. However, even when the valves are incompetent, muscle contractions continue to produce a basic heartward movement of the blood because the resistance of the larger veins in the direction of the heart is less than the resistance of the small vessels away from the heart.
VENOUS PRESSURE IN THE HEAD In the upright position, the venous pressure in the parts of the body above the heart is decreased by the force of gravity. The neck veins collapse above the point where the venous pressure is close to zero. However, the dural sinuses have rigid walls and cannot collapse. The pressure in them in the standing or sitting position is therefore subatmospheric. The magnitude of the negative pressure is proportional to the vertical distance above the top of the collapsed neck veins, and in the superior sagittal sinus may be as much as –10 mm Hg. This fact must be kept in mind by neurosurgeons. Neurosurgical procedures are sometimes performed with the patient seated. If one of the sinuses is opened during such a procedure it sucks air, causing air embolism.
AIR EMBOLISM Because air, unlike fluid, is compressible, its presence in the circulation has serious consequences. The forward movement of the blood depends on the fact that blood is incompressible. Large amounts of air fill the heart and effectively stop the circulation, causing sudden death because most of the air is compressed by the contracting ventricles rather than propelled into the arteries. Small amounts of air are swept through the heart with the blood, but the bubbles lodge in the small blood vessels. The surface capillarity of the bubbles markedly increases the resistance to blood flow, and flow is reduced or abolished. Blockage of small vessels in the brain leads to serious and even fatal neurologic abnormalities. Treatment with hyperbaric oxygen (see Chapter 37) is of value because the pressure reduces the size of the gas emboli. In experimental animals, the amount of air that produces fatal air embolism varies considerably, depending in part on the rate at which it enters the veins. Sometimes as much as 100 mL can be injected without ill effects, whereas at other times as little as 5 mL is lethal.
MEASURING VENOUS PRESSURE Central venous pressure can be measured directly by inserting a catheter into the thoracic great veins. Peripheral venous pressure correlates well with central venous pressure in most conditions. To measure peripheral venous pressure, a needle attached to a manometer containing sterile saline is inserted
into an arm vein. The peripheral vein should be at the level of the right atrium (a point half the chest diameter from the back in the supine position). The values obtained in millimeters of saline can be converted into millimeters of mercury (mm Hg) by dividing by 13.6 (the density of mercury). The amount by which peripheral venous pressure exceeds central venous pressure increases with the distance from the heart along the veins. The mean pressure in the antecubital vein is normally 7.1 mm Hg, compared with a mean pressure of 4.6 mm Hg in the central veins. A fairly accurate estimate of central venous pressure can be made without any equipment by simply noting the height to which the external jugular veins are distended when the subject lies with the head slightly above the heart. The vertical distance between the right atrium and the place the vein collapses (the place where the pressure in it is zero) is the venous pressure in mm of blood. Central venous pressure is decreased during negative pressure breathing and shock. It is increased by positive pressure breathing, straining, expansion of the blood volume, and heart failure. In advanced congestive heart failure or obstruction of the superior vena cava, the pressure in the antecubital vein may reach values of 20 mm Hg or more.
LYMPHATIC CIRCULATION & INTERSTITIAL FLUID VOLUME LYMPHATIC CIRCULATION Fluid efflux normally exceeds influx across the capillary walls, but the extra fluid enters the lymphatics and drains through them back into the blood. This keeps the interstitial fluid pressure from rising and promotes the turnover of tissue fluid. The normal 24-h lymph flow is 2 to 4 L. Lymphatic vessels can be divided into two types: initial lymphatics and collecting lymphatics (Figure 32–35). The former lack valves and smooth muscle in their walls, and they are found in regions such as the intestine or skeletal muscle. Tissue fluid appears to enter them through loose junctions between the endothelial cells that form their walls. The fluid in them apparently is massaged by muscle contractions of the organs and contraction of arterioles and venules, with which they are often associated. They drain into the collecting lymphatics, which have valves and smooth muscle in their walls and contract in a peristaltic fashion, propelling the lymph along the vessels. Flow in the collecting lymphatics is further aided by movements of skeletal muscle, the negative intrathoracic pressure during inspiration, and the suction effect of highvelocity flow of blood in the veins in which the lymphatics terminate. However, the contractions are the principal factor propelling the lymph.
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
551
TABLE 32–13 Causes of increased interstitial
Collecting lymphatic
fluid volume and edema. Increased filtration pressure Valve
Venular constriction Increased venous pressure (heart failure, incompetent valves, venous obstruction, increased total ECF volume, effect of gravity, etc) Decreased osmotic pressure gradient across capillary Decreased plasma protein level Accumulation of osmotically active substances in interstitial space Arcading arteriole
Initial lymphatics
FIGURE 32–35
Initial lymphatics draining into collecting lymphatics in the mesentery. Note the close association with arcading arterioles, indicated by the single red lines. (Reproduced with
Increased capillary permeability Substance P Histamine and related substances Kinins, etc Inadequate lymph flow
permission from Schmid Schönbein GW, Zeifach BW: Fluid pump mechanisms in initial lymphatics. News Physiol Sci 1994;9:67.)
OTHER FUNCTIONS OF THE LYMPHATIC SYSTEM Appreciable quantities of protein enter the interstitial fluid in the liver and intestine, and smaller quantities enter from the blood in other tissues. The macromolecules enter the lymphatics, presumably at the junctions between the endothelial cells, and the proteins are returned to the bloodstream via the lymphatics. The amount of protein returned in this fashion in 1 d is equal to 25–50% of the total circulating plasma protein. The transport of absorbed long-chain fatty acids and cholesterol from the intestine via the lymphatics has been discussed in Chapter 27.
INTERSTITIAL FLUID VOLUME The amount of fluid in the interstitial spaces depends on the capillary pressure, the interstitial fluid pressure, the oncotic pressure, the capillary filtration coefficient, the number of active capillaries, the lymph flow, and the total extracellular fluid (ECF) volume. The ratio of precapillary to postcapillary venular resistance is also important. Precapillary constriction lowers filtration pressure, whereas postcapillary constriction raises it. Changes in any of these variables lead to changes in the volume of interstitial fluid. Factors promoting an increase in this volume are summarized in Table 32–13. Edema is the accumulation of interstitial fluid in abnormally large amounts. In active tissues, capillary pressure rises, often to the point where it exceeds the oncotic pressure throughout the length
of the capillary. In addition, osmotically active metabolites may temporarily accumulate in the interstitial fluid because they cannot be washed away as rapidly as they are formed. To the extent that they accumulate, they exert an osmotic effect that decreases the magnitude of the osmotic gradient due to the oncotic pressure. The amount of fluid leaving the capillaries is therefore markedly increased and the amount entering them reduced. Lymph flow is increased, decreasing the degree to which the fluid would otherwise accumulate, but exercising muscle, for example, still increases in volume by as much as 25%. Interstitial fluid tends to accumulate in dependent parts because of the effect of gravity. In the upright position, the capillaries in the legs are protected from the high arterial pressure by the arterioles, but the high venous pressure is transmitted to them through the venules. Skeletal muscle contractions keep the venous pressure low by pumping blood toward the heart (see above) when the individual moves about; however, if one stands still for long periods, fluid accumulates and edema eventually develops. The ankles also swell during long trips when travelers sit for prolonged periods with their feet in a dependent position. Venous obstruction may contribute to the edema in these situations. Whenever there is abnormal retention of salt in the body, water is also retained. The salt and water are distributed throughout the ECF, and since the interstitial fluid volume is therefore increased, there is a predisposition to edema. Salt and water retention is a factor in the edema seen in heart failure, nephrosis, and cirrhosis, but there are also variations in the mechanisms that govern fluid movement across the capillary walls in these diseases. In congestive heart failure, for example, venous pressure is usually elevated, with a consequent elevation in capillary pressure. In cirrhosis of the liver, oncotic
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pressure is low because hepatic synthesis of plasma proteins is depressed; and in nephrosis, oncotic pressure is low because large amounts of protein are lost in the urine. Another cause of edema is inadequate lymphatic drainage. Edema caused by lymphatic obstruction is called lymphedema, and the edema fluid has a high protein content. If it persists, it causes a chronic inflammatory condition that leads to fibrosis of the interstitial tissue. One cause of lymphedema is radical mastectomy, during which removal of the axillary lymph nodes leads to reduced lymph drainage. In filariasis, parasitic worms migrate into the lymphatics and obstruct them. Fluid accumulation plus tissue reaction lead in time to massive swelling, usually of the legs or scrotum (elephantiasis).
CHAPTER SUMMARY ■
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Blood consists of a suspension of red blood cells (erythrocytes), white blood cells, and platelets in a protein-rich fluid known as plasma. Blood cells arise in the bone marrow and are subject to regular renewal; the majority of plasma proteins are synthesized by the liver. Hemoglobin, stored in red blood cells, transports oxygen to peripheral tissues. Fetal hemoglobin is specialized to facilitate diffusion of oxygen from mother to fetus during development. Mutated forms of hemoglobin lead to red cell abnormalities and anemia. Complex oligosaccharide structures, specific to groups of individuals, form the basis of the ABO blood group system. AB blood group oligosaccharides, as well as other blood group molecules, can trigger the production of antibodies in naïve individuals following inappropriate transfusions, with potentially serious consequences due to erythrocyte agglutination. Blood flows from the heart to arteries and arterioles, thence to capillaries, and eventually to venules and veins and back to the heart. Each segment of the vasculature has specific contractile properties and regulatory mechanisms that subserve physiologic function. Physical principles of pressure, wall tension, and vessel caliber govern the flow of blood through each segment of the circulation. Transfer of oxygen and nutrients from the blood to tissues, as well as collection of metabolic wastes, occurs exclusively in the capillary beds. Fluid also leaves the circulation across the walls of capillaries. Some is reabsorbed; the remainder enters the lymphatic system, which eventually drains into the subclavian veins to return fluid to the bloodstream. Hypertension is an increase in mean blood pressure that is usually chronic and is common in humans. Hypertension can result in serious health consequences if left untreated. The majority of hypertension is of unknown cause, but several gene mutations underlie rare forms of the disease and are informative about mechanisms that control the dynamics of the circulatory system and its integration with other organs.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. Which of the following has the highest total cross-sectional area in the body? A) arteries B) arterioles C) capillaries D) venules E) veins 2. Lymph flow from the foot is A) increased when an individual rises from the supine to the standing position. B) increased by massaging the foot. C) increased when capillary permeability is decreased. D) decreased when the valves of the leg veins are incompetent. E) decreased by exercise. 3. The pressure in a capillary in skeletal muscle is 35 mm Hg at the arteriolar end and 14 mm Hg at the venular end. The interstitial pressure is 0 mm Hg. The colloid osmotic pressure is 25 mm Hg in the capillary and 1 mm Hg in the interstitium. The net force producing fluid movement across the capillary wall at its arteriolar end is A) 3 mm Hg out of the capillary. B) 3 mm Hg into the capillary. C) 10 mm Hg out of the capillary. D) 11 mm Hg out of the capillary. E) 11 mm Hg into the capillary. 4. The velocity of blood flow A) is higher in the capillaries than the arterioles. B) is higher in the veins than in the venules. C) is higher in the veins than the arteries. D) falls to zero in the descending aorta during diastole. E) is reduced in a constricted area of a blood vessel. 5. When the radius of the resistance vessels is increased, which of the following is increased? A) systolic blood pressure B) diastolic blood pressure C) viscosity of the blood D) hematocrit E) capillary blood flow 6. When the viscosity of the blood is increased, which of the following is increased? A) mean blood pressure B) radius of the resistance vessels C) radius of the capacitance vessels D) central venous pressure E) capillary blood flow 7. A pharmacologist discovers a drug that stimulates the production of VEGF receptors. He is excited because his drug might be of value in the treatment of A) coronary artery disease. B) cancer. C) emphysema. D) diabetes insipidus. E) dysmenorrhea.
CHAPTER 32 Blood as a Circulatory Fluid & the Dynamics of Blood & Lymph Flow
CHAPTER RESOURCES de Montalembert M: Management of sickle cell disease. Brit Med J 2008;337:626. Miller JL: Signaled expression of fetal hemoglobin during development. Transfusion 2005;45:1229.
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Perrotta S, Gallagher PG, Mohandas N: Hereditary spherocytosis. Lancet 2008;372:1411. Semenza GL: Vasculogenesis, angiogenesis, and arteriogenesis: Mechanisms of blood vessel formation and remodeling. J Cell Biochem 2007;102:840.
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6 33 C
Cardiovascular Regulatory Mechanisms
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■
■ ■ ■
Outline the neural mechanisms that control arterial blood pressure and heart rate, including the receptors, afferent and efferent pathways, central integrating pathways, and effector mechanisms involved. Describe the direct effects of CO2 and hypoxia on the vasomotor areas in the medulla oblongata. Describe how the process of autoregulation contributes to control of vascular caliber. Identify the paracrine factors and hormones that regulate vascular tone, their sources, and their mechanisms of action.
INTRODUCTION In humans and other mammals, multiple cardiovascular regulatory mechanisms have evolved. These mechanisms increase the blood supply to active tissues and increase or decrease heat loss from the body by redistributing the blood. In the face of challenges such as hemorrhage, they maintain the blood flow to the heart and brain. When the challenge faced is severe, flow to these vital organs is maintained at the expense of the circulation to the rest of the body. Circulatory adjustments are effected by altering the output of the pump (the heart), changing the diameter of the resistance vessels (primarily the arterioles), or altering the amount of blood pooled in the capacitance vessels (the veins). Regulation of cardiac output is discussed in Chapter 31. The caliber of the
arterioles is adjusted in part by autoregulation (Table 33–1). It is also increased in active tissues by locally produced vasodilator metabolites, is affected by substances secreted by the endothelium, and is regulated systemically by circulating vasoactive substances and the nerves that innervate the arterioles. The caliber of the capacitance vessels is also affected by circulating vasoactive substances and by vasomotor nerves. The systemic regulatory mechanisms synergize with the local mechanisms and adjust vascular responses throughout the body. The terms vasoconstriction and vasodilation are generally used to refer to constriction and dilation of the resistance vessels. Changes in the caliber of the veins are referred to specifically as venoconstriction or venodilation.
NEURAL CONTROL OF THE CARDIOVASCULAR SYSTEM
and venules contain smooth muscle and receive motor nerve fibers from the sympathetic division of the autonomic nervous system. The fibers to the resistance vessels regulate tissue blood flow and arterial pressure. The fibers to the venous capacitance vessels vary the volume of blood “stored” in the veins. The innervation of most veins is sparse, but the splanchnic veins are well innervated. Venoconstriction is produced by stimuli that also activate the vasoconstrictor nerves to the arterioles. The
NEURAL REGULATORY MECHANISMS Although the arterioles and the other resistance vessels are most densely innervated, all blood vessels except capillaries
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resultant decrease in venous capacity increases venous return, shifting blood to the arterial side of the circulation.
INNERVATION OF THE BLOOD VESSELS Sympathetic noradrenergic fibers end on blood vessels in all parts of the body to mediate vasoconstriction. In addition to their vasoconstrictor innervation, resistance vessels in skeletal muscles are innervated by vasodilator fibers, which, although they travel with the sympathetic nerves, are cholinergic (sympathetic cholinergic vasodilator system). There is no tonic activity in the vasodilator fibers, but the vasoconstrictor fibers to most vascular beds have some tonic activity. When the sympathetic nerves are cut (sympathectomy), the blood vessels dilate. In most tissues, vasodilation is produced by decreasing the rate of tonic discharge in the vasoconstrictor nerves, although in skeletal muscles it can also be produced by activating the sympathetic cholinergic vasodilator system (Table 33–1).
TABLE 33–1 Summary of factors affecting the caliber of the arterioles. Constriction
Dilation
Local factors Decreased local temperature
Increased CO2 and decreased O2
Autoregulation
Increased K+, adenosine, lactate, etc Decreased local pH Increased local temperature
Endothelial products Endothelin-1
NO
Locally released platelet serotonin
Kinins
Thromboxane A2
Prostacyclin
Circulating hormones
CARDIAC INNERVATION Impulses in the sympathetic nerves to the heart increase the cardiac rate (chronotropic effect), rate of transmission in the cardiac conductive tissue (dromotropic effect), and the force of contraction (inotropic effect). They also inhibit the effects of vagal parasympathetic stimulation, probably by release of neuropeptide Y, which is a cotransmitter in the sympathetic endings. Impulses in vagal fibers decrease heart rate. A moderate amount of tonic discharge takes place in the cardiac sympathetic nerves at rest, but there is a good deal of tonic vagal discharge (vagal tone) in humans and other large animals. After the administration of parasympatholytic drugs such as atropine, the heart rate in humans increases from 70, its normal resting value, to 150 to 180 beats/min because the sympathetic tone is unopposed. In humans in whom both noradrenergic and cholinergic systems are blocked, the heart rate is approximately 100 beats/min.
CARDIOVASCULAR CONTROL The cardiovascular system is under neural influences coming from several parts of the brain (see Figure 17–6), which in turn receive feedback from sensory receptors in the vasculature (eg, baroreceptors). A simplified model of the feedback control circuit is shown in Figure 33–1. An increase in neural output from the brain stem to sympathetic nerves leads to a decrease in blood vessel diameter (arteriolar constriction) and increases in stroke volume and heart rate, which contribute to a rise in blood pressure. This in turn causes an increase in baroreceptor activity, which signals the brain stem to reduce the neural output to sympathetic nerves. Venoconstriction and a decrease in the stores of blood in the venous reservoirs usually accompany increases in arteriolar constriction, although changes in the capacitance vessels
Epinephrine (except in skeletal muscle and liver)
Epinephrine in skeletal muscle and liver
Norepinephrine
CGRPα
AVP
Substance P
Angiotensin II
Histamine
Circulating Na+-K+ ATPase inhibitor
ANP
Neuropeptide Y
VIP
Neural factors Increased discharge of sympathetic nerves
Decreased discharge of sympathetic nerves Activation of sympathetic cholinergic vasodilator nerves to skeletal muscle
do not always parallel changes in the resistance vessels. In the presence of an increase in sympathetic nerve activity to the heart and vasculature, there is usually an associated decrease in the activity of vagal fibers to the heart. Conversely, a decrease in sympathetic activity causes vasodilation, a fall in blood pressure, and an increase in the storage of blood in the venous reservoirs. There is usually a concomitant decrease in heart rate, but this is mostly due to stimulation of the vagal innervation of the heart.
MEDULLARY CONTROL OF THE CARDIOVASCULAR SYSTEM One of the major sources of excitatory input to sympathetic nerves controlling the vasculature is neurons located near the pial surface of the medulla in the rostral ventrolateral medulla
CHAPTER 33 Cardiovascular Regulatory Mechanisms
Baroreceptors
Blood pressure
Brain stem
Heart rate
Stroke volume
Vessel diameter
FIGURE 33–1
Feedback control of blood pressure. Brain stem excitatory input to sympathetic nerves to the heart and vasculature increases heart rate and stroke volume and reduces vessel diameter. Together these increase blood pressure, which activates the baroreceptor reflex to reduce the activity in the brain stem.
557
(RVLM; Figure 33–2). This region is sometimes called a vasomotor area. The axons of RVLM neurons course dorsally and medially and then descend in the lateral column of the spinal cord to the thoracolumbar intermediolateral gray column (IML). They contain phenylethanolamine-N-methyltransferase (PNMT; see Chapter 7), but it appears that the excitatory transmitter they secrete is glutamate rather than epinephrine. Neurovascular compression of the RVLM has been linked to some cases of essential hypertension in humans (see Clinical Box 33–1). The activity of RVLM neurons is determined by many factors (see Table 33–2). They include not only the very important fibers from arterial and venous baroreceptors, but also fibers from other parts of the nervous system and from the carotid and aortic chemoreceptors. In addition, some stimuli act directly on the vasomotor area. There are descending tracts to the vasomotor area from the cerebral cortex (particularly the limbic cortex) that relay in the hypothalamus. These fibers are responsible for the blood
Medulla
Baroreceptor afferents (Glu)
NTS
(GABA) IX RVLM
(Glu) X
CVLM IVLM Bulbospinal pathway (Glu)
Carotid sinus Thoracic cord
IML Aortic arch Preganglionic sympathetic neuron (Ach) Adrenal medulla
Heart
Postganglionic sympathetic neuron (NE)
Arteriole or venule
FIGURE 33–2 Basic pathways involved in the medullary control of blood pressure. The vagal efferent pathways that slow the heart are not shown. The putative neurotransmitters in the pathways are indicated in parentheses. Glu, glutamate; GABA,γ-aminobutyric acid; Ach, acetylcholine; NE, norepinephrine; IML, intermediolateral gray column; NTS, nucleus of the tractus solitarius; CVLM, IVLM, RVLM, caudal, intermediate, and rostral ventrolateral medulla; IX and X, glossopharyngeal and vagus nerves.
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CLINICAL BOX 33–1
TABLE 33–2 Factors affecting the activity of the RVLM. Direct stimulation
Essential Hypertension & Neurovascular Compression of the RVLM In about 88% of patients with elevated blood pressure, the cause of the hypertension is unknown, and they are said to have essential hypertension. There are data available to support the view that neurovascular compression of the RVLM is associated with essential hypertension in some subjects. In the 1970s, Dr. Peter Jannetta, a neurosurgeon in Pittsburgh, PA, developed a technique for “microvascular decompression” of the medulla to treat trigeminal neuralgia and hemifacial spasm, which he attributed to pulsatile compression of the vertebral and posterior inferior cerebellar arteries impinging on the fifth and seventh cranial nerves. Moving the arteries away from the nerves led to reversal of the neurologic symptoms in many cases. Some of these patients were also hypertensive, and they showed reductions in blood pressure postoperatively. Later, a few human studies claimed that surgical decompression of the RVLM could sometimes relieve hypertension. There are several reports of patients with a schwannoma or meningioma lying close to the RVLM whose hypertension has been reversed by surgical decompression. Magnetic resonance angiography (MRA) has been used to compare the incidence of neurovascular compression in hypertensive and normotensive individuals and to correlate indices of sympathetic nerve activity with the presence or absence of compression. Some of these studies showed a higher incidence of coexistence of neurovascular compression with essential hypertension than in other forms of hypertension or normotension, but others showed the presences of a compression in normotensive subjects. On the other hand, there was a strong positive relationship between the presence of neurovascular compression and increased sympathetic activity.
pressure rise and tachycardia produced by emotions such as sexual excitement and anger. The connections between the hypothalamus and the vasomotor area are reciprocal, with afferents from the brain stem closing the loop. Inflation of the lungs causes vasodilation and a decrease in blood pressure. This response is mediated via vagal afferents from the lungs that inhibit vasomotor discharge. Pain usually causes a rise in blood pressure via afferent impulses in the reticular formation converging in the RVLM. However, prolonged severe pain may cause vasodilation and fainting. The activity in afferents from exercising muscles probably exerts a similar pressor effect via pathway to the RVLM. The pressor response to stimulation of somatic afferent nerves is called the somatosympathetic reflex.
CO2 Hypoxia Excitatory inputs Cortex via hypothalamus Mesencephalic periaqueductal gray Brain stem reticular formation Pain pathways Somatic afferents (somatosympathetic reflex) Carotid and aortic chemoreceptors Inhibitory inputs Cortex via hypothalamus Caudal ventrolateral medulla Caudal medullary raphé nuclei Lung inflation afferents Carotid, aortic, and cardiopulmonary baroreceptors
Unlike the vasculature, the heart is controlled by both sympathetic and parasympathetic (vagal) nerves. The medulla is also a major site of origin of excitatory input to cardiac vagal motor neurons in the nucleus ambiguus (Figure 33–3). Table 33–3 is a summary of conditions that affect the heart rate. In general, stimuli that increase the heart rate also increase blood pressure, whereas those that decrease the heart rate lower blood pressure. However, there are exceptions, such as the production of hypotension and tachycardia by stimulation of atrial stretch receptors and the production of hypertension and bradycardia by increased intracranial pressure.
BARORECEPTORS The baroreceptors are stretch receptors in the walls of the heart and blood vessels. The carotid sinus and aortic arch receptors monitor the arterial circulation. Receptors are also located in the walls of the right and left atria at the entrance of the superior and inferior venae cavae and the pulmonary veins, as well as in the pulmonary circulation. These receptors in the low-pressure part of the circulation are referred to collectively as the cardiopulmonary receptors. The carotid sinus is a small dilation of the internal carotid artery just above the bifurcation of the common carotid into external and internal carotid branches (Figure 33–4). Baroreceptors are located in this dilation. They are also found in the
CHAPTER 33 Cardiovascular Regulatory Mechanisms
NTS
Dorsal motor nucleus
AP
559
TABLE 33–3 Factors affecting heart rate. Heart rate accelerated by: Decreased activity of arterial baroreceptors Increased activity of atrial stretch receptors
XII
Inspiration Excitement Anger
Pyr Nucleus ambiguus
Most painful stimuli Vagus nerve Hypoxia Heart
Exercise Thyroid hormones Fever Heart rate slowed by:
FIGURE 33–3
Basic pathways involved in the medullary control of heart rate by the vagus nerves. NTS neurons (dashed lines) project to and inhibit cardiac preganglionic parasympathetic neurons primarily in the nucleus ambiguus. Some are also located in the dorsal motor nucleus of the vagus; however, this nucleus primarily contains vagal motor neurons that project to the gastrointestinal tract. AP, area postrema; Pyr, pyramid; XII, hypoglossal nucleus.
Increased activity of arterial baroreceptors Expiration Fear Grief Stimulation of pain fibers in trigeminal nerve
wall of the arch of the aorta. The receptors are located in the adventitia of the vessels. The afferent nerve fibers from the carotid sinus form a distinct branch of the glossopharyngeal nerve, the carotid sinus nerve. The fibers from the aortic arch form a branch of the vagus nerve, the aortic depressor nerve. The baroreceptors are stimulated by distention of the structures in which they are located, and so they discharge at an increased rate when the pressure in these structures rises. Their afferent fibers pass via the glossopharyngeal and vagus nerves to the medulla. Most of them end in the nucleus of the tractus solitarius (NTS), and the excitatory transmitter they secrete is glutamate (Figure 33–2). Excitatory (glutamate) projections extend from the NTS to the caudal ventrolateral medulla (CVLM), where they stimulate γ-aminobutyrate (GABA)secreting inhibitory neurons that project to the RVLM. Excitatory projections also extend from the NTS to the vagal motor neurons in the nucleus ambiguus and dorsal motor nucleus (Figure 33–3). Thus, increased baroreceptor discharge inhibits the tonic discharge of sympathetic nerves and excites the vagal innervation of the heart. These neural changes produce vasodilation, venodilation, a drop in blood pressure, bradycardia, and a decrease in cardiac output.
Increased intracranial pressure
Carotid body
X X XX X X X XXX X X
Internal carotid artery
External carotid artery Carotid sinus
Common carotid artery Left common carotid artery
Left subclavian artery Aortic body
Aortic body Innominate artery X X X X X X X X X X X X
Aortic arch (viewed from behind)
FIGURE 33–4 Baroreceptor areas in the carotid sinus and aortic arch. X, sites where receptors are located. The carotid and aortic bodies, which contain chemoreceptors, are also shown.
SECTION VI Cardiovascular Physiology
BARORECEPTOR NERVE ACTIVITY Baroreceptors are more sensitive to pulsatile pressure than to constant pressure. A decline in pulse pressure without any change in mean pressure decreases the rate of baroreceptor discharge and provokes a rise in systemic blood pressure and tachycardia. At normal blood pressure levels (about 100 mm Hg mean pressure), a burst of action potentials appears in a single baroreceptor fiber during systole, but there are few action potentials in early diastole (Figure 33–5). At lower mean pressures, this phasic change in firing is even more dramatic with activity only occurring during systole. At these lower pressures, the overall firing rate is considerably reduced. The threshold for eliciting activity in the carotid sinus nerve is about 50 mm Hg; maximal activity occurs at about 200 mm Hg. When one carotid sinus is isolated and perfused and the other baroreceptors are denervated, there is no discharge in the afferent fibers from the perfused sinus and no drop in the animal’s arterial pressure or heart rate when the perfusion pressure is below 30 mm Hg (Figure 33–6). At carotid sinus perfusion pressures of 70–110 mm Hg, there is a near linear relationship between perfusion pressure and the fall in systemic blood pressure and heart rate. At perfusion pressures above 150 mm Hg there is no further increase in response, presumably because the rate of baroreceptor discharge and the degree of inhibition of sympathetic nerve activity are maximal. From the foregoing discussion, it is apparent that the baroreceptors on the arterial side of the circulation, their afferent connections to the medullary cardiovascular areas, and the efferent pathways from these areas constitute a reflex feedback mechanism that operates to stabilize blood pressure and heart Phasic aortic pressure Mean arterial pressures (mm Hg)
70 60 50 40 30 20 10 0
50
100
150
200
Pressure in carotid sinus (mm Hg)
FIGURE 33–6
Fall in systemic blood pressure produced by raising the pressure in the isolated carotid sinus to various values. Solid line: Response in a normal monkey. Dashed line: Response in a hypertensive monkey, demonstrating baroreceptor resetting (arrow).
rate. Any drop in systemic arterial pressure decreases the inhibitory discharge in the buffer nerves, and there is a compensatory rise in blood pressure and cardiac output. Any rise in pressure produces dilation of the arterioles and decreases cardiac output until the blood pressure returns to its previous normal level.
BARORECEPTOR RESETTING In chronic hypertension, the baroreceptor reflex mechanism is “reset” to maintain an elevated rather than a normal blood pressure. In perfusion studies on hypertensive experimental animals, raising the pressure in the isolated carotid sinus lowers the elevated systemic pressure, and decreasing the perfusion pressure raises the elevated pressure (Figure 33–6). Little is known about how and why this occurs, but resetting occurs rapidly in experimental animals. It is also rapidly reversible, both in experimental animals and in clinical situations.
50
75
ROLE OF BARORECEPTORS IN SHORTTERM CONTROL OF BLOOD PRESSURE
100 125 200 0
80 % fall in systemic blood pressure
560
0.5
1.0
1.5
2.0
Time (s)
FIGURE 33–5
Discharges (vertical lines) in a single afferent nerve fiber from the carotid sinus at various levels of mean arterial pressures, plotted against changes in aortic pressure with time. Baroreceptors are very sensitive to changes in pulse pressure as shown by the record of phasic aortic pressure. (Reproduced with
permission from Berne RM, Levy MN: Cardiovascular Physiology, 3rd ed. Mosby, 1977.)
The changes in pulse rate and blood pressure that occur in humans on standing up or lying down are due for the most part to baroreceptor reflexes. The function of the receptors can be tested by monitoring changes in heart rate as a function of increasing arterial pressure during infusion of the α-adrenergic agonist phenylephrine. A normal response is shown in Figure 33–7; from a systolic pressure of about 120 to 150 mm Hg, there is a linear relation between pressure and lowering of the heart rate (greater RR interval). Baroreceptors are very important in short-term control of arterial pressure. Activation of the reflex allows for rapid adjustments in blood pressure in
CHAPTER 33 Cardiovascular Regulatory Mechanisms
1800
CLINICAL BOX 33–2
1600 RR interval (ms)
561
Slope = 33.3 ms mm Hg−1 Threshold = 124 mm Hg
1400
Cardiopulmonary Chemosensitive Receptors
1200 1000 800 600 80
100
120
140
160
Systolic pressure (mm Hg)
FIGURE 33–7 Baroreflex-mediated lowering of the heart rate during infusion of phenylephrine in a human subject. Note that the values for the RR interval of the electrocardiogram, which are plotted on the vertical axis, are inversely proportionate to the heart rate. (Reproduced with permission from Kotrly K et al: Effects of fentanyl-diazepamnitrous oxide anaesthesia on arterial baroreflex control of heart rate in man. Br J Anaesth 1986;58:406.)
response to abrupt changes in blood volume, cardiac output, or peripheral resistance during exercise. Blood pressure initially rises dramatically after bilateral section of baroreceptor nerves or bilateral lesions of the NTS. However, after a period of time, mean blood pressure returns to near control levels, but there are huge fluctuations in pressure during the course of a day. Removal of the baroreceptor reflex prevents an individual from responding to stimuli that cause abrupt changes in blood volume, cardiac output, or peripheral resistance, including exercise and postural changes. A long-term change in blood pressure resulting from loss of baroreceptor reflex control is called neurogenic hypertension.
ATRIAL STRETCH RECEPTORS The stretch receptors in the atria are of two types: those that discharge primarily during atrial systole (type A), and those that discharge primarily late in diastole, at the time of peak atrial filling (type B). The discharge of type B baroreceptors is increased when venous return is increased and decreased by positivepressure breathing, indicating that these baroreceptors respond primarily to distention of the atrial walls. The reflex circulatory adjustments initiated by increased discharge from most if not all of these receptors include vasodilation and a fall in blood pressure. However, the heart rate is increased rather than decreased.
CARDIOPULMONARY RECEPTORS Receptors in the endocardial surfaces of the ventricles are activated during ventricular distention. The response is a vagal bradycardia and hypotension, comparable to a baroreceptor reflex. Left ventricular stretch receptors may play a role in the maintenance of vagal tone that keeps the heart rate low at rest.
For nearly 150 years, it has been known that activation of chemosensitive vagal C fibers in the cardiopulmonary region (eg, juxtacapillary region of alveoli, ventricles, atria, great veins, and pulmonary artery) causes profound bradycardia, hypotension, and a brief period of apnea followed by rapid shallow breathing. This response pattern is called the Bezold–Jarisch reflex and was named after the individuals who first reported these findings. This reflex can be elicited by a variety of substances including capsaicin, serotonin, phenylbiguanide, and veratridine in cats, rabbits, and rodents. Although originally viewed as a pharmacologic curiosity, there is a growing body of evidence supporting the view that the Bezold–Jarisch reflex is activated during certain pathophysiologic conditions. For example, this reflex may be activated during myocardial ischemia and reperfusion as a result of increased production of oxygen radicals and by agents used as radio-contrast for coronary angiography. This can contribute to the hypotension that is frequently a stubborn complication of this disease. Activation of cardiopulmonary chemosensitive receptors may also be part of a defense mechanism protecting individuals from toxic chemical hazards. Activation of cardiopulmonary reflexes may help reduce the amount of inspired pollutants that get absorbed into the blood, protecting vital organs from potential toxicity of these pollutants, and facilitating the elimination of the pollutants. Finally, the syndrome of cardiac slowing with hypotension (vasovagal syncope) has also been attributed to activation of the Bezold–Jarisch reflex. Vasovagal syncope can occur after prolonged upright posture that results in pooling of blood in the lower extremities and diminished intracardiac blood volume (also called postural syncope). This phenomenon is exaggerated if combined with dehydration. The resultant arterial hypotension is sensed in the carotid sinus baroreceptors, and afferent fibers from these receptors trigger autonomic signals that increase cardiac rate and contractility. However, pressure receptors in the wall of the left ventricle respond by sending signals that trigger paradoxical bradycardia and decreased contractility, resulting in sudden marked hypotension. The individual also feels lightheaded and may experience a brief episode of loss of consciousness.
Various chemicals are known to elicit reflexes due to activation of cardiopulmonary chemoreceptors and may play a role in various cardiovascular disorders (see Clinical Box 33–2).
VALSALVA MANEUVER The function of the receptors can also be tested by monitoring the changes in pulse and blood pressure that occur in response
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+40
Esophageal pressure (cm H2O)
0 −40
Start
Stop
10 s
200 Arterial pressure (mm Hg) 0
FIGURE 33–8
Diagram of the response to straining (the Valsalva maneuver) in a normal man, recorded with a needle in the brachial artery. Blood pressure rises at the onset of straining because increased intrathoracic pressure is added to the pressure of the blood in the aorta. It then falls because the high intrathoracic pressure compresses veins, decreasing venous return and cardiac output. (Courtesy of M Mcllroy.)
to brief periods of straining (forced expiration against a closed glottis: the Valsalva maneuver). Valsalva maneuvers occur regularly during coughing, defecation, and heavy lifting. The blood pressure rises at the onset of straining (Figure 33–8) because the increase in intrathoracic pressure is added to the pressure of the blood in the aorta. It then falls because the high intrathoracic pressure compresses the veins, decreasing venous return and cardiac output. The decreases in arterial pressure and pulse pressure inhibit the baroreceptors, causing tachycardia and a rise in peripheral resistance. When the glottis is opened and the intrathoracic pressure returns to normal, cardiac output is restored but the peripheral vessels are constricted. The blood pressure therefore rises above normal, and this stimulates the baroreceptors, causing bradycardia and a drop in pressure to normal levels. In sympathectomized patients, heart rate changes still occur because the baroreceptors and the vagi are intact. However, in patients with autonomic insufficiency, a syndrome in which autonomic function is widely disrupted, the heart rate changes are absent. For reasons that are still obscure, patients with primary hyperaldosteronism also fail to show the heart rate changes and the blood pressure rise when the intrathoracic pressure returns to normal. Their response to the Valsalva maneuver returns to normal after removal of the aldosterone-secreting tumor.
PERIPHERAL CHEMORECEPTOR REFLEX Peripheral arterial chemoreceptors in the carotid and aortic bodies (Figure 33–2) have very high rates of blood flow. These receptors are primarily activated by a reduction in partial pressure of oxygen (PaO2), but they also respond to an increase in the partial pressure of carbon dioxide (PaCO2) and pH. Chemoreceptors exert their main effects on respiration; however, their activation also leads to vasoconstriction. Heart rate changes are variable and depend on various factors, including
changes in respiration. A direct effect of chemoreceptor activation is to increase vagal nerve activity. However, hypoxia also produces hyperpnea and increased catecholamine secretion from the adrenal medulla, both of which produce tachycardia and an increase in cardiac output. Hemorrhage that produces hypotension leads to chemoreceptor stimulation due to decreased blood flow to the chemoreceptors and consequent stagnant anoxia of these organs. Chemoreceptor discharge may also contribute to the production of Mayer waves. These should not be confused with Traube–Hering waves, which are fluctuations in blood pressure synchronized with respiration. The Mayer waves are slow, regular oscillations in arterial pressure that occur at the rate of about one per 20–40 s during hypotension. Under these conditions, hypoxia stimulates the chemoreceptors. The stimulation raises the blood pressure, which improves the blood flow in the receptor organs and eliminates the stimulus to the chemoreceptors, so that the pressure falls and a new cycle is initiated.
DIRECT EFFECTS ON THE RVLM When intracranial pressure is increased, the blood supply to RVLM neurons is compromised, and the local hypoxia and hypercapnia increase their discharge. The resultant rise in systemic arterial pressure (Cushing reflex) tends to restore the blood flow to the medulla and over a considerable range, the blood pressure rise is proportional to the increase in intracranial pressure. The rise in blood pressure causes a reflex decrease in heart rate via the arterial baroreceptors. This is why bradycardia rather than tachycardia is characteristically seen in patients with increased intracranial pressure. A rise in arterial PCO2 stimulates the RVLM, but the direct peripheral effect of hypercapnia is vasodilation. Therefore, the peripheral and central actions tend to cancel each other out. Moderate hyperventilation, which significantly lowers the CO2 tension of the blood, causes cutaneous and cerebral
CHAPTER 33 Cardiovascular Regulatory Mechanisms vasoconstriction in humans, but there is little change in blood pressure. Exposure to high concentrations of CO2 is associated with marked cutaneous and cerebral vasodilation, but vasoconstriction occurs elsewhere and usually there is a slow rise in blood pressure.
LOCAL REGULATION AUTOREGULATION The capacity of tissues to regulate their own blood flow is referred to as autoregulation. Most vascular beds have an intrinsic capacity to compensate for moderate changes in perfusion pressure by changes in vascular resistance, so that blood flow remains relatively constant. This capacity is well developed in the kidneys (see Chapter 38), but it has also been observed in the mesentery, skeletal muscle, brain, liver, and myocardium. It is probably due in part to the intrinsic contractile response of smooth muscle to stretch (myogenic theory of autoregulation). As the pressure rises, the blood vessels are distended and the vascular smooth muscle fibers that surround the vessels contract. If it is postulated that the muscle responds to the tension in the vessel wall, this theory could explain the greater degree of contraction at higher pressures; the wall tension is proportional to the distending pressure times the radius of the vessel (law of Laplace; see Chapter 32), and the maintenance of a given wall tension as the pressure rises would require a decrease in radius. Vasodilator substances tend to accumulate in active tissues, and these “metabolites” also contribute to autoregulation (metabolic theory of autoregulation). When blood flow decreases, they accumulate and the vessels dilate; when blood flow increases, they tend to be washed away.
VASODILATOR METABOLITES The metabolic changes that produce vasodilation include, in most tissues, decreases in O2 tension and pH. These changes cause relaxation of the arterioles and precapillary sphincters. A local fall in O2 tension, in particular, can initiate a program of vasodilatory gene expression secondary to production of hypoxia-inducible factor-1α (HIF-1α), a transcription factor with multiple targets. Increases in CO2 tension and osmolality also dilate the vessels. The direct dilator action of CO2 is most pronounced in the skin and brain. The neurally mediated vasoconstrictor effects of systemic as opposed to local hypoxia and hypercapnia have been discussed above. A rise in temperature exerts a direct vasodilator effect, and the temperature rise in active tissues (due to the heat of metabolism) may contribute to the vasodilation. K+ is another substance that accumulates locally, and has demonstrated dilator activity secondary to the hyperpolarization of vascular smooth muscle cells. Lactate may also contribute to the dilation. In injured tissues, histamine released from damaged cells increases capillary permeability. Thus, it is probably responsible for some of the swelling in areas of inflammation. Adenosine may play a
563
vasodilator role in cardiac muscle but not in skeletal muscle. It also inhibits the release of norepinephrine.
LOCALIZED VASOCONSTRICTION Injured arteries and arterioles constrict strongly. The constriction appears to be due in part to the local liberation of serotonin from platelets that stick to the vessel wall in the injured area. Injured veins also constrict. A drop in tissue temperature causes vasoconstriction, and this local response to cold plays a part in temperature regulation (see Chapter 18).
SUBSTANCES SECRETED BY THE ENDOTHELIUM ENDOTHELIAL CELLS As noted in Chapter 32, the endothelial cells constitute a large and important tissue. They secrete many growth factors and vasoactive substances. The vasoactive substances include prostaglandins and thromboxanes, nitric oxide, and endothelins.
PROSTACYCLIN & THROMBOXANE A2 Prostacyclin is produced by endothelial cells and thromboxane A2 by platelets from their common precursor arachidonic acid via the cyclooxygenase pathway. Thromboxane A2 promotes platelet aggregation and vasoconstriction, whereas prostacyclin inhibits platelet aggregation and promotes vasodilation. The balance between platelet thromboxane A2 and prostacyclin fosters localized platelet aggregation and consequent clot formation (see Chapter 32) while preventing excessive extension of the clot and maintaining blood flow around it. The thromboxane A2–prostacyclin balance can be shifted toward prostacyclin by administration of low doses of aspirin. Aspirin produces irreversible inhibition of cyclooxygenase by acetylating a serine residue in its active site. Obviously, this reduces production of both thromboxane A2 and prostacyclin. However, endothelial cells produce new cyclooxygenase in a matter of hours, whereas platelets cannot manufacture the enzyme, and the level rises only as new platelets enter the circulation. This is a slow process because platelets have a half-life of about 4 days. Therefore, administration of small amounts of aspirin for prolonged periods reduces clot formation and has been shown to be of value in preventing myocardial infarctions, unstable angina, transient ischemic attacks, and stroke.
NITRIC OXIDE A chance observation two decades ago led to the discovery that the endothelium plays a key role in vasodilation. Many
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different stimuli act on the endothelial cells to produce endothelium-derived relaxing factor (EDRF), a substance that is now known to be nitric oxide (NO). NO is synthesized from arginine (Figure 33–9) in a reaction catalyzed by nitric oxide synthase (NO synthase, NOS). Three isoforms of NOS have been identified: NOS 1, found in the nervous system; NOS 2, found in macrophages and other immune cells; and NOS 3, found in endothelial cells. NOS 1 and NOS 3 are activated by agents that increase intracellular Ca2+ concentrations, including the vasodilators acetylcholine and bradykinin. The NOS in immune cells is not activated by Ca2+ but is induced by cytokines. The NO that is formed in the endothelium diffuses to smooth muscle cells, where it activates soluble guanylyl cyclase, producing cyclic 3,5-guanosine monophosphate (cGMP; see Figure 33–9), which in turn mediates the relaxation of vascular smooth muscle. NO is inactivated by hemoglobin. Adenosine, atrial natriuretic peptide (ANP), and histamine via H2 receptors produce relaxation of vascular smooth muscle that is independent of the endothelium. However, acetylcholine, histamine via H1 receptors, bradykinin, vasoactive intestinal peptide (VIP), substance P, and some other polypeptides act via the endothelium, and various vasoconstrictors that act directly on vascular smooth muscle would produce much greater constriction if their effects were not limited by their ability simultaneously to cause release of NO. When flow to a tissue is suddenly increased by arteriolar dilation, the large arteries to the tissue also dilate. This flow-induced dilation is due to local release of NO. Products of platelet aggregation also cause release of NO, and the resulting vasodilation helps keep blood vessels with an intact endothelium patent. This is in contrast to injured blood vessels, where the endothelium is
damaged at the site of injury and platelets therefore aggregate and produce vasoconstriction (see Chapter 32). Further evidence for a physiologic role of NO is the observation that mice lacking NOS 3 are hypertensive. This suggests that tonic release of NO is necessary to maintain normal blood pressure. NO is also involved in vascular remodeling and angiogenesis, and NO may be involved in the pathogenesis of atherosclerosis. It is interesting in this regard that some patients with heart transplants develop an accelerated form of atherosclerosis in the vessels of the transplant, and there is reason to believe that this is triggered by endothelial damage. Nitroglycerin and other nitrovasodilators that are of great value in the treatment of angina act by stimulating guanylyl cyclase in the same manner as NO. Penile erection is also produced by release of NO, with consequent vasodilation and engorgement of the corpora cavernosa (see Chapter 25). This accounts for the efficacy of drugs such as Viagra, which slow the breakdown of cGMP.
OTHER FUNCTIONS OF NO NO is present in the brain and, acting via cGMP, it is important in brain function (see Chapter 7). It is necessary for the antimicrobial and cytotoxic activity of various inflammatory cells, although the net effect of NO in inflammation and tissue injury depends on the amount and kinetics of release, which in turn may depend on the specific NOS isoform involved. In the gastrointestinal tract, it is important in the relaxation of smooth muscle. Other functions of NO are mentioned in other parts of this book.
CARBON MONOXIDE L-Arginine
+ O2 + NADPH
Ach Bradykinin Shear stress
Ca2+
NOS
Thiol Tetrahydrobiopterin FAD FMN
Citruline + NO + NADP
Soluble guanylyl cyclase
The production of carbon monoxide (CO) from heme is shown in Figure 29–4. HO2, the enzyme that catalyzes the reaction, is present in cardiovascular tissues, and there is growing evidence that CO as well as NO produces local dilation in blood vessels. Interestingly, hydrogen sulfide is likewise emerging as a third gaseotransmitter that regulates vascular tone, although the relative roles of NO, CO, and H2S have yet to be established.
GTP
ENDOTHELINS cGMP
Smooth muscle relaxation
FIGURE 33–9 Synthesis of NO from arginine in endothelial cells and its action via stimulation of soluble guanylyl cyclase and generation of cGMP to produce relaxation in vascular smooth muscle cells. The endothelial form of nitric oxide synthase (NOS) is activated by increased intracellular Ca 2+ concentration, and an increase is produced by acetylcholine (Ach), bradykinin, or shear stress acting on the cell membrane. Thiol, tetrahydrobiopterin, FAD, and FMN are requisite cofactors.
Endothelial cells also produce endothelin-1, one of the most potent vasoconstrictor agents yet isolated. Endothelin-1 (ET1), endothelin-2 (ET-2), and endothelin-3 (ET-3) are the members of a family of three similar 21-amino-acid polypeptides (Figure 33–10). Each is encoded by a different gene. The unique structure of the endothelins resembles that of the sarafotoxins, polypeptides found in the venom of a snake, the Israeli burrowing asp.
CHAPTER 33 Cardiovascular Regulatory Mechanisms
TABLE 33–4 Regulation of endothelin-1
S
L
565
S C
M
S
secretion via transcription of its gene.
Endothelin-1
C
Stimulators
D K C
E
V
Y
F
C
H
L
D
I
I
W
Angiotensin II Catecholamines
S
W
Growth factors
S C
L
S
Endothelin-2
C
Hypoxia
D
Insulin
K C
E
V
Y
F
C
H
L
D
I
I
W
Oxidized LDL HDL
T
Y
F C
K
T
Shear stress
Endothelin-3
C
Thrombin
D K C
E
V
Y
Y
C
H
L
D
I
I
W
Inhibitors NO
D
M
ANP
K C
T
S
Sarafotoxin b
C
PGE2
D
Prostacyclin
K E
C
L
Y
F
C
H
Q
D
V
I
W
FIGURE 33–10
Structure of human endothelins and one of the snake venom sarafotoxins. The amino acid residues that differ from endothelin-1 are indicated in pink.
ENDOTHELIN-1 In endothelial cells, the product of the endothelin-1 gene is processed to a 39-amino-acid prohormone, big endothelin-1, which has about 1% of the activity of endothelin-1. The prohormone is cleaved at a tryptophan-valine (Trp-Val) bond to form endothelin-1 by endothelin-converting enzyme. Small amounts of big endothelin-1 and endothelin-1 are secreted into the blood, but for the most part, they are secreted locally and act in a paracrine fashion. Two different endothelin receptors have been cloned, both of which are coupled via G proteins to phospholipase C (see Chapter 2). The ETA receptor, which is specific for endothelin-1, is found in many tissues and mediates the vasoconstriction produced by endothelin-1. The ETB receptor responds to all three endothelins, and is coupled to Gi. It may mediate vasodilation, and it appears to mediate the developmental effects of the endothelins (see below).
REGULATION OF SECRETION Endothelin-1 is not stored in secretory granules, and most regulatory factors alter the transcription of its gene, with changes in secretion occurring promptly thereafter. Factors activating and inhibiting the gene are summarized in Table 33–4.
CARDIOVASCULAR FUNCTIONS As noted above, endothelin-1 appears to be primarily a paracrine regulator of vascular tone. However, endothelin-1 is not increased in hypertension, and in mice in which one allele of the endothelin-1 gene is knocked out, blood pressure is actually elevated rather than reduced. The concentration of circulating endothelin-1 is, however, elevated in congestive heart failure and after myocardial infarction, so it may play a role in the pathophysiology of these diseases.
OTHER FUNCTIONS OF ENDOTHELINS Endothelin-1 is found in the brain and kidneys as well as the endothelial cells. Endothelin-2 is produced primarily in the kidneys and intestine. Endothelin-3 is present in the blood and is found in high concentrations in the brain. It is also found in the kidneys and gastrointestinal tract. In the brain, endothelins are abundant and, in early life, are produced by both astrocytes and neurons. They are found in the dorsal root ganglia, ventral horn cells, the cortex, the hypothalamus, and cerebellar Purkinje cells. They also play a role in regulating transport across the blood–brain barrier. There are endothelin receptors on mesangial cells (see Chapter 38), and the polypeptide participates in tubuloglomerular feedback. Mice that have both alleles of the endothelin-1 gene deleted have severe craniofacial abnormalities and die of respiratory failure at birth. They also have megacolon (Hirschsprung
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disease), apparently because the cells that normally form the myenteric plexus fail to migrate to the distal colon. In addition, endothelins play a role in closing the ductus arteriosus at birth.
SYSTEMIC REGULATION BY HORMONES Many circulating hormones affect the vascular system. The vasodilator hormones include kinins, VIP, and ANP. Circulating vasoconstrictor hormones include vasopressin, norepinephrine, epinephrine, and angiotensin II.
KININS Two related vasodilator peptides called kinins are found in the body. One is the nonapeptide bradykinin, and the other is the decapeptide lysylbradykinin, also known as kallidin (Figure 33–11). Lysylbradykinin can be converted to bradykinin by aminopeptidase. Both peptides are metabolized to inactive fragments by kininase I, a carboxypeptidase that removes the carboxyl terminal arginine (Arg). In addition, the dipeptidylcarboxypeptidase kininase II inactivates bradykinin and lysylbradykinin by removing phenylalanine-arginine (Phe-Arg) from the carboxyl terminal. Kininase II is the same enzyme as angiotensin-converting enzyme (see Chapter 39), which removes histidine-leucine (His-Leu) from the carboxyl terminal end of angiotensin I. Bradykinin and lysylbradykinin are formed from two precursor proteins: high-molecular-weight kininogen and lowmolecular-weight kininogen (Figure 33–12). They are formed by alternative splicing of a single gene located on chromosome 3. Proteases called kallikreins release the peptides from their precursors. They are produced in humans by a family of three genes located on chromosome 19. There are two types of kallikreins: plasma kallikrein, which circulates in an inactive form, and tissue kallikrein, which appears to be located primarily on the apical membranes of cells concerned with transcellular electrolyte transport. Tissue kallikrein is found in many tissues, including sweat and salivary glands, the pancreas, the prostate, the intestine, and the kidneys. Tissue kallikrein acts on high-molecular-weight kininogen to form bradykinin and low-molecular-weight kininogen
KII KI Lys Arg Pro Pro Gly Phe Ser Pro Phe Arg Aminopeptidase Arg Pro Pro Gly Phe Ser Pro Phe Arg KII KI
FIGURE 33–11
Kinins. Lysylbradykinin (top) can be converted to bradykinin (bottom) by aminopeptidase. The peptides are inactivated by kininase I (KI) or kininase II (KII) at the sites indicated by the short arrows.
XIIa
XII Plasma kallikrein
Clotting
Prekallikrein
HMW kininogen
Bradykinin
LMW kininogen
Lysylbradykinin Tissue kallikrein
FIGURE 33–12
Formation of kinins from high-molecularweight (HMW) and low-molecular-weight (LMW) kininogens.
to form lysylbradykinin. When activated, plasma kallikrein acts on high-molecular-weight kininogen to form bradykinin. Inactive plasma kallikrein (prekallikrein) is converted to the active form, kallikrein, by active factor XII, the factor that initiates the intrinsic blood clotting cascade. Kallikrein also activates factor XII in a positive feedback loop, and highmolecular-weight kininogen has a factor XII-activating action (see Figure 32–13). The actions of both kinins resemble those of histamine. They are primarily tissue hormones, although small amounts are also found in the circulating blood. They cause contraction of visceral smooth muscle, but they relax vascular smooth muscle via NO, lowering blood pressure. They also increase capillary permeability, attract leukocytes, and cause pain upon injection under the skin. They are formed during active secretion in sweat glands, salivary glands, and the exocrine portion of the pancreas, and they are probably responsible for the increase in blood flow when these tissues are actively secreting their products. Two bradykinin receptors, B1 and B2, have been identified. Their amino acid residues are 36% identical, and both are coupled to G proteins. The B1 receptor may mediate the painproducing effects of the kinins, but little is known about its distribution and function. The B2 receptor has strong homology to the H2 receptor and is found in many different tissues.
NATRIURETIC HORMONES There is a family of natriuretic peptides involved in vascular regulation, including atrial natriuretic peptide (ANP) secreted by the heart, brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). They are released in response to hypervolemia. ANP and BNP circulate, whereas CNP acts predominantly in a paracrine fashion. In general, these peptides antagonize the action of various vasoconstrictor agents and lower blood pressure. ANP and BNP also serve to coordinate the control of vascular tone with fluid and electrolyte homeostasis via actions on the kidney.
CHAPTER 33 Cardiovascular Regulatory Mechanisms
CIRCULATING VASOCONSTRICTORS Vasopressin is a potent vasoconstrictor, but when it is injected in normal individuals, there is a compensating decrease in cardiac output, so that there is little change in blood pressure. Its role in blood pressure regulation is discussed in Chapter 18. Norepinephrine has a generalized vasoconstrictor action, whereas epinephrine dilates the vessels in skeletal muscle and the liver. The relative unimportance of circulating norepinephrine, as opposed to norepinephrine released from vasomotor nerves, is pointed out in Chapter 22, where the cardiovascular actions of catecholamines are discussed in detail. Angiotensin II has a generalized vasoconstrictor action. It is formed by the action of angiotensin converting enzyme (ACE) on angiotensin I, which itself is liberated by the action of renin from the kidney on circulating angiotensinogen (see Chapter 39). Renin secretion, in turn, is increased when the blood pressure falls or extracellular fluid (ECF) volume is reduced, and angiotensin II therefore helps to maintain blood pressure. Angiotensin II also increases water intake and stimulates aldosterone secretion, and increased formation of angiotensin II is part of a homeostatic mechanism that operates to maintain ECF volume (see Chapter 22). In addition, there are rennin–angiotensin systems in many different organs, and there may be one in the walls of blood vessels. Angiotensin II produced in blood vessel walls could be important in some forms of clinical hypertension. The role of angiotensin II in cardiovascular regulation is also amply demonstrated in the widespread use of so-called ACE inhibitors as antihypertensive medications. Urotensin-II, a polypeptide first isolated from the spinal cord of fish, is present in human cardiac and vascular tissue. It is one of the most potent mammalian vasoconstrictors known, but its pathophysiogic and physiologic roles are currently the subject of intense interest.
CHAPTER SUMMARY ■
■ ■
■
■
RVLM neurons project to the thoracolumbar IML and release glutamate on preganglionic sympathetic neurons that innervate the heart and vasculature. The NTS is the major excitatory input to cardiac vagal motor neurons in the nucleus ambiguus. Carotid sinus and aortic depressor baroreceptors are innervated by branches of the 9th and 10th cranial nerves, respectively (glossopharyngeal and aortic depressor nerves). These receptors are most sensitive to changes in pulse pressure but also respond to changes in mean arterial pressure. Baroreceptor nerves terminate in the NTS and release glutamate. NTS neurons project to the CVLM and nucleus ambiguus and release glutamate. CVLM neurons project to RVLM and release GABA. This leads to a reduction in sympathetic activity and an increase in vagal activity (ie, the baroreceptor reflex). Activation of peripheral chemoreceptors in the carotid and aortic bodies by a reduction in PaO2 or an increase in PaCO2 leads to an increase in vasoconstriction. Heart rate changes are vari-
■ ■
■
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567
able and depend on a number of factors including changes in respiration. In addition to various neural inputs, RVLM neurons are directly activated by hypoxia and hypercapnia. Most vascular beds have an intrinsic capacity to respond to changes in blood pressure within a certain range by altering vascular resistance to maintain stable blood flow. This property is known as autoregulation. Local factors such as oxygen tension, pH, temperature, and metabolic products contribute to vascular regulation; many produce vasodilation to restore blood flow. The endothelium is an important source of vasoactive mediators that act to either contract or relax vascular smooth muscle. Three gaseous mediators—NO, CO, and H2S—are important regulators of vasodilation. Endothelins and angiotensin II induce vasoconstriction and may be involved in the pathogenesis of some forms of hypertension.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. When a pheochromocytoma (tumor of the adrenal medulla) suddenly discharges a large amount of epinephrine into the circulation, the patient’s heart rate would be expected to A) increase, because the increase in blood pressure stimulates the carotid and aortic baroreceptors. B) increase, because epinephrine has a direct chronotropic effect on the heart. C) increase, because of increased tonic parasympathetic discharge to the heart. D) decrease, because the increase in blood pressure stimulates the carotid and aortic chemoreceptors. E) decrease, because of increased tonic parasympathetic discharge to the heart. 2. Activation of the baroreceptor reflex A) is primarily involved in short-term regulation of systemic blood pressure. B) leads to an increase in heart rate because of inhibition of the vagal cardiac motor neurons. C) inhibits neurons in the CVLM. D) excites neurons in the RVLM. E) all of the above 3. Sympathetic nerve activity would be expected to increase A) if glutamate receptors were blocked in the NTS. B) if GABA receptors were blocked in the RVLM. C) if there was a compression of the RVLM. D) during hypoxia. E) for all of the above. 4. Why is the dilator response to injected acetylcholine changed to a constrictor response when the endothelium is damaged? A) More Na+ is generated. B) More bradykinin is generated. C) The damage lowers the pH of the remaining layers of the artery. D) The damage augments the production of endothelin by the endothelium. E) The damage interferes with the production of NO by the endothelium.
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CHAPTER RESOURCES Ahluwalia A, MacAllister RJ, Hobbs AJ: Vascular actions of natriuretic peptides. Cyclic GMP-dependent and -independent mechanisms. Basic Res Cardiol 2004;99:83. Benarroch EE: Central Autonomic Network. Functional Organization and Clinical Correlations. Futura Publishing, 1997. Chapleau MW, Abboud F (editors): Neuro-cardiovascular regulation: From molecules to man. Ann NY Acad Sci 2001;940. de Burgh Daly M: Peripheral Arterial Chemoreceptors and Respiratory-Cardiovascular Integration. Clarendon Press, 1997. Haddy FJ, Vanhouttee PM, Feletou M: Role of potassium in regulating blood flow and blood pressure. Am J Physiol Regul Integr Comp Physiol 2006;290:R546.
Loewy AD, Spyer KM (editors): Central Regulation of Autonomic Function. Oxford University Press, 1990. Marshall JM: Peripheral chemoreceptors and cardiovascular regulation. Physiol Rev 1994;74:543. Paffett ML, Walker BR: Vascular adaptations to hypoxia: Molecular and cellular mechanisms regulating vascular tone. Essays Biochem 2007;43:105. Squire LR, Bloom FE, Spitzer NC, du Lac S, Ghosh A, Berg D (editors): Fundamental Neuroscience, 3rd ed. Academic Press, 2008. Trouth CO, Millis RM, Kiwull-Schöne HF, Schläfke ME: Ventral Brainstem Mechanisms and Control of Respiration and Blood Pressure. Marcel Dekker, 1995.
34 C
Circulation Through Special Regions
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■
Define the special features of the circulation in the brain, coronary vessels, skin, and fetus, and how these are regulated. Describe how cerebrospinal fluid (CSF) is formed and reabsorbed, and its role in protecting the brain from injury. Understand how the blood–brain barrier impedes the entry of specific substances into the brain. Delineate how the oxygen needs of the contracting myocardium are met by the coronary arteries and the consequences of their occlusion. List the vascular reactions of the skin and the reflexes that mediate them. Understand how the fetus is supplied with oxygen and nutrients in utero, and the circulatory events required for a transition to independent life after birth.
INTRODUCTION The distribution of the cardiac output to various parts of the body at rest in a normal man is shown in Table 34–1. The general principles described in preceding chapters apply to the circulation of all these regions, but the vascular supplies of many organs have additional special features that are important to their physiology. The portal circulation of the anterior
pituitary is discussed in Chapter 24, the pulmonary circulation in Chapter 35, the renal circulation in Chapter 38, and the circulation of the splanchnic area, particularly the intestines and liver, in Chapters 26 and 29. This chapter is concerned with the special circulations of the brain, the heart, and the skin, as well as the placenta and fetus.
CEREBRAL CIRCULATION: ANATOMIC CONSIDERATIONS
one carotid artery are distributed almost exclusively to the cerebral hemisphere on that side. Normally no crossing over occurs, probably because the pressure is equal on both sides. Even when it is not, the anastomotic channels in the circle do not permit a very large flow. Occlusion of one carotid artery, particularly in older patients, often causes serious symptoms of cerebral ischemia. There are precapillary anastomoses between the cerebral vessels, but flow through these channels is generally insufficient to maintain the circulation and prevent infarction when a cerebral artery is occluded. Venous drainage from the brain by way of the deep veins and dural sinuses empties principally into the internal jugular veins in humans, although a small amount of venous blood
VESSELS The principal arterial inflow to the brain in humans is via four arteries: two internal carotids and two vertebrals. In humans, the carotid arteries are quantitatively the most significant. The vertebral arteries unite to form the basilar artery, and the basilar artery and the carotids form the circle of Willis below the hypothalamus. The circle of Willis is the origin of the six large vessels supplying the cerebral cortex. Substances injected into
569
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SECTION VI Cardiovascular Physiology
TABLE 34–1 Resting blood flow and O2 consumption of various organs in a 63-kg adult man with a mean arterial blood pressure of 90 mm Hg and an O2 consumption of 250 mL/min. Blood Flow
per kg
Percentage of Total
Liver
2.6
1500
57.7
34
51
2.0
3.6
9.4
27.8
20.4
Kidneys
0.3
1260
420.0
14
18
6.0
4.3
1.3
23.3
7.2
Brain
1.4
750
54.0
62
46
3.3
7.2
10.1
13.9
18.4
Skin
3.6
462
12.8
25
12
0.3
11.7
42.1
8.6
4.8
31.0
840
2.7
60
50
0.2
6.4
198.4
15.6
20.0
Heart muscle
0.3
250
84.0
114
29
9.7
21.4
6.4
4.7
11.6
Rest of body
23.8
336
1.4
129
44
0.2
16.1
383.2
6.2
17.6
Whole body
63.0
5400
8.6
46
250
0.4
1.0
63.0
100.0
100.0
mL/min
mL/100 g/min
Resistance (R units)a
mL/min
Skeletal muscle
a
Oxygen Consumption
Mass (kg)
Region
mL/100 g/min
Arteriovenous Oxygen Difference (mL/L)
Absolute
Cardiac Output
Oxygen Consumption
R units are pressure (mm Hg) divided by blood flow (mL/s).
Reproduced with permission from Bard P (editor): Medical Physiology, 11th ed. Mosby, 1961.
drains through the ophthalmic and pterygoid venous plexuses, through emissary veins to the scalp, and down the system of paravertebral veins in the spinal canal. The cerebral vessels have a number of unique anatomic features. In the choroid plexuses, there are gaps between the endothelial cells of the capillary wall, but the choroid epithelial cells that separate them from the cerebrospinal fluid (CSF) are connected to one another by tight junctions. The capillaries in the brain substance resemble nonfenestrated capillaries in muscle (see Chapter 32), but there are tight junctions between the endothelial cells that limit the passage of substances through the junctions. In addition, there are relatively few vesicles in the endothelial cytoplasm, and presumably little vesicular transport. However, multiple transport systems are present in the capillary cells. The brain capillaries are surrounded by the endfeet of astrocytes (Figure 34–1). These endfeet are closely applied to the basal lamina of the capillaries, but they do not cover the entire capillary wall, and gaps of about 20 nm occur between endfeet (Figure 34–2). However, the endfeet induce the tight junctions in the capillaries (see Chapter 32). The protoplasm of astrocytes is also found around synapses, where it appears to isolate the synapses in the brain from one another.
also innervate the cerebral vessels, and the postganglionic cholinergic neurons on the blood vessels contain acetylcholine. Many also contain vasoactive intestinal peptide (VIP) and peptide histidyl methionine (PHM-27) (see Chapter 7). These nerves end primarily on large arteries. Sensory nerves are found on more distal arteries. They have their cell bodies in the trigeminal ganglia and contain substance P, neurokinin A, and calcitonin gene-related peptide (CGRP). Substance P, CGRP, VIP, and PHM-27 cause vasodilation, whereas
1 2 4
3
INNERVATION Three systems of nerves innervate the cerebral blood vessels. Postganglionic sympathetic neurons have their cell bodies in the superior cervical ganglia, and their endings contain norepinephrine. Many also contain neuropeptide Y. Cholinergic neurons that probably originate in the sphenopalatine ganglia
FIGURE 34–1
Relation of fibrous astrocyte (3) to a capillary (2) and neuron (4) in the brain. The endfeet of the astrocyte processes form a discontinuous membrane around the capillary (1). Astrocyte processes also envelop the neuron. (Adapted from Krstic RV: Die Gewebe des
Menschen und der Säugetiere. Springer, 1978.)
CHAPTER 34 Circulation Through Special Regions
Nucleus
TABLE 34–2 Concentration of various substances in human CSF and plasma.
Mitochondrion
Substance Lipid-soluble diffusion, carrier-mediated transport
Glucose, etc
Glial endfoot Tight junction
FIGURE 34–2
571
Transport across cerebral capillaries.
CSF
Plasma
Ratio CSF/Plasma
Na+
(meq/kg H2O)
147.0
150.0
0.98
K+
(meq/kg H2O)
2.9
4.6
0.62
Mg2+
(meq/kg H2O)
2.2
1.6
1.39
Ca2+
(meq/kg H2O)
2.3
4.7
0.49
Cl–
(meq/kg H2O)
113.0
99.0
1.14
HCO3–
(meq/L)
25.1
24.8
1.01
PCO2
(mm Hg)
50.2
39.5
1.28
pH
7.33
289.0
1.00
(mg/dL)
20.0
6000.0
0.003
Glucose
(mg/dL)
64.0
100.0
0.64
CEREBROSPINAL FLUID
Inorganic P
(mg/dL)
3.4
4.7
0.73
FORMATION & ABSORPTION
Urea
(mg/dL)
12.0
15.0
0.80
Creatinine
(mg/dL)
1.5
1.2
1.25
Uric acid
(mg/dL)
1.5
5.0
0.30
Cholesterol
(mg/dL)
0.2
175.0
CSF fills the ventricles and subarachnoid space. In humans, the volume of CSF is about 150 mL and the rate of CSF production is about 550 mL/d. Thus the CSF turns over about 3.7 times a day. In experiments on animals, it has been estimated that 50– 70% of the CSF is formed in the choroid plexuses and the remainder is formed around blood vessels and along ventricular walls. Presumably, the situation in humans is similar. The CSF in the ventricles flows through the foramens of Magendie and Luschka to the subarachnoid space and is absorbed through the arachnoid villi into veins, primarily the cerebral venous sinuses. The villi consist of projections of the fused arachnoid membrane and endothelium of the sinuses into the venous sinuses. Similar, smaller villi project into veins around spinal nerve routes. These projections may contribute to the outflow of CSF into venous blood by a process known as bulk flow, which is unidirectional. However, recent studies suggest that, at least in animals, a more important route for CSF reabsorption into the bloodstream in health is via the cribriform plate above the nose and thence into the cervical lymphatics. However, reabsorption via one-way valves (of uncertain structural basis) in the arachnoid villi may assume a greater role if CSF pressure is elevated. Likewise, when CSF builds up abnormally, aquaporin water channels may be expressed in the choroid plexus and brain microvessels as a compensatory adaptation. CSF is formed continuously by the choroid plexus in two stages. First, plasma is passively filtered across the choroidal capillary endothelium. Next, secretion of water and ions across the choroidal epithelium provides for active control of CSF composition and quantity. Bicarbonate, chloride, and potassium ions enter the CSF via channels in the epithelial cell
(mosm/kg H2O)
Protein
...
289.0
neuropeptide Y is a vasoconstrictor. Touching or pulling on the cerebral vessels causes pain.
Osmolality
7.40
0.001
apical membranes. Aquaporins provide for water movement to balance osmotic gradients. The composition of CSF (Table 34–2) is essentially the same as that of brain extracellular fluid (ECF), which in living humans makes up 15% of the brain volume. In adults, free communication appears to take place between the brain interstitial fluid and CSF, although the diffusion distances from some parts of the brain to the CSF are appreciable. Consequently, equilibration may take some time to occur, and local areas of the brain may have extracellular microenvironments that are transiently different from CSF. Lumbar CSF pressure is normally 70 to 180 mm H2O. Up to pressures well above this range, the rate of CSF formation is independent of intraventricular pressure. However, absorption is proportional to the pressure (Figure 34–3). At a pressure of 112 mm H2O, which is the average normal CSF pressure, filtration and absorption are equal. Below a pressure of approximately 68 mm H2O, absorption stops. Large amounts of fluid accumulate when the capacity for CSF reabsorption is decreased (external hydrocephalus, communicating hydrocephalus). Fluid also accumulates proximal to the block and distends the ventricles when the foramens of Luschka and Magendie are blocked or there is obstruction within the ventricular system (internal hydrocephalus, noncommunicating hydrocephalus).
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SECTION VI Cardiovascular Physiology
Outer table of skull
Flow (mL/min)
1.6
Trabecular bone
1.2
0.8
Ab
r so
pt
ion
Formation
0.4
Inner table of skull Dura mater Subdural (potential) space Arachnoid
0 0
68 100 112 200 Outflow pressure (mm CSF)
FIGURE 34–3
CSF formation and absorption in humans at various CSF pressures. Note that at 112 mm CSF, formation and absorption are equal, and at 68 mm CSF, absorption is zero. (Modified and reproduced with permission from Cutler RWP, et al: Formation and absorption of
Subarachnoid space Arachnoid trabeculae Artery Pia mater Perivascular spaces
cerebrospinal fluid in man. Brain 1968;91:707.)
Brain
PROTECTIVE FUNCTION The most critical role for CSF (and the meninges) is to protect the brain. The dura is attached firmly to bone. Normally, there is no “subdural space,” with the arachnoid being held to the dura by the surface tension of the thin layer of fluid between the two membranes. As shown in Figure 34–4, the brain itself is supported within the arachnoid by the blood vessels and nerve roots and by the multiple fine fibrous arachnoid trabeculae. The brain weighs about 1400 g in air, but in its “water bath” of CSF it has a net weight of only 50 g. The buoyancy of the brain in the CSF permits its relatively flimsy attachments to suspend it very effectively. When the head receives a blow, the arachnoid slides on the dura and the brain moves, but its motion is gently checked by the CSF cushion and by the arachnoid trabeculae. The pain produced by spinal fluid deficiency illustrates the importance of CSF in supporting the brain. Removal of CSF during lumbar puncture can cause a severe headache after the fluid is removed, because the brain hangs on the vessels and nerve roots, and traction on them stimulates pain fibers. The pain can be relieved by intrathecal injection of sterile isotonic saline.
HEAD INJURIES Without the protection of the spinal fluid and the meninges, the brain would probably be unable to withstand even the minor traumas of everyday living; but with the protection afforded, it takes a fairly severe blow to produce cerebral damage. The brain is damaged most commonly when the skull is fractured and bone is driven into neural tissue (depressed skull fracture), when the brain moves far enough to tear the delicate bridging veins from the cortex to the bone, or when the brain is accelerated by a blow on the head and is driven against the skull or the tentorium at a point opposite where the blow was struck (contrecoup injury).
FIGURE 34–4 Investing membranes of the brain, showing their relation to the skull and to brain tissue. (Reproduced with permission from Wheater PR et al: Functional Histology. Churchill Livingstone, 1979.)
THE BLOOD–BRAIN BARRIER The tight junctions between capillary endothelial cells in the brain and between the epithelial cells in the choroid plexus effectively prevent proteins from entering the brain in adults and slow the penetration of some smaller molecules as well. An example is the slow penetration of urea (Figure 34–5). This uniquely limited exchange of substances into the brain is referred to as the blood–brain barrier, a term most commonly used to encompass this barrier overall and more specifically the barrier in the choroid epithelium between blood and CSF. Passive diffusion across the tight cerebral capillaries is very limited, and little vesicular transport takes place. However, there are numerous carrier-mediated and active transport systems in the cerebral capillaries. These move substances out of as well as into the brain, though movement out of the brain is generally more free than movement into it.
PENETRATION OF SUBSTANCES INTO THE BRAIN Water, CO2, and O2 penetrate the brain with ease, as do the lipid-soluble free forms of steroid hormones, whereas their
CHAPTER 34 Circulation Through Special Regions
Tissue concentration Plasma
1.0
Muscle Brain CSF
0.8
0.6
GLUT 3 GLUT 1 55K GLUT 1 45K GLUT 5
Oligodendroglia
Astroglia GLUT 1 55K GLUT 1 Endothelial 45K cell
0.4
Neuron Microglia
0.2
0
573
30
60 90 120 150 Min after start of infusion
180
Microvessel - Lumen -
FIGURE 34–5
GLUT 3
Penetration of urea into muscle, brain, spinal cord, and CSF. Urea was administered by constant infusion.
GLUT 5
FIGURE 34–6
Localization of the various GLUT transporters in the brain. (Adapted from Maher F, Vannucci SJ, Simpson IA: Glucose
protein-bound forms and, in general, all proteins and polypeptides do not. The rapid passive penetration of CO2 contrasts with the regulated transcellular penetration of H+ and HCO3– and has physiologic significance in the regulation of respiration (see Chapter 37). Glucose is the major ultimate source of energy for nerve cells. Its diffusion across the blood–brain barrier would be very slow, but the rate of transport into the CSF is markedly enhanced by the presence of specific transporters, including the glucose transporter 1 (GLUT 1). The brain contains two forms of GLUT 1: GLUT 1 55K and GLUT 1 45K. Both are encoded by the same gene, but they differ in the extent to which they are glycosylated. GLUT 1 55K is present in high concentration in brain capillaries (Figure 34–6). Infants with congenital GLUT 1 deficiency develop low CSF glucose concentrations in the presence of normal plasma glucose, and they have seizures and delayed development. In addition, transporters for thyroid hormones; several organic acids; choline; nucleic acid precursors; and neutral, basic, and acidic amino acids are present at the blood–brain barrier. A variety of drugs and peptides actually cross the cerebral capillaries but are promptly transported back into the blood by a multidrug nonspecific transporter in the apical membranes of the endothelial cells. This P-glycoprotein is a member of the family of adenosine triphosphate (ATP) binding cassettes that transport various proteins and lipids across cell membranes (see Chapter 2). In the absence of this transporter in mice, much larger proportions of systemically administered doses of various chemotherapeutic drugs, analgesics, and opioid peptides are found in the brain than in controls. If pharmacologic agents that inhibit this transporter can be developed, they could be of value in the treatment of brain tumors and other central nervous system (CNS) diseases in which it is difficult to introduce adequate amounts of therapeutic agents into the brain.
transporter proteins in brain. FASEB J 1994;8:1003.)
CIRCUMVENTRICULAR ORGANS When dyes that bind to proteins in the plasma are injected, they stain many tissues but spare most of the brain. However, four small areas in or near the brain stem do take up the stain. These areas are (1) the posterior pituitary (neurohypophysis) and the adjacent ventral part of the median eminence of the hypothalamus, (2) the area postrema, (3) the organum vasculosum of the lamina terminalis (OVLT, supraoptic crest), and (4) the subfornical organ (SFO). These areas are referred to collectively as the circumventricular organs (Figure 34–7). All have fenestrated capillaries, and because of their permeability they are said to be “outside the blood–brain barrier.” Some of them function as neurohemal organs; that is, areas in which polypeptides secreted by neurons enter the circulation. Others contain receptors for many different peptides and other substances, and function as chemoreceptor zones in which substances in the circulating blood can act to trigger changes in brain function without penetrating the blood–brain barrier. For example, the area postrema is a chemoreceptor trigger zone that initiates vomiting in response to chemical changes in the plasma (see Chapter 28). It is also concerned with cardiovascular control, and in many species circulating angiotensin II acts on the area postrema to produce a neurally mediated increase in blood pressure. Angiotensin II also acts on the SFO and possibly on the OVLT to increase water intake. In addition, it appears that the OVLT is the site of the osmoreceptor controlling vasopressin secretion (see Chapter 39), and evidence suggests that circulating interleukin-1 (IL-1) produces fever by acting here too. The subcommissural organ (Figure 34–7) is closely associated with the pineal gland and histologically resembles the circumventricular organs. However, it does not have fenestrated capillaries, is not highly permeable, and has no established
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SECTION VI Cardiovascular Physiology
CLINICAL BOX 34–1 Clinical Implications of the Blood–Brain Barrier
SFO SCO
PI
OVLT
NH
AP
FIGURE 34–7 Circumventricular organs. The neurohypophysis (NH), organum vasculosum of the lamina terminalis (OVLT, organum vasculosum of the lamina terminalis), subfornical organ (SFO), and area postrema (AP) are shown projected on a sagittal section of the human brain. SCO, subcommissural organ; PI, pineal. function. Conversely, the pineal and the anterior pituitary do have fenestrated capillaries and are outside the blood–brain barrier, but both are endocrine glands and are not part of the brain.
FUNCTION OF THE BLOOD–BRAIN BARRIER The blood–brain barrier strives to maintain the constancy of the environment of the neurons in the central nervous system (see Clinical Box 34–1). Even minor variations in the concentrations of K+, Ca2+, Mg2+, H+, and other ions can have farreaching consequences. The constancy of the composition of the ECF in all parts of the body is maintained by multiple homeostatic mechanisms (see Chapters 1 and 39), but because of the sensitivity of the cortical neurons to ionic change, it is not surprising that an additional defense has evolved to protect them. Other functions of the blood–brain barrier include protection of the brain from endogenous and exogenous toxins in the blood and prevention of the escape of neurotransmitters into the general circulation.
DEVELOPMENT OF THE BLOOD–BRAIN BARRIER In experimental animals, many small molecules penetrate the brain more readily during the fetal and neonatal period than they do in the adult. On this basis, it is often stated that the blood–brain barrier is immature at birth. Humans are more
Physicians must know the degree to which drugs penetrate the brain in order to treat diseases of the nervous system intelligently. For example, it is clinically relevant that the amines dopamine and serotonin penetrate brain tissue to a very limited degree but their corresponding acid precursors, L-dopa and 5-hydroxytryptophan, respectively, enter with relative ease (see Chapters 7 and 16). Another important clinical consideration is the fact that the blood–brain barrier tends to break down in areas of infection or injury. Tumors develop new blood vessels, and the capillaries that are formed lack contact with normal astrocytes. Therefore, there are no tight junctions, and the vessels may even be fenestrated. The lack of a barrier helps in identifying the location of tumors; substances such as radioactive iodine-labeled albumin penetrate normal brain tissue very slowly, but they enter tumor tissue, making the tumor stand out as an island of radioactivity in the surrounding normal brain. The blood–brain barrier can also be temporarily disrupted by sudden marked increases in blood pressure or by intravenous injection of hypertonic fluids.
mature at birth than rats and various other experimental animals, and detailed data on passive permeability of the human blood–brain barrier are not available. However, in severely jaundiced infants with high plasma levels of free bilirubin and an immature hepatic bilirubin-conjugating system, free bilirubin enters the brain and, in the presence of asphyxia, damages the basal ganglia (kernicterus). The counterpart of this situation in later life is the Crigler–Najjar syndrome in which there is a congenital deficiency of glucuronyl transferase. These individuals can have very high free bilirubin levels in the blood and develop encephalopathy. In other conditions, free bilirubin levels are generally not high enough to produce brain damage.
CEREBRAL BLOOD FLOW & ITS REGULATION KETY METHOD According to the Fick principle (see Chapter 31), the blood flow of any organ can be measured by determining the amount of a given substance (Qx) removed from the bloodstream by the organ per unit of time and dividing that value by the difference between the concentration of the substance in arterial blood and the concentration in the venous blood from the organ ([Ax] – [Vx]). Thus: Qx Cerebral blood flow (CBF) = ---------------------------[ Ax ] – [ Vx ]
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This can be applied clinically using inhaled nitrous oxide (N2O) (Kety method). The average cerebral blood flow in young adults is 54 mL/100 g/min. The average adult brain weighs about 1400 g, so the flow for the whole brain is about 756 mL/min. Note that the Kety method provides an average value for perfused areas of brain because it gives no information about regional differences in blood flow. It also can only measure flow to perfused parts of the brain. If the blood flow to a portion of the brain is occluded, the measured flow does not change because the nonperfused area does not take up any N2O. In spite of the marked local fluctuations in brain blood flow with neural activity, the cerebral circulation is regulated in such a way that total blood flow remains relatively constant. The factors involved in regulating the flow are summarized in Figure 34–8.
creases and blood flow is much less severely compromised than it would otherwise be. Conversely, during acceleration downward, force acting toward the head (negative g) increases arterial pressure at head level, but intracranial pressure also rises, so that the vessels are supported and do not rupture. The cerebral vessels are protected during the straining associated with defecation or delivery in the same way.
ROLE OF INTRACRANIAL PRESSURE
ROLE OF VASOMOTOR & SENSORY NERVES
In adults, the brain, spinal cord, and spinal fluid are encased, along with the cerebral vessels, in a rigid bony enclosure. The cranial cavity normally contains a brain weighing approximately 1400 g, 75 mL of blood, and 75 mL of spinal fluid. Because brain tissue and spinal fluid are essentially incompressible, the volume of blood, spinal fluid, and brain in the cranium at any time must be relatively constant (Monro–Kellie doctrine). More importantly, the cerebral vessels are compressed whenever the intracranial pressure rises. Any change in venous pressure promptly causes a similar change in intracranial pressure. Thus, a rise in venous pressure decreases cerebral blood flow both by decreasing the effective perfusion pressure and by compressing the cerebral vessels. This relationship helps to compensate for changes in arterial blood pressure at the level of the head. For example, if the body is accelerated upward (positive g), blood moves toward the feet and arterial pressure at the level of the head decreases. However, venous pressure also falls and intracranial pressure falls, so that the pressure on the vessels de-
Local constriction and dilation of cerebral arterioles
As seen in other vascular beds, autoregulation is prominent in the brain (Figure 34–9). This process, by which the flow to many tissues is maintained at relatively constant levels despite variations in perfusion pressure, is discussed in Chapter 32. In the brain, autoregulation maintains a normal cerebral blood flow at arterial pressures of 65 to 140 mm Hg.
The innervation of large cerebral blood vessels by postganglionic sympathetic and parasympathetic nerves and the additional distal innervation by sensory nerves have been described above. The nerves may also modulate tone indirectly, via the release of paracine substances from astrocytes. The precise role of these nerves, however, remains a matter of debate. It has been argued that noradrenergic discharge occurs when the blood pressure is markedly elevated. This reduces the resultant passive increase in blood flow and helps protect the blood– brain barrier from the disruption that could otherwise occur (see above). Thus, vasomotor discharges affect autoregulation. With sympathetic stimulation, the constant-flow, or plateau, part of the pressure-flow curve is extended to the right (Figure 34–9); that is, greater increases in pressure can occur without an increase in flow. On the other hand, the vasodilator hydralazine and the angiotensin-converting enzyme (ACE) inhibitor captopril reduce the length of the plateau. Finally, neurovascular coupling may adjust local perfusion in response to changes in brain activity (see below).
Cranium Brain, spinal cord, and spinal fluid 100 CBF
Intracranial pressure
AUTOREGULATION
50
Mean arterial pressure at brain level Viscosity of blood Mean venous pressure at brain level
FIGURE 34–8
Vertebral column
Diagrammatic summary of the factors affecting overall cerebral blood flow.
70 140 Arterial pressure (mm Hg)
FIGURE 34–9
Autoregulation of cerebral blood flow (CBF) during steady-state conditions. The blue line shows the alteration produced by sympathetic stimulation during autoregulation.
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FIGURE 34–10 Activity in the human brain at five different horizontal levels while a subject generates a verb that is appropriate for each noun presented by an examiner. This mental task activates the frontal cortex (slices 1–4), anterior cingulate gyrus (slice 1), and posterior temporal lobe (slice 3) on the left side and the cerebellum (slices 4 and 5) on the right side. Light purple, moderate activation; dark purple, marked activation. (Based on PET scans in Posner MI, Raichle ME: Images of Mind. Scientific American Library, 1994.)
BLOOD FLOW IN VARIOUS PARTS OF THE BRAIN A major advance in recent decades has been the development of techniques for monitoring regional blood flow in living, conscious humans. Among the most valuable methods are positron emission tomography (PET) and related techniques in which a short-lived radioisotope is used to label a compound and the compound is injected. The arrival and clearance of the tracer are monitored by scintillation detectors placed over the head. Because blood flow is tightly coupled to brain metabolism, local uptake of 2-deoxyglucose is also a good index of blood flow (see below and Chapter 1). If the 2deoxyglucose is labeled with a short-half-life positron emitter such as 18F, 11O, or 15O, its concentration in any part of the brain can be monitored. Another valuable technique involves magnetic resonance imaging (MRI). MRI is based on detecting resonant signals from different tissues in a magnetic field. Functional magnetic resonance imaging (fMRI) measures the amount of blood in a tissue area. When neurons become active, their increased discharge alters the local field potential. A still unsettled mechanism triggers an increase in local blood flow and oxygen. The increase in oxygenated blood is detected by fMRI. PET scanning can be used to measure not only blood flow but the concentration of molecules, such as dopamine, in various regions of the living brain. On the other hand, fMRI does not involve the use of radioactivity. Consequently, it can be used at frequent intervals to measure changes in regional blood flow in a single individual. In resting humans, the average blood flow in gray matter is 69 mL/100 g/min compared with 28 mL/100 g/min in white matter. A striking feature of cerebral function is the marked variation in local blood flow with changes in brain activity. An example is shown in Figure 34–10. In subjects who are awake but at rest, blood flow is greatest in the premotor and frontal regions. This is the part of the brain that is believed to be concerned with decoding and analyzing afferent input and with intellectual activity. During voluntary clenching of the right hand, flow is increased in the hand area of the left motor cortex and the corresponding sensory areas in the postcentral gyrus.
Especially when the movements being performed are sequential, the flow is also increased in the supplementary motor area. When subjects talk, there is a bilateral increase in blood flow in the face, tongue, and mouth-sensory and motor areas and the upper premotor cortex in the categorical (usually the left) hemisphere. When the speech is stereotyped, Broca’s and Wernicke’s areas do not show increased flow, but when the speech is creative—that is, when it involves ideas—flow increases in both these areas. Reading produces widespread increases in blood flow. Problem solving, reasoning, and motor ideation without movement produce increases in selected areas of the premotor and frontal cortex. In anticipation of a cognitive task, many of the brain areas that will be activated during the task are activated beforehand, as if the brain produces an internal model of the expected task. In right-handed individuals, blood flow to the left hemisphere is greater when a verbal task is being performed and blood flow to the right hemisphere is greater when a spatial task is being performed (see Clinical Box 34–2).
BRAIN METABOLISM & OXYGEN REQUIREMENTS UPTAKE & RELEASE OF SUBSTANCES BY THE BRAIN If the cerebral blood flow is known, it is possible to calculate the consumption or production by the brain of O2, CO2, glucose, or any other substance present in the bloodstream by multiplying the cerebral blood flow by the difference between the concentration of the substance in arterial blood and its concentration in cerebral venous blood (Table 34–3). When calculated in this fashion, a negative value indicates that the brain is producing the substance.
OXYGEN CONSUMPTION O2 consumption by the human brain (cerebral metabolic rate for O2, CMRO2) averages approximately 20% of the total body resting O2 consumption (Table 34–1). The brain is
CHAPTER 34 Circulation Through Special Regions
CLINICAL BOX 34–2 Changes in Cerebral Blood Flow in Disease Several disease states are now known to be associated with localized or general changes in cerebral blood flow, as revealed by PET scanning and fMRI techniques. For example, epileptic foci are hyperemic during seizures, whereas flow is reduced in other parts of the brain. Between seizures, flow is sometimes reduced in the foci that generate the seizures. Parietooccipital flow is decreased in patients with symptoms of agnosia (see Chapter 14). In Alzheimer disease, the earliest change is decreased metabolism and blood flow in the superior parietal cortex, with later spread to the temporal and finally the frontal cortex. The pre- and postcentral gyri, basal ganglia, thalamus, brain stem, and cerebellum are relatively spared. In Huntington disease, blood flow is reduced bilaterally in the caudate nucleus, and this alteration in flow occurs early in the disease. In manic depressives (but interestingly, not in patients with unipolar depression), there is a general decrease in cortical blood flow when the patients are depressed. In schizophrenia, some evidence suggests decreased blood flow in the frontal lobes, temporal lobes, and basal ganglia. Finally, during the aura in patients with migraine, a bilateral decrease in blood flow starts in the occipital cortex and spreads anteriorly to the temporal and parietal lobes.
extremely sensitive to hypoxia, and occlusion of its blood supply produces unconsciousness in a period as short as 10 s. The vegetative structures in the brain stem are more resistant to hypoxia than the cerebral cortex, and patients may recover from accidents such as cardiac arrest and other conditions causing fairly prolonged hypoxia with normal vegetative functions but severe, permanent intellectual deficiencies. The basal
TABLE 34–3 Utilization and production of substances by the adult human brain in vivo. Substance
Uptake (+) or Output (–) per 100 g of Brain/min
Total/min
Substances utilized Oxygen
+3.5 mL
+49 mL
Glucose
+5.5 mg
+77 mg
Glutamate
+0.4 mg
+5.6 mg
Substances produced Carbon dioxide
–3.5 mL
–49 mL
Glutamine
–0.6 mL
–8.4 mg
Substances not used or produced in the fed state: lactate, pyruvate, total ketones, and α-ketoglutarate.
577
ganglia use O2 at a very high rate, and symptoms of Parkinson disease as well as intellectual deficits can be produced by chronic hypoxia. The thalamus and the inferior colliculus are also very susceptible to hypoxic damage (see Clinical Box 34–3).
ENERGY SOURCES Glucose is the major ultimate source of energy for the brain; under normal conditions, 90% of the energy needed to maintain ion gradients across cell membranes and transmit electrical impulses comes from this source. Glucose enters the brain via GLUT 1 in cerebral capillaries (see above). Other transporters then distribute it to neurons and glial cells. Glucose is taken up from the blood in large amounts, and the RQ (respiratory quotient; see Chapter 21) of cerebral tissue is 0.95–0.99 in normal individuals. Importantly, insulin is not required for most cerebral cells to utilize glucose. In general, glucose utilization at rest parallels blood flow and O2 consumption. This does not mean that the total source of energy is always glucose. During prolonged starvation, appreciable utilization of other substances occurs. Indeed, evidence indicates that as much as 30% of the glucose taken up under normal conditions is converted to amino acids, lipids, and proteins, and that substances other than glucose are metabolized for energy during convulsions. Some utilization of amino acids from the circulation may also take place even though the amino acid arteriovenous difference across the brain is normally minute. The consequences of hypoglycemia in terms of neural function are discussed in Chapter 21.
GLUTAMATE & AMMONIA REMOVAL The brain’s uptake of glutamate is approximately balanced by its output of glutamine. Glutamate entering the brain takes up ammonia and leaves as glutamine. The glutamate– glutamine conversion in the brain—the opposite of the reaction in the kidney that produces some of the ammonia entering the tubules—serves as a detoxifying mechanism to keep the brain free of ammonia. Ammonia is very toxic to nerve cells, and ammonia intoxication is believed to be a major cause of the bizarre neurologic symptoms in hepatic coma (see Chapter 29).
CORONARY CIRCULATION ANATOMIC CONSIDERATIONS The two coronary arteries that supply the myocardium arise from the sinuses behind two of the cusps of the aortic valve at the root of the aorta (Figure 34–11). Eddy currents keep the valves away from the orifices of the arteries, and they are patent throughout the cardiac cycle. Most of the venous blood returns to the heart through the coronary sinus and anterior
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CLINICAL BOX 34–3
Right coronary artery
Left coronary artery Circumflex branch
Stroke When the blood supply to a part of the brain is interrupted, ischemia damages or kills the cells in the area, producing the signs and symptoms of a stroke. There are two general types of strokes: hemorrhagic and ischemic. Hemorrhagic stroke occurs when a cerebral artery or arteriole ruptures, sometimes but not always at the site of a small aneurysm. Ischemic stroke occurs when flow in a vessel is compromised by atherosclerotic plaques on which thrombi form. Thrombi may also be produced elsewhere (eg, in the atria in patients with atrial fibrillation) and pass to the brain as emboli where they then lodge and interrupt flow. In the past, little could be done to modify the course of a stroke and its consequences. However, it has now become clear that in the penumbra, the area surrounding the most severe brain damage, ischemia reduces glutamate uptake by astrocytes, and the increase in local glutamate causes excitotoxic damage and death to neurons (see Chapter 7). In experimental animals, and perhaps in humans, drugs that prevent this excitotoxic damage significantly reduce the effects of strokes. In addition, clot-lysing drugs such as tissuetype plasminogen activator (t-PA) (see Chapter 32) are of benefit in ischemic strokes. Both antiexcitotoxic treatment and t-PA must be given early in the course of a stroke to be of maximum benefit, and this is why stroke has become a condition in which rapid diagnosis and treatment have become important. In addition, of course, it is important to determine if a stroke is thrombotic or hemorrhagic, since clot lysis is contraindicated in the latter.
cardiac veins (Figure 34–12), which drain into the right atrium. In addition, there are other vessels that empty directly into the heart chambers. These include arteriosinusoidal vessels, sinusoidal capillary-like vessels that connect arterioles to the chambers; thebesian veins that connect capillaries to the chambers; and a few arterioluminal vessels that are small arteries draining directly into the chambers. A few anastomoses occur between the coronary arterioles and extracardiac arterioles, especially around the mouths of the great veins. Anastomoses between coronary arterioles in humans only pass particles less than 40 μm in diameter, but evidence indicates that these channels enlarge and increase in number in patients with coronary artery disease.
Anterior descending branch Septal branches Marginal branch Marginal branch Posterior descending branch
FIGURE 34–11
Coronary arteries and their principal branches in humans. (Reproduced with permission from Ross G: The
cardiovascular system. In: Essentials of Human Physiology. Ross G [editor]. Copyright © 1978 by Year Book Medical Publishers.)
(Table 34–4). Consequently, flow occurs in the arteries supplying the subendocardial portion of the left ventricle only during diastole, although the force is sufficiently dissipated in the more superficial portions of the left ventricular myocardium to permit some flow in this region throughout the cardiac cycle. Because diastole is shorter when the heart rate is high, left ventricular coronary flow is reduced during tachycardia. On the other hand, the pressure differential between the aorta and the right ventricle, and the differential between the aorta and the atria, are somewhat greater during systole than during diastole. Consequently, coronary flow in those parts of the heart is not appreciably reduced during systole. Flow in the right and left coronary arteries is shown in Figure 34–13. Because no blood flow occurs during systole in the subendocardial portion of the left ventricle, this region is prone to ischemic damage and is the most common site of myocardial infarction. Blood flow to the left ventricle is decreased in
Extracoronary arteries
Coronary arteries
Arterioles
Arterioles Capillaries
Arteriosinusoidal vessels
PRESSURE GRADIENTS & FLOW IN THE CORONARY VESSELS The heart is a muscle that, like skeletal muscle, compresses its blood vessels when it contracts. The pressure inside the left ventricle is slightly higher than in the aorta during systole
Veins Coronary sinus or anterior cardiac veins
Arterioluminal vessels Thebesian veins
Heart chambers
FIGURE 34–12
Diagram of the coronary circulation.
CHAPTER 34 Circulation Through Special Regions
TABLE 34–4 Pressure in aorta and left and right ventricles (vent) in systole and diastole. Pressure Differential (mm Hg) between Aorta and
Pressure (mm Hg) in
Aorta
Left Vent
Right Vent
Left Vent
Right Vent
Systole
120
121
25
–1
95
Diastole
80
0
0
80
80
patients with stenotic aortic valves because the pressure in the left ventricle must be much higher than that in the aorta to eject the blood. Consequently, the coronary vessels are severely compressed during systole. Patients with this disease are particularly prone to develop symptoms of myocardial ischemia, in part because of this compression and in part because the myocardium requires more O2 to expel blood through the stenotic aortic valve. Coronary flow is also decreased when the aortic diastolic pressure is low. The rise in venous pressure in conditions such as congestive heart failure reduces coronary flow because it decreases effective coronary perfusion pressure (see Clinical Box 34–4). Coronary blood flow has been measured by inserting a catheter into the coronary sinus and applying the Kety method to
Aortic pressure (mm Hg)
the heart on the assumption that the N2O content of coronary venous blood is typical of the entire myocardial effluent. Coronary flow at rest in humans is about 250 mL/min (5% of the cardiac output). A number of techniques utilizing radionuclides, radioactive tracers that can be detected with radiation detectors over the chest, have been used to study regional blood flow in the heart and to detect areas of ischemia and infarct as well as to evaluate ventricular function. Radionuclides such as thallium-201 (201T1) are pumped into cardiac muscle cells by Na, K ATPase and equilibrate with the intracellular K+ pool. For the first 10–15 min after intravenous injection, 201T1 distribution is directly proportional to myocardial blood flow, and areas of ischemia can be detected by their low uptake. The uptake of this isotope is often determined soon after exercise and again several hours later to bring out areas in which exertion leads to compromised flow. Conversely, radiopharmaceuticals such as technetium-99m stannous pyrophosphate (99mTc-PYP) are selectively taken up by infarcted tissue by an incompletely understood mechanism and make infarcts stand out as “hot spots” on scintigrams of the chest. Coronary angiography can be combined with measurement of 133Xe washout (see above) to provide detailed analysis of coronary blood flow. Radiopaque contrast medium is first injected into the coronary arteries, and x-rays are used to outline their distribution. The angiographic camera is then replaced with a scintillation camera, and 133Xe washout is measured.
VARIATIONS IN CORONARY FLOW
120
At rest, the heart extracts 70–80% of the O2 from each unit of blood delivered to it (Table 34–1). O2 consumption can be increased significantly only by increasing blood flow. Therefore, it is not surprising that blood flow increases when the metabolism of the myocardium is increased. The caliber of the coronary vessels, and consequently the rate of coronary blood flow, is influenced not only by pressure changes in the aorta but also by chemical and neural factors. The coronary circulation also shows considerable autoregulation.
100
80
100 80 Phasic coronary blood flow (mL/min)
579
60 40
CHEMICAL FACTORS
Left coronary
20 0
15 10 Right coronary
5 0 0.2
0.4 0.6 Time (s)
FIGURE 34–13
0.8
1.0
Blood flow in the left and right coronary arteries during various phases of the cardiac cycle. Systole occurs between the two vertical dashed lines. (Reproduced with permission from
Berne RM, Levy MN: Physiology, 2nd ed. Mosby, 1988.)
The close relationship between coronary blood flow and myocardial O2 consumption indicates that one or more of the products of metabolism cause coronary vasodilation. Factors suspected of playing this role include O2 lack and increased local concentrations of CO2, H+, K+, lactate, prostaglandins, adenine nucleotides, and adenosine. Likely several or all of these vasodilator metabolites act in an integrated fashion, redundant fashion, or both. Asphyxia, hypoxia, and intracoronary injections of cyanide all increase coronary blood flow 200– 300% in denervated as well as intact hearts, and the feature common to these three stimuli is hypoxia of the myocardial fibers. A similar increase in flow is produced in the area supplied by a coronary artery if the artery is occluded and then released. This reactive hyperemia is similar to that seen in the
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CLINICAL BOX 34–4 Coronary Artery Disease When flow through a coronary artery is reduced to the point that the myocardium it supplies becomes hypoxic, angina pectoris develops (see Chapter 31). If the myocardial ischemia is severe and prolonged, irreversible changes occur in the muscle, and the result is myocardial infarction. Many individuals have angina only on exertion, and blood flow is normal at rest. Others have more severe restriction of blood flow and have anginal pain at rest as well. Partially occluded coronary arteries can be constricted further by vasospasm, producing myocardial infarction. However, it is now clear that the most common cause of myocardial infarction is rupture of an atherosclerotic plaque, or hemorrhage into it, which triggers the formation of a coronary-occluding clot at the site of the plaque. The electrocardiographic changes in myocardial infarction are discussed in Chapter 30. When myocardial cells actually die, they leak enzymes into the circulation, and measuring the rises in serum enzymes and isoenzymes produced by infarcted myocardial cells also plays an important role in the diagnosis of myocardial infarction. The enzymes most commonly measured today are the MB isomer of creatine kinase (CK-MB), troponin T, and troponin I. Myocardial infarction is a very common cause of death in developed countries because of the widespread occurrence of atherosclerosis. In addition, there is a relation between atherosclerosis and circulating levels of lipoprotein(a) (Lp[a]). Lp(a) has an outer coat count of apo(a). It interferes with fibrinolysis by down-regulating plasmin generation (see Chapter 32). There is also a strong positive correlation between atherosclerosis and circulating levels of homocysteine. This substance damages endothelial cells. It is converted to nontoxic methionine in the presence of folate and vitamin B12, and clinical trials are under way to determine whether supplements of folate and B12 lower the incidence of coronary disease. It now appears that atherosclerosis has an important inflammatory component as well. The lesions of the disease contain inflammatory cells, and there is a positive correlation between increased levels of C-reactive protein and other inflammatory markers in the circulation and subsequent myocardial infarction. Treatment of myocardial infarction aims to restore flow to the affected area as rapidly as possible while minimizing reperfusion injury. Needless to say, it should be started as promptly as possible to avoid irreversible changes in heart function.
skin (see below). Evidence suggests that in the heart it is due to release of adenosine.
NEURAL FACTORS The coronary arterioles contain α-adrenergic receptors, which mediate vasoconstriction, and β-adrenergic receptors, which mediate vasodilation. Activity in the noradrenergic nerves to the heart and injections of norepinephrine cause coronary vasodilation. However, norepinephrine increases the heart rate and the force of cardiac contraction, and the vasodilation is due to production of vasodilator metabolites in the myocardium secondary to the increase in its activity. When the inotropic and chronotropic effects of noradrenergic discharge are blocked by a β-adrenergic blocking drug, stimulation of the noradrenergic nerves or injection of norepinephrine in unanesthetized animals elicits coronary vasoconstriction. Thus, the direct effect of noradrenergic stimulation is constriction rather than dilation of the coronary vessels. On the other hand, stimulation of vagal fibers to the heart dilates the coronaries. When the systemic blood pressure falls, the overall effect of the reflex increase in noradrenergic discharge is increased coronary blood flow secondary to the metabolic changes in the myocardium at a time when the cutaneous, renal, and splanchnic vessels are constricted. In this way the circulation of the heart, like that of the brain, is preserved when flow to other organs is compromised.
CUTANEOUS CIRCULATION The amount of heat lost from the body is regulated to a large extent by varying the amount of blood flowing through the skin. The fingers, toes, palms, and earlobes contain wellinnervated anastomotic connections between arterioles and venules (arteriovenous anastomoses; see Chapter 32). Blood flow in response to thermoregulatory stimuli can vary from 1 to as much as 150 mL/100 g of skin/min, and it has been postulated that these variations are possible because blood can be shunted through the anastomoses. The subdermal capillary and venous plexus is a blood reservoir of some importance, and the skin is one of the few places where the reactions of blood vessels can be observed visually.
WHITE REACTION When a pointed object is drawn lightly over the skin, the stroke lines become pale (white reaction). The mechanical stimulus apparently initiates contraction of the precapillary sphincters, and blood drains out of the capillaries and small veins. The response appears in about 15 s.
TRIPLE RESPONSE When the skin is stroked more firmly with a pointed instrument, instead of the white reaction there is reddening at the site that appears in about 10 s (red reaction). This is followed in a few minutes by local swelling and diffuse, mottled reddening
CHAPTER 34 Circulation Through Special Regions around the injury. The initial redness is due to capillary dilation, a direct response of the capillaries to pressure. The swelling (wheal) is local edema due to increased permeability of the capillaries and postcapillary venules, with consequent extravasation of fluid. The redness spreading out from the injury (flare) is due to arteriolar dilation. This three-part response—the red reaction, wheal, and flare—is called the triple response and is part of the normal reaction to injury (see Chapter 3). It persists after total sympathectomy. On the other hand, the flare is absent in locally anesthetized skin and in denervated skin after the sensory nerves have degenerated, but it is present immediately after nerve block or section above the site of the injury. This, plus other evidence, indicates that it is due to an axon reflex, a response in which impulses initiated in sensory nerves by the injury are relayed antidromically down other branches of the sensory nerve fibers (Figure 34–14). This is the one situation in the body in which there is substantial evidence for a physiologic effect due to antidromic conduction. The transmitter released at the central termination of the sensory C fiber neurons is substance P (see Chapter 7), and substance P and CGRP are present in all parts of the neurons. Both dilate arterioles and, in addition, substance P causes extravasation of fluid. Effective nonpeptide antagonists to substance P have now been developed, and they reduce the extravasation. Thus, it appears that these peptides produce the wheal.
REACTIVE HYPEREMIA A response of the blood vessels that occurs in many organs but is visible in the skin is reactive hyperemia, an increase in the amount of blood in a region when its circulation is reestablished after a period of occlusion. When the blood supply to a limb is occluded, the cutaneous arterioles below the occlusion dilate. When the circulation is reestablished, blood flowing
Spinal cord
Sensory neuron
581
into the dilated vessels makes the skin become fiery red. O2 in the atmosphere can diffuse a short distance through the skin, and reactive hyperemia is prevented if the circulation of the limb is occluded in an atmosphere of 100% O2. Therefore, the arteriolar dilation is apparently due to a local effect of hypoxia.
GENERALIZED RESPONSES Noradrenergic nerve stimulation and circulating epinephrine and norepinephrine constrict cutaneous blood vessels. No known vasodilator nerve fibers extend to the cutaneous vessels, and thus vasodilation is brought about by a decrease in constrictor tone as well as the local production of vasodilator metabolites. Skin color and temperature also depend on the state of the capillaries and venules. A cold blue or gray skin is one in which the arterioles are constricted and the capillaries dilated; a warm red skin is one in which both are dilated. Because painful stimuli cause diffuse noradrenergic discharge, a painful injury causes generalized cutaneous vasoconstriction in addition to the local triple response. When the body temperature rises during exercise, the cutaneous blood vessels dilate in spite of continuing noradrenergic discharge in other parts of the body. Dilation of cutaneous vessels in response to a rise in hypothalamic temperature overcomes other reflex activity. Cold causes cutaneous vasoconstriction; however, with severe cold, superficial vasodilation may supervene. This vasodilation is the cause of the ruddy complexion seen on a cold day. Shock is more profound in patients with elevated temperatures because of cutaneous vasodilation, and patients in shock should not be warmed to the point that their body temperature rises. This is sometimes a problem because well-meaning laymen have read in first-aid books that “injured patients should be kept warm,” and they pile blankets on accident victims who are in shock.
PLACENTAL & FETAL CIRCULATION UTERINE CIRCULATION
Endings in skin
Orthodromic conduction Antidromic conduction Direction taken by impulses
FIGURE 34–14
Axon reflex.
Endings near arteriole
The blood flow of the uterus parallels the metabolic activity of the myometrium and endometrium and undergoes cyclic fluctuations that correlate with the menstrual cycle in nonpregnant women. The function of the spiral and basilar arteries of the endometrium in menstruation is discussed in Chapter 25. During pregnancy, blood flow increases rapidly as the uterus increases in size (Figure 34–15). Vasodilator metabolites are undoubtedly produced in the uterus, as they are in other active tissues. In early pregnancy, the arteriovenous O2 difference across the uterus is small, and it has been suggested that estrogens act on the blood vessels to increase uterine blood flow in excess of tissue O2 needs. However, even though uterine blood flow increases 20-fold during pregnancy, the
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Relative units
Parturition
Amnion Septum Umbilical arteries Umbilical vein
Uterine blood flow
Fetal weight Umbilical cord Chorion
O2 saturation
Villus Systemic venous blood
Intervillous space Spiral arteriole
Uterine venous blood Parturition Time after conception
Basal plate Chorionic plate Endometrium
FIGURE 34–15
Myometrium
Changes in uterine blood flow and the amount of O2 in uterine venous blood during pregnancy. (After
Barcroft H. Modified and redrawn with permission from Keele CA, Neil E: Samson Wright’s Applied Physiology, 12th ed. Oxford University Press, 1971.)
size of the conceptus increases much more, changing from a single cell to a fetus plus a placenta that weighs 4 to 5 kg at term in humans. Consequently, more O2 is extracted from the uterine blood during the latter part of pregnancy, and the O2 saturation of uterine blood falls. Corticotrophin-releasing hormone appears to play an important role in up-regulating uterine blood flow, as well as in the eventual timing of birth.
PLACENTA The placenta is the “fetal lung” (Figures 34–16 and 34–17). Its maternal portion is in effect a large blood sinus. Into this “lake” project the villi of the fetal portion containing the small branches of the fetal umbilical arteries and vein (Figure 34–16). O2 is taken up by the fetal blood and CO2 is discharged into the maternal circulation across the walls of the villi in a fashion analogous to O2 and CO2 exchange in the lungs (see Chapter 36). However, the cellular layers covering the villi are thicker and less permeable than the alveolar membranes in the lungs, and exchange is much less efficient. The placenta is also the route by which all nutritive materials enter the fetus and by which fetal wastes are discharged to the maternal blood.
FETAL CIRCULATION The arrangement of the circulation in the fetus is shown diagrammatically in Figure 34–17. Fifty-five percent of the fetal cardiac output goes through the placenta. The blood in the umbilical vein in humans is believed to be about 80% saturated with O2, compared with 98% saturation in the arterial circulation of the adult. The ductus venosus (Figure 34–18) diverts
FIGURE 34–16
Diagram of a section through the human placenta, showing the way the fetal villi project into the maternal sinuses. (Reproduced with permission from Benson RC: Handbook of Obstetrics and
Gynecology, 8th ed. Originally published by Appleton & Lange. Copyright © 1983 McGraw-Hill.)
some of this blood directly to the inferior vena cava, and the remainder mixes with the portal blood of the fetus. The portal and systemic venous blood of the fetus is only 26% saturated, and the saturation of the mixed blood in the inferior vena cava is approximately 67%. Most of the blood entering the heart through the inferior vena cava is diverted directly to the left atrium via the patent foramen ovale. Most of the blood from the superior vena cava enters the right ventricle and is expelled into the pulmonary artery. The resistance of the collapsed lungs is high, and the pressure in the pulmonary artery is several mm Hg higher than it is in the aorta, so that most of the blood in the pulmonary artery passes through the ductus arteriosus to the aorta. In this fashion, the relatively unsaturated blood from the right ventricle is diverted to the trunk and lower body of the fetus, while the head of the fetus receives the betteroxygenated blood from the left ventricle. From the aorta, some of the blood is pumped into the umbilical arteries and back to the placenta. The O2 saturation of the blood in the lower aorta and umbilical arteries of the fetus is approximately 60%.
FETAL RESPIRATION The tissues of fetal and newborn mammals have a remarkable but poorly understood resistance to hypoxia. However, the O2 saturation of the maternal blood in the placenta is so low that the fetus might suffer hypoxic damage if fetal red cells did not
CHAPTER 34 Circulation Through Special Regions
583
Placenta
Body
Body
Body
L heart
L heart
Lungs
Lungs
Lungs
R heart
R heart
R heart
the heart and lungs at birth. Cold Spring Harbor Symp Quant
FETUS
NEWBORN
ADULT
Biol 1954;19:102.)
FO L heart
DA
DA
FIGURE 34–17
Diagram of the circulation in the fetus, the newborn infant, and the adult. DA, ductus arteriosus; FO, foramen ovale. (Redrawn and
reproduced with permission from Born GVR et al: Changes in
Left atrium Superior vena cava
Ductus arteriosus
Foramen ovale
Right atrium
Pulmonary artery
Right ventricle
Left ventricle
Ductus venosus
Inferior vena cava
have a greater O2 affinity than adult red cells (Figure 34–19). The fetal red cells contain fetal hemoglobin (hemoglobin F), whereas the adult cells contain adult hemoglobin (hemoglobin A). The cause of the difference in O2 affinity between the two is that hemoglobin F binds 2, 3-DPG less effectively than hemoglobin A does. The decrease in O2 affinity due to the binding of 2, 3-DPG is discussed in Chapter 32). Some hemoglobin A is present in blood during fetal life (see Chapter 32). After birth, production of hemoglobin F normally ceases, and by the age of 4 mo 90% of the circulating hemoglobin is hemoglobin A.
Aorta Fetus
22 20
Portal vein Umbilical vein
Umbilical arteries
O2 content (mL/dL)
18
Mother
16 14 12 10 8 6 4 2
From placenta
0 To placenta
10 20 30 40 50 60 70 80 90 100 PO2 (mm Hg)
FIGURE 34–18
Circulation in the fetus. Most of the oxygenated blood reaching the heart via the umbilical vein and inferior vena cava is diverted through the foramen ovale and pumped out the aorta to the head, while the deoxygenated blood returned via the superior vena cava is mostly pumped through the pulmonary artery and ductus arteriosus to the feet and the umbilical arteries.
FIGURE 34–19
Dissociation curves of hemoglobin in human maternal and fetal blood.
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SECTION VI Cardiovascular Physiology
CHANGES IN FETAL CIRCULATION & RESPIRATION AT BIRTH Because of the patent ductus arteriosus and foramen ovale (Figure 34–18), the left heart and right heart pump in parallel in the fetus rather than in series as they do in the adult. At birth, the placental circulation is cut off and the peripheral resistance suddenly rises. The pressure in the aorta rises until it exceeds that in the pulmonary artery. Meanwhile, because the placental circulation has been cut off, the infant becomes increasingly asphyxial. Finally, the infant gasps several times, and the lungs expand. The markedly negative intrapleural pressure (–30 to –50 mm Hg) during the gasps contributes to the expansion of the lungs, but other factors are likely also involved. The sucking action of the first breath plus constriction of the umbilical veins squeezes as much as 100 mL of blood from the placenta (the “placental transfusion”). Once the lungs are expanded, the pulmonary vascular resistance falls to less than 20% of the value in utero, and pulmonary blood flow increases markedly. Blood returning from the lungs raises the pressure in the left atrium, closing the foramen ovale by pushing the valve that guards it against the interatrial septum. The ductus arteriosus constricts within a few hours after birth, producing functional closure, and permanent anatomic closure follows in the next 24–48 h due to extensive intimal thickening. The mechanism producing the initial constriction is not completely understood, but the increase in arterial O2 tension plays an important role. Relatively high concentrations of vasodilators are present in the ductus in utero—especially prostaglandin F2a—and synthesis of these prostaglandins is blocked by inhibition of cyclooxygenase at birth. In many premature infants the ductus fails to close spontaneously, but closure can be produced by infusion of drugs that inhibit cyclooxygenase. NO may also be involved in maintaining ductal patency in this setting.
CHAPTER SUMMARY ■
■
■
■
Cerebrospinal fluid is produced predominantly in the choroid plexus of the brain, in part via active transport mechanisms in the choroid epithelial cells. Fluid is reabsorbed into the bloodstream to maintain appropriate pressure in the setting of continuous production. The permeation of circulating substances into the brain is tightly controlled. Water, CO2, and O2 permeate freely. Other substances (such as glucose) require specific transport mechanisms, whereas entry of macromolecules is negligible. The effectiveness of the blood–brain barrier in preventing entry of xenobiotics is bolstered by active efflux mediated by P-glycoprotein. The coronary circulation supplies oxygen to the contracting myocardium. Metabolic products and neural input induce vasodilation as needed for oxygen demand. Blockage of coronary arteries may lead to irreversible injury to heart tissue. Control of cutaneous blood flow is a key facet of temperature regulation, and is underpinned by varying levels of shunting through arteriovenous anastomoses. Hypoxia, axon reflexes,
■
and sympathetic input are all important determinants of flow through the cutaneous vasculature. The fetal circulation cooperates with that of the placenta and uterus to deliver oxygen and nutrients to the growing fetus, as well as carrying away waste products. Unique anatomic features of the fetal circulation as well as biochemical properties of fetal hemoglobin serve to ensure adequate O2 supply, particularly to the head. At birth, the foramen ovale and the ductus arteriosus close such that the neonatal lungs now serve as the site for oxygen exchange.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. Blood in which of the following vessels normally has the lowest PO2? A) maternal artery B) maternal uterine vein C) maternal femoral vein D) umbilical artery E) umbilical vein 2. The pressure differential between the heart and the aorta is least in the A) left ventricle during systole. B) left ventricle during diastole. C) right ventricle during systole. D) right ventricle during diastole. E) left atrium during systole. 3. Injection of tissue plasminogen activator (t-PA) would probably be most beneficial A) after at least 1 y of uncomplicated recovery following occlusion of a coronary artery. B) after at least 2 mo of rest and recuperation following occlusion of a coronary artery. C) during the second week after occlusion of a coronary artery. D) during the second day after occlusion of a coronary artery. E) during the second hour after occlusion of a coronary artery. 4. Which of the following organs has the greatest blood flow per 100 g of tissue? A) brain B) heart muscle C) skin D) liver E) kidneys 5. Which of the following does not dilate arterioles in the skin? A) increased body temperature B) epinephrine C) bradykinin D) substance P E) vasopressin
CHAPTER 34 Circulation Through Special Regions 6. A baby boy is brought to the hospital because of convulsions. In the course of a workup, his body temperature and plasma glucose are found to be normal, but his cerebrospinal fluid glucose is 12 mg/dL (normal, 65 mg/dL). A possible explanation of his condition is A) constitutive activation of GLUT 3 in neurons. B) SGLT 1 deficiency in astrocytes. C) GLUT 5 deficiency in cerebral capillaries. D) GLUT 1 55K deficiency in cerebral capillaries. E) GLUT 1 45K deficiency in microglia.
585
CHAPTER RESOURCES Begley DJ, Bradbury MW, Kreater J (editors): The Blood–Brain Barrier and Drug Delivery to the CNS. Marcel Dekker, 2000. Birmingham K (editor): The heart. Nature 2002;415:197. Duncker DJ, Bache RJ: Regulation of coronary blood flow during exercise. Physiol Rev 2008;88:1009. Hamel E: Perivascular nerves and the regulation of cerebrovascular tone. J Appl Physiol 2006;100:1059. Johanson CE, et al: Multiplicity of cerebrospinal fluid functions: New challenges in health and disease. Cerebrospinal Fluid Res 2008;5:10. Ward JPT: Oxygen sensing in context. Biochim Biophys Acta 2008;1777:1.
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SECTION VII RESPIRATORY PHYSIOLOGY
35 C
Pulmonary Function
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
Define partial pressure and calculate the partial pressure of each of the important gases in the atmosphere at sea level. List the passages through which air passes from the exterior to the alveoli, and describe the cells that line each of them. List the major muscles involved in respiration, and state the role of each. Define the basic measures of lung volume and give approximate values for each in a normal adult. Define compliance, and give examples of diseases in which it is abnormal. Describe the chemical composition and function of surfactant. List the factors that determine alveolar ventilation. Define diffusion capacity, and compare the diffusion of O2 with that of CO2 in the lungs. Compare the pulmonary and systemic circulations, listing the main differences between them. Describe basic lung defense and metabolic functions.
INTRODUCTION Respiration, as the term is generally used, includes two processes: external respiration, the absorption of O2 and removal of CO2 from the body as a whole; and internal respiration, the utilization of O2 and production of CO2 by cells and the gaseous exchanges between the cells and their fluid medium. Aspects of external respiratory physiology are presented throughout this section. In this chapter, the processes
responsible for the uptake of O2 and excretion of CO2 in the lungs are explored. The next chapter is concerned with the transport of O2 and CO2 to and from the tissues. The final chapter in this section examines some key factors that regulate respiration. Throughout each chapter, clinical implications of specific physiology will be presented.
587
588
SECTION VII Respiratory Physiology
PROPERTIES OF GASES
TABLE 35–1 Standard conditions to which measurements involving gas volumes are corrected.
The pressure of a gas is proportional to its temperature and the number of moles per volume:
STPD
0 °C, 760 mm Hg, dry (standard temperature and pressure, dry)
nRT P = ---------- (from equation of state of ideal gas) V
BTPS
Body temperature and pressure, saturated with water vapor
ATPD
Ambient temperature and pressure, dry
ATPS
Ambient temperature and pressure, saturated with water vapor
where P = Pressure n = Number of moles R = Gas constant T = Absolute temperature V = Volume
PARTIAL PRESSURES Unlike liquids, gases expand to fill the volume available to them, and the volume occupied by a given number of gas molecules at a given temperature and pressure is (ideally) the same regardless of the composition of the gas. Therefore, the pressure exerted by any one gas in a mixture of gases (its partial pressure) is equal to the total pressure times the fraction of the total amount of gas it represents. The composition of dry air is 20.98% O2, 0.04% CO2, 78.06% N2, and 0.92% other inert constituents such as argon and helium. The barometric pressure (PB) at sea level is 760 mm Hg (1 atmosphere). The partial pressure (indicated by the symbol P) of O2 in dry air is therefore 0.21 × 760, or 160 mm Hg at sea level. The PN2 and the other inert gases is 0.79 × 760, or 600 mm Hg; and the PCO2 is 0.0004 × 760, or 0.3 mm Hg. The water vapor in the air in most climates reduces these percentages, and therefore the partial pressures, to a slight degree. Air equilibrated with water is saturated with water vapor, and inspired air is saturated by the time it reaches the lungs. The PH2O at body temperature (37 °C) is 47 mm Hg. Therefore, the partial pressures at sea level of the other gases in the air reaching the lungs are PO2, 149 mm Hg; PCO2, 0.3 mm Hg; and PN2 (including the other inert gases), 564 mm Hg. Gas diffuses from areas of high pressure to areas of low pressure, with the rate of diffusion depending on the concentration gradient and the nature of the barrier between the two areas. When a mixture of gases is in contact with and permitted to equilibrate with a liquid, each gas in the mixture dissolves in the liquid to an extent determined by its partial pressure and its solubility in the fluid. The partial pressure of a gas in a liquid is the pressure that, in the gaseous phase in equilibrium with the liquid, would produce the concentration of gas molecules found in the liquid.
METHODS OF QUANTITATING RESPIRATORY PHENOMENA Modern spirometers permit direct measurement of gas intake and output. Since gas volumes vary with temperature and
pressure and since the amount of water vapor in them varies, these devices have the ability to correct respiratory measurements involving volume to a stated set of standard conditions. The four most commonly used standards and their abbreviations are shown in Table 35–1. It should be noted that correct measurements are highly dependent on the ability for the practitioner to properly encourage the patient to fully utilize the device. Modern techniques for gas analysis make possible rapid, reliable measurements of the composition of gas mixtures and the gas content of body fluids. For example, O2 and CO2 electrodes, small probes sensitive to O2 or CO2, can be inserted into the airway or into blood vessels or tissues and the PO2 and PCO2 recorded continuously. Chronic assessment of oxygenation is carried out noninvasively with a pulse oximeter, which is usually attached to the ear.
ANATOMY OF THE LUNGS THE RESPIRATORY SYSTEM The respiratory system is made up of a gas-exchanging organ (the lungs) and a “pump” that ventilates the lungs. The pump consists of the chest wall; the respiratory muscles, which increase and decrease the size of the thoracic cavity; the areas in the brain that control the muscles; and the tracts and nerves that connect the brain to the muscles. At rest, a normal human breathes 12 to 15 times a minute. About 500 mL of air per breath, or 6 to 8 L/min, is inspired and expired. This air mixes with the gas in the alveoli, and, by simple diffusion, O2 enters the blood in the pulmonary capillaries while CO2 enters the alveoli. In this manner, 250 mL of O2 enters the body per minute and 200 mL of CO2 is excreted. Traces of other gases, such as methane from the intestines, are also found in expired air. Alcohol and acetone are expired when present in appreciable quantities in the body. Indeed, over 250 different volatile substances have been identified in human breath.
AIR PASSAGES After passing through the nasal passages and pharynx, where it is warmed and takes up water vapor, the inspired air passes
CHAPTER 35 Pulmonary Function
589
Trachea Left pulmonary artery Pulmonary veins Bronchiole Left main bronchus Heart
Terminal bronchiole Branch of pulmonary vein Branch of pulmonary artery
Smooth muscle
A
Respiratory bronchiole Alveoli
Capillary
B
FIGURE 35–1
Structure of the respiratory system. A) The respiratory system is diagrammed with a transparent lung to emphasize the flow of air into and out of the system. B) Enlargement of boxed area from (A) shows transition from conducting airway to the respiratory airway, with emphasis on the anatomy of the alveoli. Red and blue represent oxygenated and deoxygenated blood, respectively. (Continued )
down the trachea and through the bronchioles, respiratory bronchioles, and alveolar ducts to the alveoli, where gas exchange occurs (Figure 35–1). Between the trachea and the alveolar sacs, the airways divide 23 times. The first 16 generations of passages form the conducting zone of the airways that transports gas from and to the exterior. They are made up of bronchi, bronchioles, and terminal bronchioles. The remaining seven generations form the transitional and respiratory zones where gas exchange occurs; they are made up of respiratory bronchioles, alveolar ducts, and alveoli. These multiple divisions greatly increase the total cross-sectional area of the air-
ways, from 2.5 cm2 in the trachea to 11,800 cm2 in the alveoli (Figure 35–2). Consequently, the velocity of air flow in the small airways declines to very low values. The alveoli are surrounded by pulmonary capillaries (Figure 35–1). In most areas, air and blood are separated only by the alveolar epithelium and the capillary endothelium, so they are about 0.5 μm apart (Figure 35–3). Humans have 300 million alveoli, and the total area of the alveolar walls in contact with capillaries in both lungs is about 70 m2. The alveoli are lined by two types of epithelial cells. Type I cells are flat cells with large cytoplasmic extensions and are
590
SECTION VII Respiratory Physiology
Name of branches Trachea
500
Number of tubes in branch 1
Conducting zone
Bronchi
2
4 8 Bronchioles
16 32
Terminal bronchioles
6 x 104
Total cross section area (cm2)
400
300
200
Respiratory zone
Conducting zone
100 Respiratory zone
Respiratory bronchioles 5 x 105
Alveolar ducts
Terminal bronchioles 0
5
10
15
20
23
Airway generation Alveolar sacs
8 x 106
C
FIGURE 35–2
Total airway cross-sectional area as a function of airway generation. Note the extremely rapid increase in total cross-sectional area in the respiratory zone. As a result, forward velocity of gas during inspiration falls to a very low level in this zone.
FIGURE 35–1 (Continued ) C) The branching patterns of the airway during the transition form conducting to respiratory airway are drawn (not all divisions are drawn, and drawings are not to scale).
ed. Williams & Wilkins, 1991.)
the primary lining cells of the alveoli, covering approximately 95% of the alveolar epithelial surface area. Type II cells (granular pneumocytes) are thicker and contain numerous lamellar inclusion bodies. A primary function of these cells is to secrete surfactant; however, they are also important in alveolar repair as well as other cellular physiology. Although these cells make up approximately 5% of the surface area, they represent approximately 60% of the epithelial cells in the alveoli. The alveoli also contain other specialized cells, including pulmonary alveolar macrophages (PAMs, or AMs), lymphocytes, plasma cells, neuroendocrine cells, and mast cells. The mast cells contain heparin, various lipids, histamine, and various proteases that participate in allergic reactions (see Chapter 3).
amount relative to the thickness of the wall is present in the terminal bronchioles. The walls of the bronchi and bronchioles are innervated by the autonomic nervous system. Muscarinic receptors are abundant, and cholinergic discharge causes bronchoconstriction. The bronchial epithelium and smooth muscle contain β2-adrenergic receptors. Many of these are not innervated. Some may be located on cholinergic endings, where they inhibit acetylcholine release. The β2 receptors mediate bronchodilation. They increase bronchial secretion, while α1 adrenergic receptors inhibit secretion. There is, in addition, a noncholinergic, nonadrenergic innervation of the bronchioles that produces bronchodilation, and evidence suggests that vasoactive intestinal polypeptide (VIP) is the mediator responsible for the dilation.
THE BRONCHI & THEIR INNERVATION The trachea and bronchi have cartilage in their walls but relatively little smooth muscle. They are lined by a ciliated epithelium that contains mucous and serous glands. Cilia are present as far as the respiratory bronchioles, but glands are absent from the epithelium of the bronchioles and terminal bronchioles, and their walls do not contain cartilage. However, their walls contain more smooth muscle, of which the largest
(Reproduced with permission from West JB: Respiratory Physiology: The Essentials, 4th
ANATOMY OF BLOOD FLOW IN THE LUNG Both the pulmonary circulation and the bronchial circulation contribute to blood flow in the lung. In the pulmonary circulation, almost all the blood in the body passes via the pulmonary artery to the pulmonary capillary bed, where it is oxygenated and returned to the left atrium via the pulmonary
CHAPTER 35 Pulmonary Function
591
Capillaries
Respiratory bronchiole Alveolus
Alveolar duct Alveolus pore
Alveolus
Alveolus
ma
A Capillary endothelium
Alveolar air
Type II cell
Basement membrane
cf
a en
Erythrocyte
Interstitium
a
Plasma in capillary
epI
cap cf
Erythrocyte
B
Type I cell
Alveolar air
C
FIGURE 35–3 Portion of an interalveolar septum in the adult human lung. A) A cross-section of the respiratory zone shows the relationship between capillaries and the airway epithelium. Only 4 of the 18 alveoli are labeled. B) Enlargement of the boxed area from (A) displaying intimate relationship between capillaries, the interstitium, and the alveolar epithelium. C) Electron micrograph displaying area depicted in (B). The pulmonary capillary (cap) in the septum contains plasma with red blood cells apposed to the thin epithelial cells that line the alveoli. Note the closely apposed endothelial wall and pulmonary epithelium, separated at places by connective tissue fibers (cf); en, nucleus of endothelial cell; epl, nucleus of type I alveolar epithelial cell; a, alveolar space; ma, alveolar macrophage. (Reproduced with permission from (A, B) Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008; and (C) Burri PA: Development and growth of the human lung. In: Handbook of Physiology, Section 3, The Respiratory System. Fishman AP, Fisher AB [editors]. American Physiological Society, 1985.)
veins (Figure 35–4). The separate and much smaller bronchial circulation includes the bronchial arteries that come from systemic arteries. They form capillaries, which drain into bronchial veins or anastomose with pulmonary capillaries or veins (Figure 35–5). The bronchial veins drain into the azygos vein. The bronchial circulation nourishes the trachea down to the terminal bronchioles and also supplies the pleura and hilar lymph nodes. It should be noted that lymphatic channels are more abundant in the lungs than in any other organ.
MECHANICS OF RESPIRATION INSPIRATION & EXPIRATION The lungs and the chest wall are elastic structures. Normally, no more than a thin layer of fluid is present between the lungs and the chest wall (intrapleural space). The lungs slide easily on the chest wall, but resist being pulled away from it in the same way that two moist pieces of glass slide on each other but resist separation. The pressure in the “space” between the lungs and chest wall (intrapleural pressure) is subatmospheric (Figure 35–6). The lungs are stretched when they expand at birth, and at the end of quiet expiration their tendency to recoil from the chest wall is just balanced by the tendency of the
592
SECTION VII Respiratory Physiology
12
InspiExpiration ration Pressure in alveoli
12
12
12
+2 +1
14
14
120/80 24 9
2
0 −1
8
25 0
−2
Intrapleural pressure
120 0
Pressure (mm Hg)
−3 −4 −5 −6
120 80
10
0.6
Volume of breath
20 30
0.4 0.2
FIGURE 35–4
Pulmonary and systemic circulations. Representative areas of blood flow are labeled with corresponding blood pressure (mm Hg). (Modified from Comroe JH Jr.: Physiology of Respiration, 2nd ed.
1
2 3 Time (s)
4
0
FIGURE 35–6
Year Book, 1974.)
chest wall to recoil in the opposite direction. If the chest wall is opened, the lungs collapse; and if the lungs lose their elasticity, the chest expands and becomes barrel-shaped. Inspiration is an active process. The contraction of the inspiratory muscles increases intrathoracic volume. The intrapleural pressure at the base of the lungs, which is normally about –2.5 mm Hg (relative to atmospheric) at the start of inspiration, decreases to about –6 mm Hg. The lungs
A Pulmonary vein
Pulmonary artery B
C
Bronchopulmonary arterial anastomosis Bronchopulmonary vein
D Bronchial artery
0
Volume (L)
Bronchial vein
Azygos vein
FIGURE 35–5
Relationship between the bronchial and pulmonary circulations. The pulmonary artery supplies pulmonary capillary network A. The bronchial artery supplies capillary networks B, C, and D. Blue-colored areas represent blood of low O 2 content. (Reproduced with permission from Murray JF: The Normal Lung. Saunders, 1986.)
Pressure in the alveoli and the plural space relative to atmospheric pressure during inspiration and expiration. The dashed line indicates what the intrapleural pressure would be in the absence of airway and tissue resistance; the actual curve (solid line) is skewed to the left by the resistance. Volume of breath during inspiration/expiration is graphed for comparison.
are pulled into a more expanded position. The pressure in the airway becomes slightly negative, and air flows into the lungs. At the end of inspiration, the lung recoil begins to pull the chest back to the expiratory position, where the recoil pressures of the lungs and chest wall balance. The pressure in the airway becomes slightly positive, and air flows out of the lungs. Expiration during quiet breathing is passive in the sense that no muscles that decrease intrathoracic volume contract. However, some contraction of the inspiratory muscles occurs in the early part of expiration. This contraction exerts a braking action on the recoil forces and slows expiration. Strong inspiratory efforts reduce intrapleural pressure to values as low as –30 mm Hg, producing correspondingly greater degrees of lung inflation. When ventilation is increased, the extent of lung deflation is also increased by active contraction of expiratory muscles that decrease intrathoracic volume.
CHAPTER 35 Pulmonary Function
LUNG VOLUMES The amount of air that moves into the lungs with each inspiration (or the amount that moves out with each expiration) is called the tidal volume. The air inspired with a maximal inspiratory effort in excess of the tidal volume is the inspiratory reserve volume. The volume expelled by an active expiratory effort after passive expiration is the expiratory reserve volume, and the air left in the lungs after a maximal expiratory effort is the residual volume. Normal values for these lung volumes, and names applied to combinations of them, are shown in Figure 35–7. The space in the conducting zone of the airways occupied by gas that does not exchange with blood in the pulmonary vessels is the respiratory dead space. The forced vital capacity (FVC), the
Inspiration Lung volume (ml)
RV
ERV TV
Expiration
IRV
largest amount of air that can be expired after a maximal inspiratory effort, is frequently measured clinically as an index of pulmonary function. It gives useful information about the strength of the respiratory muscles and other aspects of pulmonary function. The fraction of the vital capacity expired during the first second of a forced expiration is referred to as FEV1 (formerly the timed vital capacity) (Figure 35–8). The FEV1 to FVC ratio (FEV1/FVC) is a useful tool in the diagnosis of airway disease (Clinical Box 35–1). The amount of air inspired per minute (pulmonary ventilation, respiratory minute volume) is normally about 6 L (500 mL/ breath × 12 breaths/min). The maximal voluntary ventilation (MVV) is the largest volume of gas that can be moved into and out of the lungs in 1 min by voluntary effort. The normal MVV is 125 to 170 L/min.
Maximum possible inspiration
6000
Dead space
593
5000
2
Inspiratory reserve volume
Vital capacity
5
6
Inspiratory capacity
4000
3000
8
Total lung capacity 2000
1000
0
1
Tidal volume
3
Expiratory reserve volume
Maximum voluntary expiration
4
Residual volume
Functional residual capacity 7
Respiratory Volumes and Capacities for an Average Young Adult Male Measurement
Typical Value
Definition Respiratory Volumes
4
Tidal volume (TV) Inspiratory reserve volume (IRV) Expiratory reserve volume (ERV) Residual volume (RV)
500 ml 3000 ml 1200 ml 1200 ml
Amount of air inhaled or exhaled in one breath during relaxed, quiet breathing Amount of air in excess of tidal inspiration that can be inhaled with maximum effort Amount of air in excess of tidal expiration that can be exhaled with maximum effort Amount of air remaining in the lungs after maximum expiration; keeps alveoli inflated between breaths and mixes with fresh air on next inspiration
5
Vital capacity (VC)
4700 ml
6
Inspiratory capacity (IC) Functional residual capacity (FRC) Total lung capacity (TLC)
3500 ml 2400 ml 5900 ml
Amount of air that can be exhaled with maximum effort after maximum inspiration (ERV + TV + IRV); used to assess strength of thoracic muscles as well as pulmonary function Maximum amount of air that can be inhaled after a normal tidal expiration (TV + IRV) Amount of air remaining in the lungs after a normal tidal expiration (RV + ERV) Maximum amount of air the lungs can contain (RV + VC)
1 2 3
Respiratory Capacities
7 8
FIGURE 35–7 Lung volumes and capacity measurements. Top left: A cartoon figure representing lung space divided into lung volumes. Dead space refers to areas where gas exchange does not occur; all other spaces are defined in the accompanying table. Top right: Spirometer recordings are shown with marked lung volumes and capacities. Table at bottom defines individual measurements and values from the top graphs. Note that residual volume, total lung capacity, and function residual capacity cannot be measure with a spirometer. (Right figure reproduced with permission from Widmaier EP, Raff H, Strang KT: Vander’s Human Physiology: The Mechanisms of Body Function, 11th ed. McGraw-Hill, 2008.)
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SECTION VII Respiratory Physiology
6
Volume (L)
5 4 3
VC FEV1
2 1 0
1
2
3
4
5
6
7
8
9
10
Time (s)
FIGURE 35–8
Volume of gas expired by a normal adult man during a forced expiration, demonstrating the FEV1 and the total vital capacity (VC). (Reproduced, with permission, from Crapo RO: Pulmonary-
The scalene and sternocleidomastoid muscles in the neck are accessory inspiratory muscles that help to elevate the thoracic cage during deep labored respiration. A decrease in intrathoracic volume and forced expiration result when the expiratory muscles contract. The internal intercostals have this action because they pass obliquely downward and posteriorly from rib to rib and therefore pull the rib cage downward when they contract. Contractions of the muscles of the anterior abdominal wall also aid expiration by pulling the rib cage downward and inward and by increasing the intra-abdominal pressure, which pushes the diaphragm upward.
GLOTTIS
function testing. N Engl J Med 1994;331:25. Copyright © 1994, Massachusetts Medical Society.)
RESPIRATORY MUSCLES Movement of the diaphragm accounts for 75% of the change in intrathoracic volume during quiet inspiration. Attached around the bottom of the thoracic cage, this muscle arches over the liver and moves downward like a piston when it contracts. The distance it moves ranges from 1.5 cm to as much as 7 cm with deep inspiration (Figure 35–9). The diaphragm has three parts: the costal portion, made up of muscle fibers that are attached to the ribs around the bottom of the thoracic cage; the crural portion, made up of fibers that are attached to the ligaments along the vertebrae; and the central tendon, into which the costal and the crural fibers insert. The central tendon is also the inferior part of the pericardium. The crural fibers pass on either side of the esophagus and can compress it when they contract. The costal and crural portions are innervated by different parts of the phrenic nerve and can contract separately. For example, during vomiting and eructation, intra-abdominal pressure is increased by contraction of the costal fibers but the crural fibers remain relaxed, allowing material to pass from the stomach into the esophagus. The other important inspiratory muscles are the external intercostal muscles, which run obliquely downward and forward from rib to rib. The ribs pivot as if hinged at the back, so that when the external intercostals contract they elevate the lower ribs. This pushes the sternum outward and increases the anteroposterior diameter of the chest. The transverse diameter also increases, but to a lesser degree. Either the diaphragm or the external intercostal muscles alone can maintain adequate ventilation at rest. Transection of the spinal cord above the third cervical segment is fatal without artificial respiration, but transection below the fifth cervical segment is not, because it leaves the phrenic nerves that innervate the diaphragm intact; the phrenic nerves arise from cervical segments 3–5. Conversely, in patients with bilateral phrenic nerve palsy but intact innervation of their intercostal muscles, respiration is somewhat labored but adequate to maintain life.
The abductor muscles in the larynx contract early in inspiration, pulling the vocal cords apart and opening the glottis. During swallowing or gagging, a reflex contraction of the adductor muscles closes the glottis and prevents aspiration of food, fluid, or vomitus into the lungs. In unconscious or anesthetized patients, glottic closure may be incomplete and vomitus may enter the trachea, causing an inflammatory reaction in the lung (aspiration pneumonia). The laryngeal muscles are supplied by the vagus nerves. When the abductors are paralyzed, there is inspiratory stridor. When the adductors are paralyzed, food and fluid enter the trachea, causing aspiration pneumonia and edema. Bilateral cervical vagotomy in animals causes the slow development of fatal pulmonary congestion and edema. The edema is due at least in part to aspiration, although some edema develops even if a tracheostomy is performed before the vagotomy.
BRONCHIAL TONE In general, the smooth muscle in the bronchial walls aids respiration. The bronchi dilate during inspiration and constrict during expiration. Dilation is produced by sympathetic discharge and constriction by parasympathetic discharge. Stimulation of sensory receptors in the airways by irritants and chemicals such as sulfur dioxide produces reflex bronchoconstriction that is mediated via cholinergic pathways. Cool air also causes bronchoconstriction, and so does exercise, possibly because the increased respiration associated with it cools the airways. In addition, the bronchial muscles protect the bronchi during coughing. There is a circadian rhythm in bronchial tone, with maximal constriction at about 6:00 AM and maximal dilation at about 6:00 PM. Many chemical substances including VIP, substance P, adenosine, and many cytokines and inflammatory modulators can affect bronchial tone, although their full roles in the physiologic regulation of bronchial tone is still unsettled.
CHAPTER 35 Pulmonary Function
595
CLINICAL BOX 35–1 Airway Diseases That Alter Airflow Obstructive Disease: Asthma Asthma is characterized by episodic or chronic wheezing, cough, and a feeling of tightness in the chest as a result of bronchoconstriction. Although the disease is not fully understood, three airway abnormalities are present: airway obstruction that is at least partially reversible, airway inflammation, and airway hyperresponsiveness to a variety of stimuli. A link to allergy has long been recognized, and plasma IgE levels are often elevated. Proteins released from eosinophils in the inflammatory reaction may damage the airway epithelium and contribute to the hyperresponsiveness. Leukotrienes are released from eosinophils and mast cells, and can enhance bronchoconstriction. Numerous other amines, neuropeptides, chemokines, and interleukins have effects on bronchial smooth muscle or produce inflammation, and they may be involved in asthma. Because β2-adrenergic receptors mediate bronchodilation, β2-adrenergic agonists have long been the mainstay of treatment for mild to moderate asthma attacks. Inhaled and systemic steroids are used even in mild to moderate cases to reduce inflammation; they are very effective, but their side effects can be a problem. Agents that block synthesis of leukotrienes or their CysLT1 receptor have also proved useful in some cases.
Restrictive Disease: Emphysema Emphysema is a degenerative and potentially fatal pulmonary disease that is characterized by a loss of lung elasticity and replacement of alveoli with large air sacs. This loss of elasticity prevents full expansion of the lung, or airway restriction, during breathing. The most common cause of emphysema is heavy cigarette smoking. The smoke causes an increase
COMPLIANCE OF THE LUNGS & CHEST WALL The interaction between the recoil of the lungs and recoil of the chest can be demonstrated in living subjects through a spirometer that has a valve just beyond the mouthpiece. The mouthpiece contains a pressure-measuring device. After the subject inhales a given amount, the valve is shut, closing off the airway. The respiratory muscles are then relaxed while the pressure in the airway is recorded. The procedure is repeated after inhaling or actively exhaling various volumes. The curve of airway pressure obtained in this way, plotted against volume, is the relaxation pressure curve of the total respiratory system (Figure 35–10). The pressure is zero at a lung volume that corresponds to the volume of gas in the lungs at the end of quiet expiration (functional residual capacity, or FRC;
in the number of pulmonary alveolar macrophages, and these macrophages release a chemical substance that attracts leukocytes to the lungs. The leukocytes in turn release proteases including elastase, which attacks the elastic tissue in the lungs. At the same time, α1-antitrypsin, a plasma protein that normally inactivates elastase and other proteases, is itself inhibited. The α1-antitrypsin is inactivated by oxygen radicals, and these are released by the leukocytes. The final result is a protease–antiprotease imbalance with increased destruction of lung tissue. Similar protease–antiprotease imbalance can occur through congenital deficiency α1-antitrypsin.
Airflow Measurements of Obstructive & Restrictive Disease In a healthy normal adult male, FVC is approximately 5.0 L, FEV1 is approximately 4.0 L, and thus, the calculated FEV1/FVC is 80%. As would be expected, patients with obstructive or restrictive diseases display reduced FVC, on the order of 3.0 L, and this measurement alone does not differentiate between the two. However, measurement of FEV1 can significantly vary between the two diseases. In obstructive disorders, patients tend to show a slow, steady slope to the FVC, resulting in a small FEV1, on the order of 1.3 L. However, in the restrictive disorder patients, air flow tends to be fast at first, and then due to the loss of elasticity, quickly levels out to approach FVC. The resultant FEV1 is much greater, on the order of 2.8 L, even though FVC is equivalent. A quick calculation of FEV1/FVC for obstructive (42%) versus restrictive (90%) patients defines the hallmark measurements in evaluating these two diseases. Obstructive disorders result in a marked decrease in both FVC and FEV1/ FVC, whereas restrictive disorders result in a loss of FVC without loss in FEV1/FVC.
also known as relaxation volume). It is positive at greater volumes and negative at smaller volumes. The change in lung volume per unit change in airway pressure (ΔV/ΔP) is the compliance (stretchability) of the lungs and chest wall. It is normally measured in the pressure range where the relaxation pressure curve is steepest, and the normal value is approximately 0.2 L/cm H2O. However, compliance depends on lung volume; an individual with only one lung has approximately half the ΔV for a given ΔP. Compliance is also slightly greater when measured during deflation than when measured during inflation. Consequently, it is more informative to examine the whole pressure–volume curve. The curve is shifted downward and to the right (compliance is decreased) by pulmonary congestion and interstitial pulmonary fibrosis (Figure 35–11). Pulmonary fibrosis is a progressive restrictive airway disease of unknown cause in which there is stiffening and scarring of
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SECTION VII Respiratory Physiology
FIGURE 35–9
X-ray of chest in full expiration (left) and full inspiration (right). The dashed white line on the right is an outline of the lungs in full expiration. Note the difference in intrathoracic volume. (Reproduced with permission from Comroe JH Jr.: Physiology of Respiration, 2nd ed., Year Book, 1974.)
the lung. The curve is shifted upward and to the left (compliance is increased) in emphysema. It should be noted that compliance is a static measure of lung and chest recoil. The resistance of the lung and chest is the pressure difference required for a unit of air flow; this measurement, which is dynamic rather than static, also takes into account the resistance to air flow in the airways.
ALVEOLAR SURFACE TENSION An important factor affecting the compliance of the lungs is the surface tension of the film of fluid that lines the alveoli. The magnitude of this component at various lung volumes can be measured by removing the lungs from the body of an experimental
Liters
Liters 6 Relaxation pressure curve
Change from resting volume
+3
Maximal inspiratory curve
Maximal expiratory curve Relaxation volume
0 Vital capacity
−1 −2 −120
FIGURE 35–10
5
+2 +1
4 3 2
1
−80
Total lung capacity
−40 0 40 80 120 Intrapulmonary pressure (mm Hg)
160
Functional residual capacity Residual volume
200
Intrapulmonary pressure and volume relationship, the relaxation pressure curve. The middle curve is the relaxation pressure curve of the total respiratory system; that is, the static pressure curve of values obtained when the lungs are inflated or deflated by various amounts and the intrapulmonary pressure (elastic recoil pressure) is measured with the airway closed. The relaxation volume is the point where the recoil of the chest and the recoil of the lungs balance. The slope of the curve is the compliance of the lungs and chest wall. The maximal inspiratory and expiratory curves are the airway pressures that can be developed during maximal inspiratory and expiratory efforts.
CHAPTER 35 Pulmonary Function
8
Emphysema
597
TABLE 35–2 Approximate composition of surfactant.
7 Component Lung volume (L)
6
Normal
Dipalmitoylphosphatidylcholine
Percentage Composition 62
5
Phosphatidylglycerol
5
4
Other phospholipids
10
Neutral lipids
13
Fibrosis
3 2 1 0
10 20 30 Transpulmonary pressure (cm H2O)
40
FIGURE 35–11
Static expiratory pressure–volume curves of lungs in normal subjects and subjects with severe emphysema and pulmonary fibrosis. (Modified and reproduced with permission from
Proteins
8
Carbohydrate
2
tension is also much lower than the expected surface tension at a water–air interface of the same dimensions.
SURFACTANT
Pride NB, Macklem PT: Lung mechanics in disease. In: Handbook of Physiology. Section 3, The Respiratory System. Vol III, part 2. Fishman AP [editor]. American Physiological Society, 1986.)
animal and distending them alternately with saline and with air while measuring the intrapulmonary pressure. Because saline reduces the surface tension to nearly zero, the pressure–volume curve obtained with saline measures only the tissue elasticity (Figure 35–12), whereas the curve obtained with air measures both tissue elasticity and surface tension. The difference between the two curves, the elasticity due to surface tension, is much smaller at small than at large lung volumes. The surface
Saline
Volume (% maximum inflation)
100
Air
Def
50
Inf
0
10
20
30
40
Pressure (cm H2O)
FIGURE 35–12
Pressure–volume relations in the lungs of a cat after removal from the body. Saline: lungs inflated and deflated with saline to reduce surface tension, resulting in a measurement of tissue elasticity. Air: lungs inflated (Inf) and deflated (Def) with air results in a measure of both tissue elasticity and surface tension.
(Reproduced with permission from Morgan TE: Pulmonary surfactant. N Engl J Med 1971;284:1185.)
The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent. Surfactant is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins (Table 35–2). If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace. In spherical structures like the alveoli, the distending pressure equals two times the tension divided by the radius (P = 2T/r); if T is not reduced as r is reduced, the tension overcomes the distending pressure. Surfactant also helps to prevent pulmonary edema. It has been calculated that if it were not present, the unopposed surface tension in the alveoli would produce a 20 mm Hg force favoring transudation of fluid from the blood into the alveoli. Surfactant is produced by type II alveolar epithelial cells (Figure 35–13). Typical lamellar bodies, membrane-bound organelles containing whorls of phospholipid, are formed in these cells and secreted into the alveolar lumen by exocytosis. Tubes of lipid called tubular myelin form from the extruded bodies, and the tubular myelin in turn forms the phospholipid film. Following secretion, the phospholipids of surfactant line up in the alveoli with their hydrophobic fatty acid tails facing the alveolar lumen. Surface tension is inversely proportional to their concentration per unit area. The surfactant molecules move further apart as the alveoli enlarge during inspiration, and surface tension increases, whereas it decreases when they move closer together during expiration. Some of the protein– lipid complexes in surfactant are taken up by endocytosis in type II alveolar cells and recycled. Formation of the phospholipid film is greatly facilitated by the proteins in surfactant. This material contains four unique proteins: surfactant protein (SP)-A, SP-B, SP-C, and SP-D. SPA is a large glycoprotein and has a collagen-like domain within its structure. It has multiple functions, including regulation of the feedback uptake of surfactant by the type II alveolar
SECTION VII Respiratory Physiology
Air space
CLINICAL BOX 35–2
SF TM
Surfactant
N N LB
Alveolar macrophage CB
Type II cell
Golgi N
RER
Type I cell Fatty acids Choline Glycerol Amino acids Etc
FIGURE 35–13
Formation and metabolism of surfactant. Lamellar bodies (LB) are formed in type II alveolar epithelial cells and secreted by exocytosis into the fluid lining the alveoli. The released lamellar body material is converted to tubular myelin (TM), and the TM is the source of the phospholipid surface film (SF). Surfactant is taken up by endocytosis into alveolar macrophages and type II epithelial cells. N, nucleus; RER, rough endoplasmic reticulum; CB, composite body. (Reproduced with permission from Wright JR: Metabolism and turnover of
lung surfactant. Am Rev Respir Dis 1987;136:426.)
epithelial cells that secrete it. SP-B and SP-C are smaller proteins, which facilitate formation of the monomolecular film of phospholipid. A mutation of the gene for SP-C has been reported to be associated with familial interstitial lung disease. Like SP-A, SP-D is a glycoprotein. Its full function is uncertain. However, SP-A and SP-D are members of the collectin family of proteins that are involved in innate immunity in the conducting airway as well as in the alveoli. For other roles of surfactant, see Clinical Box 35–2.
WORK OF BREATHING Work is performed by the respiratory muscles in stretching the elastic tissues of the chest wall and lungs (elastic work; approximately 65% of the total work), moving inelastic tissues (viscous resistance; 7% of total), and moving air through the respiratory passages (airway resistance; 28% of total). Because pressure times volume (g/cm2 × cm3 = g × cm) has the same dimensions as work (force × distance), the work of breathing can be calculated from the relaxation pressure curve (Figures 35–10 and 35–14). The total elastic work required for inspiration is represented by the area ABCA in Figure 35–14. Note that the relaxation pressure curve of the total respiratory system differs from that of the lungs alone. The actual elastic work required to increase the volume of the lungs alone is area ABDEA. The amount of elastic work required to inflate the whole respiratory system is less than the amount required to inflate the lungs alone because part of the work comes from elastic energy stored in the thorax. The elastic energy lost from the thorax (area AFGBA) is equal to that gained by the lungs (area AEDCA).
Surfactant is important at birth. The fetus makes respiratory movements in utero, but the lungs remain collapsed until birth. After birth, the infant makes several strong inspiratory movements and the lungs expand. Surfactant keeps them from collapsing again. Surfactant deficiency is an important cause of infant respiratory distress syndrome (IRDS, also known as hyaline membrane disease), the serious pulmonary disease that develops in infants born before their surfactant system is functional. Surface tension in the lungs of these infants is high, and the alveoli are collapsed in many areas (atelectasis). An additional factor in IRDS is retention of fluid in the lungs. During fetal life, Cl– is secreted with fluid by the pulmonary epithelial cells. At birth, there is a shift to Na+ absorption by these cells via the epithelial Na+ channels (ENaCs), and fluid is absorbed with the Na+. Prolonged immaturity of the ENaCs contributes to the pulmonary abnormalities in IRDS. Patchy atelectasis is also associated with surfactant deficiency in patients who have undergone cardiac surgery involving use of a pump oxygenator and interruption of the pulmonary circulation. In addition, surfactant deficiency may play a role in some of the abnormalities that develop following occlusion of a main bronchus, occlusion of one pulmonary artery, or long-term inhalation of 100% O2. Cigarette smoking also decreases lung surfactant.
6
Lung volume (L)
598
PW
PL
PTR 4 H G 2
0 −20
B C
F A
D E
0 Transmural pressure (cm H2O)
FIGURE 35–14
+20
Relaxation pressure curves in the lung. The relaxation pressure curves of the total respiratory system (P TR), the lungs (PL), and the chest (PW) are plotted together with standard volumes for functional residual capacity and tidal volume. The transmural pressure is intrapulmonary pressure minus intrapleural pressure in the case of the lungs, intrapleural pressure minus outside (barometric) pressure in the case of the chest wall, and intrapulmonary pressure minus barometric pressure in the case of the total respiratory system. From these curves, the total and actual elastic work associated with breathing can be derived (see text). (Modified from Mines AH: Respiratory Physiology, 3rd ed. Raven Press, 1993.)
CHAPTER 35 Pulmonary Function
500
C
B
599
–10 cm H2O
Tidal volume (mL)
Intrapleural pressure –2.5 cm H2O
Z Y
100%
0
−2 A −4 Intrapleural pressure (mm Hg)
50%
−6
FIGURE 35–15
Pressure volume relationships in breathing. Diagrammatic representation of pressure and volume changes during quiet inspiration (line AXB) and expiration (line BZA). Line AYB is the compliance line.
+10
0
–10
–20
Lung volume
X
0 –30
Intrapleural pressure (cm H2O)
FIGURE 35–16 The frictional resistance to air movement is relatively small during quiet breathing, but it does cause the intrapleural pressure changes to lead the lung volume changes during inspiration and expiration (Figure 35–6), producing a hysteresis loop rather than a straight line when pressure is plotted against volume (Figure 35–15). In this diagram, area AXBYA represents the work done to overcome airway resistance and lung viscosity. If the air flow becomes turbulent during rapid respiration, the energy required to move the air is greater than when the flow is laminar. Estimates of the total work of quiet breathing range from 0.3 up to 0.8 kg-m/min. The value rises markedly during exercise, but the energy cost of breathing in normal individuals represents less than 3% of the total energy expenditure during exercise. The work of breathing is greatly increased in diseases such as emphysema, asthma, and congestive heart failure with dyspnea and orthopnea. The respiratory muscles have length– tension relations like those of other skeletal and cardiac muscles, and when they are severely stretched, they contract with less strength. They can also become fatigued and fail (pump failure), leading to inadequate ventilation.
Intrapleural pressures in the upright position and their effect on ventilation. Note that because intrapulmonary pressure is atmospheric, the more negative intrapleural pressure at the apex holds the lung in a more expanded position at the start of inspiration. Further increases in volume per unit increase in intrapleural pressure are smaller than at the base because the expanded lung is stiffer. (Reproduced with permission from West JB: Ventilation/Blood Flow
and Gas Exchange, 3rd ed. Blackwell, 1977.)
ventilation is consequently greater at the base. Blood flow is also greater at the base than the apex. The relative change in blood flow from the apex to the base is greater than the relative change in ventilation, so the ventilation/perfusion ratio is low at the base and high at the apex. The ventilation and perfusion differences from the apex to the base of the lung have usually been attributed to gravity; they tend to disappear in the supine position, and the weight of the lung would be expected to make the intrapleural pressure lower at the base in the upright position. However, the inequalities of ventilation and blood flow in humans were found to persist to a remarkable degree in the weightlessness of space. Therefore, other factors also play a role in producing the inequalities.
DIFFERENCES IN VENTILATION & BLOOD FLOW IN DIFFERENT PARTS OF THE LUNG
DEAD SPACE & UNEVEN VENTILATION
In the upright position, ventilation per unit lung volume is greater at the base of the lung than at the apex. The reason for this is that at the start of inspiration, intrapleural pressure is less negative at the base than at the apex (Figure 35–16), and since the intrapulmonary intrapleural pressure difference is less than at the apex, the lung is less expanded. Conversely, at the apex, the lung is more expanded; that is, the percentage of maximum lung volume is greater. Because of the stiffness of the lung, the increase in lung volume per unit increase in pressure is smaller when the lung is initially more expanded, and
Because gaseous exchange in the respiratory system occurs only in the terminal portions of the airways, the gas that occupies the rest of the respiratory system is not available for gas exchange with pulmonary capillary blood. Normally, the volume (in mL) of this anatomic dead space is approximately equal to the body weight in pounds. As an example, in a man who weighs 150 lb (68 kg), only the first 350 mL of the 500 mL inspired with each breath at rest mixes with the air in the alveoli. Conversely, with each expiration, the first 150 mL expired is gas that occupied the dead space, and only the last 350 mL is gas from the alveoli. Consequently, the alveolar ventilation;
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SECTION VII Respiratory Physiology
TABLE 35–3 Effect of variations in respiratory rate and depth on alveolar ventilation. Respiratory rate
30/min
10/min
Tidal volume
200 mL
600 mL
Minute volume
6L
6L
Alveolar ventilation
(200 – 150) × 30 = 1500 mL
(600 – 150) × 10 = 4500 mL
that is, the amount of air reaching the alveoli per minute, is less than the respiratory minute volume. Note in addition that because of the dead space, rapid shallow breathing produces much less alveolar ventilation than slow deep breathing at the same respiratory minute volume (Table 35–3). It is important to distinguish between the anatomic dead space (respiratory system volume exclusive of alveoli) and the total (physiologic) dead space (volume of gas not equilibrating with blood; ie, wasted ventilation). In healthy individuals, the two dead spaces are identical and can be estimated by body weight. However, in disease states, no exchange may take place between the gas in some of the alveoli and the blood, and some of the alveoli may be overventilated. The volume of gas in nonperfused alveoli and any volume of air in the alveoli in excess of that necessary to arterialize the blood in the alveolar capillaries is part of the dead space (nonequilibrating) gas volume. The anatomic dead space can be measured by analysis of the single-breath N2 curves (Figure 35–17). From mid-inspiration, the subject takes as deep a breath as possible of pure O2, then exhales steadily while the N2 content of the expired gas is continuously measured. The initial gas exhaled (phase I) is the gas that filled the dead space and that consequently contains no N2. This is followed by a mixture of dead space and alveolar gas (phase II) and then by alveolar gas (phase III). The volume of the dead space is the volume of the gas expired from peak inspiration to the midportion of phase II.
N2 concentration (%)
6
PECO2 × VT = PaCO2 × (VT – VD) + PICO2 × VD The term PICO2 × VD is so small that it can be ignored and the equation solved for VD. If, for example, PECO2 = 28 mm Hg PaCO2 = 40 mm Hg VT = 500 mL then, Vd = 150 mL The equation can also be used to measure the anatomic dead space if one replaces PaCO2 with alveolar PCO2 (PACO2), which is the PCO2 of the last 10 mL of expired gas. PCO2 is an average of gas from different alveoli in proportion to their ventilation regardless of whether they are perfused. This is in contrast to PaCO2, which is gas equilibrated only with perfused alveoli, and consequently, in individuals with unperfused alveoli, is greater than PCO2.
0
Lung volume (L)
GAS EXCHANGE IN THE LUNGS
30
SAMPLING ALVEOLAR AIR
IV
III
II 0
Phase III of the single-breath N2 curve terminates at the closing volume (CV) and is followed by phase IV, during which the N2 content of the expired gas is increased. The CV is the lung volume above residual volume at which airways in the lower, dependent parts of the lungs begin to close off because of the lesser transmural pressure in these areas. The gas in the upper portions of the lungs is richer in N2 than the gas in the lower, dependent portions because the alveoli in the upper portions are more distended at the start of the inspiration of O2 and, consequently, the N2 in them is less diluted with O2. It is also worth noting that in most normal individuals, phase III has a slight positive slope even before phase IV is reached. This indicates that even during phase III there is a gradual increase in the proportion of the expired gas coming from the relatively N2-rich upper portions of the lungs. The total dead space can be calculated from the PCO2 of expired air, the PCO2 of arterial blood, and the tidal volume. The tidal volume (VT) times the PCO2 of the expired gas (PECO2) equals the arterial PCO2 (PaCO2) times the difference between the tidal volume and the dead space (VD) plus the PCO2 of inspired air (PICO2) times VD (Bohr’s equation):
I DS
FIGURE 35–17
CV
RV
Single-breath N2 curve. From mid-inspiration, the subject takes a deep breath of pure O 2 then exhales steadily. The changes in the N2 concentration of expired gas during expiration are shown, with the various phases of the curve indicated by roman numerals. Notably, region I is representative of the dead space (DS); from I–II is a mixture of DS and alveolar gas; the transition form III–IV is the closing volume (CV), and the end of IV is the residual volume (RV).
Theoretically, all but the first 150 mL expired from a healthy 150-lb man (ie, the dead space) with each expiration is the gas that was in the alveoli (alveolar air), but some mixing always occurs at the interface between the dead-space gas and the alveolar air (Figure 35–17). A later portion of expired air is therefore the portion taken for analysis. Using modern apparatus with a suitable automatic valve, it is possible to collect the last 10 mL expired during quiet breathing. The composition of alveolar gas is compared with that of inspired and expired air in Figure 35–18.
CHAPTER 35 Pulmonary Function
Inspired air
Expired gas
O2 158.0 CO2 0.3 H2O 5.7 N2 596.0
O2 CO2 H2O N2
Dead space Alveoli Right heart
O2 CO2 H 2O N2
O2 CO2 H 2O N2
100.0 40.0 47.0 573.0
40.0 46.0 Veins 47.0 573.0 Capillaries
116.0 32.0 47.0 565.0 Physiologic shunt Left heart
O2 Arteries CO2 H2O N2
95.0 40.0 47.0 573.0
40.0− O2 CO2 46.0+ H 2O 47.0 N2 573.0 Tissues
FIGURE 35–18
Partial pressures of gases (mm Hg) in various parts of the respiratory system and in the circulatory system.
PAO2 can also be calculated from the alveolar gas equation: 1– FIO2 R
(
(
PAO2 = PIO2 – PACO2 FIO2 +
where FIO2 is the fraction of O2 molecules in the dry gas, PIO2 is the inspired PO2, and R is the respiratory exchange ratio; that is, the flow of CO2 molecules across the alveolar membrane per minute divided by the flow of O2 molecules across the membrane per minute.
601
DIFFUSION ACROSS THE ALVEOLOCAPILLARY MEMBRANE Gases diffuse from the alveoli to the blood in the pulmonary capillaries or vice versa across the thin alveolocapillary membrane made up of the pulmonary epithelium, the capillary endothelium, and their fused basement membranes (Figure 35–3). Whether or not substances passing from the alveoli to the capillary blood reach equilibrium in the 0.75 s that blood takes to traverse the pulmonary capillaries at rest depends on their reaction with substances in the blood. Thus, for example, the anesthetic gas nitrous oxide (N2O) does not react and reaches equilibrium in about 0.1 s (Figure 35–19). In this situation, the amount of N2O taken up is not limited by diffusion but by the amount of blood flowing through the pulmonary capillaries; that is, it is flow-limited. On the other hand, carbon monoxide (CO) is taken up by hemoglobin in the red blood cells at such a high rate that the partial pressure of CO in the capillaries stays very low and equilibrium is not reached in the 0.75 s the blood is in the pulmonary capillaries. Therefore, the transfer of CO is not limited by perfusion at rest and instead is diffusion-limited. O2 is intermediate between N2O and CO; it is taken up by hemoglobin, but much less avidly than CO, and it reaches equilibrium with capillary blood in about 0.3 s. Thus, its uptake is perfusion-limited. The diffusing capacity of the lung for a given gas is directly proportionate to the surface area of the alveolocapillary membrane and inversely proportionate to its thickness. The diffusing capacity for CO (DLCO) is measured as an index of diffusing capacity because its uptake is diffusion-limited. DLCO is proportionate to the amount of CO entering the blood (VCO) divided by the partial pressure of CO in the alveoli minus the partial pressure of CO in the blood entering the pulmonary capillaries. Except in habitual cigarette smokers,
COMPOSITION OF ALVEOLAR AIR Alveolar level
N2O
Partial pressure
Oxygen continuously diffuses out of the gas in the alveoli into the bloodstream, and CO2 continuously diffuses into the alveoli from the blood. In the steady state, inspired air mixes with the alveolar gas, replacing the O2 that has entered the blood and diluting the CO2 that has entered the alveoli. Part of this mixture is expired. The O2 content of the alveolar gas then falls and its CO2 content rises until the next inspiration. Because the volume of gas in the alveoli is about 2 L at the end of expiration (functional residual capacity), each 350 mL increment of inspired and expired air has relatively little effect on PO2 and PCO2. Indeed, the composition of alveolar gas remains remarkably constant, not only at rest but also under a variety of other conditions.
O2
CO 0
0.25
0.50
0.75
Time in capillary (s)
FIGURE 35–19
Uptake of various substances during the 0.75 s they are in transit through a pulmonary capillary. N2O is not bound in blood, so its partial pressure in blood rises rapidly to its partial pressure in the alveoli. Conversely, CO is avidly taken up by red blood cells, so its partial pressure reaches only a fraction of its partial pressure in the alveoli. O2 is intermediate between the two.
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SECTION VII Respiratory Physiology
this latter term is close to zero, so it can be ignored and the equation becomes: •
VCO DLCO = PACO The normal value of DLCO at rest is about 25 mL/min/mm Hg. It increases up to threefold during exercise because of capillary dilation and an increase in the number of active capillaries. The PO2 of alveolar air is normally 100 mm Hg (Figure 35–18), and the PO2 of the blood entering the pulmonary capillaries is 40 mm Hg. The diffusing capacity for O2, like that for CO at rest, is about 25 mL/min/mm Hg, and the PO2 of blood is raised to 97 mm Hg, a value just under the alveolar PO2. This falls to 95 mm Hg in the aorta because of the physiologic shunt. DLO2 increases to 65 mL/min/mm Hg or more during exercise and is reduced in diseases such as sarcoidosis and beryllium poisoning (berylliosis) that cause fibrosis of the alveolar walls. The PCO2 of venous blood is 46 mm Hg, whereas that of alveolar air is 40 mm Hg, and CO2 diffuses from the blood into the alveoli along this gradient. The PCO2 of blood leaving the lungs is 40 mm Hg. CO2 passes through all biological membranes with ease, and the diffusing capacity of the lung for CO2 is much greater than the capacity for O2. It is for this reason that CO2 retention is rarely a problem in patients with alveolar fibrosis even when the reduction in diffusing capacity for O2 is severe.
PULMONARY CIRCULATION PULMONARY BLOOD VESSELS The pulmonary vascular bed resembles the systemic one, except that the walls of the pulmonary artery and its large branches are about 30% as thick as the wall of the aorta, and the small arterial vessels, unlike the systemic arterioles, are endothelial tubes with relatively little muscle in their walls. The walls of the postcapillary vessels also contain some smooth muscle. The pulmonary capillaries are large, and there are multiple anastomoses, so that each alveolus sits in a capillary basket.
PRESSURE, VOLUME, & FLOW With two quantitatively minor exceptions, the blood put out by the left ventricle returns to the right atrium and is ejected by the right ventricle, making the pulmonary vasculature unique in that it accommodates a blood flow that is almost equal to that of all the other organs in the body. One of the exceptions is part of the bronchial blood flow. As shown in Figure 35–5, there are anastomoses between the bronchial capillaries and the pulmonary capillaries and veins, and although some of the bronchial blood enters the bronchial veins, some enters the pulmonary capillaries and veins, bypassing the right ventricle. The other exception is blood that
flows from the coronary arteries into the chambers of the left side of the heart. Because of the small physiologic shunt created by those two exceptions, the blood in systemic arteries has a PO2 about 2 mm Hg lower than that of blood that has equilibrated with alveolar air, and the saturation of hemoglobin is 0.5% less. The pressure in the various parts of the pulmonary portion of the pulmonary circulation is shown in Figure 35–4. The pressure gradient in the pulmonary system is about 7 mm Hg, compared with a gradient of about 90 mm Hg in the systemic circulation. Pulmonary capillary pressure is about 10 mm Hg, whereas the oncotic pressure is 25 mm Hg, so that an inwarddirected pressure gradient of about 15 mm Hg keeps the alveoli free of all but a thin film of fluid. When the pulmonary capillary pressure is more than 25 mm Hg—as it may be, for example, in “backward failure” of the left ventricle—pulmonary congestion and edema result. The volume of blood in the pulmonary vessels at any one time is about 1 L, of which less than 100 mL is in the capillaries. The mean velocity of the blood in the root of the pulmonary artery is the same as that in the aorta (about 40 cm/s). It falls off rapidly, then rises slightly again in the larger pulmonary veins. It takes a red cell about 0.75 s to traverse the pulmonary capillaries at rest and 0.3 s or less during exercise.
EFFECT OF GRAVITY Gravity has a relatively marked effect on the pulmonary circulation. In the upright position, the upper portions of the lungs are well above the level of the heart, and the bases are at or below it. Consequently, in the upper part of the lungs, the blood flow is less, the alveoli are larger, and ventilation is less than at the base (Figure 35–20). The pressure in the capillaries at the top of the lungs is close to the atmospheric pressure in the alveoli. Pulmonary arterial pressure is normally just sufficient to maintain perfusion, but if it is reduced or if alveolar pressure is increased, some of the capillaries collapse. Under these circumstances, no gas exchange takes place in the affected alveoli and they become part of the physiologic dead space. In the middle portions of the lungs, the pulmonary arterial and capillary pressure exceeds alveolar pressure, but the pressure in the pulmonary venules may be lower than alveolar pressure during normal expiration, so they are collapsed. Under these circumstances, blood flow is determined by the pulmonary artery–alveolar pressure difference rather than the pulmonary artery–pulmonary vein difference. Beyond the constriction, blood “falls” into the pulmonary veins, which are compliant and take whatever amount of blood the constriction lets flow into them. This has been called the waterfall effect. Obviously, the compression of vessels produced by alveolar pressure decreases and pulmonary blood flow increases as the arterial pressure increases toward the base of the lung. In the lower portions of the lungs, alveolar pressure is lower than the pressure in all parts of the pulmonary circulation and blood flow is determined by the arterial–venous pressure
CHAPTER 35 Pulmonary Function
At apex Intrapleural pressure more negative Greater transmural pressure Large alveoli Lower intravascular pressure Less blood flow So less ventilation and perfusion
FIGURE 35–20
Diagram of normal differences in ventilation and perfusion of the lung in the upright position. Outlined areas are representative of changes in alveolar size (not actual size). Note the gradual change in alveolar size from top (apex) to bottom. Characteristic differences of alveoli at the apex of the lung are stated. (Modified
from Levitsky, MG: Pulmonary Physiology, 6th ed. McGraw-Hill, 2003).
difference. Examples of diseases affecting pulmonary circulation are given in Clinical Box 35–3.
VENTILATION/PERFUSION RATIOS The ratio of pulmonary ventilation to pulmonary blood flow for the whole lung at rest is about 0.8 (4.2 L/min ventilation divided by 5.5 L/min blood flow). However, relatively marked differences occur in this ventilation/perfusion ratio in various parts of the normal lung as a result of the effect of gravity, and local changes in the ventilation/perfusion ratio are common in disease. If the ventilation to an alveolus is reduced relative to its perfusion, the PO2 in the alveolus falls because less O2 is delivered to it and the PCO2 rises because less CO2 is expired. Conversely, if perfusion is reduced relative to ventilation, the PCO2 falls because less CO2 is delivered and the PO2 rises because less O2 enters the blood. These effects are summarized in Figure 35–21. As noted above, ventilation, as well as perfusion in the upright position, declines in a linear fashion from the bases to the apices of the lungs. However, the ventilation/perfusion ratios are high in the upper portions of the lungs. When widespread, nonuniformity of ventilation and perfusion in the lungs can cause CO2 retention and declines in systemic arterial PO2.
PULMONARY RESERVOIR Because of their distensibility, the pulmonary veins are an important blood reservoir. When a normal individual lies down, the pulmonary blood volume increases by up to 400 mL, and
603
CLINICAL BOX 35–3 Diseases Affecting the Pulmonary Circulation Pulmonary Hypertension Sustained primary pulmonary hypertension can occur at any age. Like systemic arterial hypertension, it is a syndrome with multiple causes. However, the causes are different from those causing systemic hypertension. They include hypoxia, inhalation of cocaine, treatment with dexfenfluramine and related appetite-suppressing drugs that increase extracellular serotonin, and systemic lupus erythematosus. Some cases are familial and appear to be related to mutations that increase the sensitivity of pulmonary vessels to growth factors or cause deformations in the pulmonary vascular system. All these conditions lead to increased pulmonary vascular resistance. If appropriate therapy is not initiated, the increased right ventricular afterload can lead eventually to right heart failure and death. Treatment with vasodilators such as prostacyclin and prostacyclin analogs is effective. Until recently, these had to be administered by continuous intravenous infusion, but aerosolized preparations that appear to be effective are now available.
Pulmonary Embolization One of the normal functions of the lungs is to filter out small blood clots, and this occurs without any symptoms. When emboli block larger branches of the pulmonary artery, they provoke a rise in pulmonary arterial pressure and rapid, shallow respiration (tachypnea). The rise in pulmonary arterial pressure may be due to reflex vasoconstriction via the sympathetic nerve fibers, but reflex vasoconstriction appears to be absent when large branches of the pulmonary artery are blocked. The tachypnea is a reflex response to activation of vagally innervated pulmonary receptors close to the vessel walls. These appear to be activated at the site of the embolization.
when the person stands up this blood is discharged into the general circulation. This shift is the cause of the decrease in vital capacity in the supine position and is responsible for the occurrence of orthopnea in heart failure.
REGULATION OF PULMONARY BLOOD FLOW It is unsettled whether pulmonary veins and pulmonary arteries are regulated separately, although constriction of the veins increases pulmonary capillary pressure and constriction of pulmonary arteries increases the load on the right side of the heart. Pulmonary blood flow is affected by both active and passive factors. There is an extensive autonomic innervation of the pulmonary vessels, and stimulation of the cervical sympathetic
604
SECTION VII Respiratory Physiology
muscle in pulmonary arteries and veins. O
A
0
50
Subtype
Response
Endothelium Dependency
α1
Contraction
No
α2
Relaxation
Yes
β2
Relaxation
Yes
Muscarinic
M3
Relaxation
Yes
Purinergic
P2x
Contraction
No
P2y
Relaxation
Yes
NK1
Relaxation
Yes
NK2
Contraction
No
VIP
?
Relaxation
?
CGRP
?
Relaxation
No
A1
Contraction
No
A2
Relaxation
No
Angiotensin II
AT1
Contraction
No
ANP
ANPA
Relaxation
No
ANPB
Relaxation
No
B1 ?
Relaxation
Yes
B2
Relaxation
Yes
ETA
Contraction
No
ETB
Relaxation
Yes
H1
Relaxation
Yes
H2
Relaxation
No
5-HT1
Contraction
No
5-HT1C
Relaxation
Yes
Thromboxane
TP
Contraction
No
Vasopressin
V1
Relaxation
Yes
Normal
Receptor
Increasing VA/Q
Autonomic
•
Decreasing VA/Q
100 PO2 (mm Hg)
•
50
•
_ V
•
PCO2 (mm Hg)
TABLE 35–4 Receptors affecting smooth
150
Adrenergic
FIGURE 35–21
Effects of decreasing or increasing the • • ventilation/perfusion ratio (VA/Q) on the PCO2 and PO2 in an alveolus. The drawings above the curve represent an alveolus and a pulmonary capillary, and the dark red areas indicate sites of blockage. With complete obstruction of the airway to the alveolus, P CO2 and PO2 – approximate the values in mixed venous blood (V ). With complete block of perfusion, PCO2 and PO2 approximate the values in inspired air. (Reproduced with permission from West JB: Ventilation/Blood Flow and Gas
Tachykinin
Exchange, 3rd ed. Blackwell, 1977.)
ganglia reduces pulmonary blood flow by as much as 30%. The vessels also respond to circulating humoral agents. Several of the receptors involved and their effect on pulmonary smooth muscle are summarized in Table 35–4. Many of the dilator responses are endothelium-dependent and presumably operate via release of nitric oxide (NO). Passive factors such as cardiac output and gravitational forces also have significant effects on pulmonary blood flow. Local adjustments of perfusion to ventilation are determined by local effects of O2 (or the lack of O2). With exercise, cardiac output increases and pulmonary arterial pressure rises proportionately with little or no vasodilation. More red cells move through the lungs without any reduction in the O2 saturation of the hemoglobin in them, and consequently, the total amount of O2 delivered to the systemic circulation is increased. Capillaries dilate, and previously underperfused capillaries are “recruited” to carry blood. The net effect is a marked increase in pulmonary blood flow with few, if any, alterations in autonomic outflow to the pulmonary vessels. When a bronchus or a bronchiole is obstructed, hypoxia develops in the underventilated alveoli beyond the obstruction. The O2 deficiency apparently acts directly on vascular smooth muscle in the area to produce constriction, shunting blood away from the hypoxic area. Accumulation of CO2 leads to a drop in pH in the area, and a decline in pH also produces vasoconstriction in the lungs, as opposed to the vasodilation it produces in other tissues. Conversely, reduction of the blood flow to a portion of the lung lowers the alveolar PCO2 in that area, and this leads to constriction of the bronchi supplying it, shifting ventilation away from the poorly perfused area. Systemic hypoxia also causes the pulmonary arterioles to constrict, with a resultant increase in pulmonary arterial pressure.
Humoral Adenosine
Bradykinin
Endothelin
Histamine
5-HT
Modified and reproduced with permission from Barnes PJ, Lin SF: Regulation of pulmonary vascular tone. Pharmacol Rev 1995;47:88.
CHAPTER 35 Pulmonary Function
OTHER FUNCTIONS OF THE RESPIRATORY SYSTEM LUNG DEFENSE MECHANISMS The respiratory passages that lead from the exterior to the alveoli do more than serve as gas conduits. They humidify and cool or warm the inspired air so that even very hot or very cold air is at or near body temperature by the time it reaches the alveoli. Airway epithelial cells can secrete a variety of molecules that aid in lung defense. Secretory immunoglobulins (IgA), collectins (including Surfactant A and D), defensins and other peptides and proteases, reactive oxygen species, and reactive nitrogen species are all generated by airway epithelial cells. These secretions can act directly as antimicrobials to help keep the airway free of infection. Airway epithelial cells also secrete a variety of chemokines and cytokines that recruit the traditional immune cells and others to site of infections. Various mechanisms operate to prevent foreign matter from reaching the alveoli. The hairs in the nostrils strain out many particles larger than 10 μm in diameter. Most of the remaining particles of this size settle on mucous membranes in the nose and pharynx; because of their momentum, they do not follow the airstream as it curves downward into the lungs, and they impact on or near the tonsils and adenoids, large collections of immunologically active lymphoid tissue in the back of the pharynx. Particles 2 to 10 μm in diameter generally fall on the walls of the bronchi as the air flow slows in the smaller passages. There they can initiate reflex bronchial constriction and coughing. Alternatively, they can be moved away from the lungs by the “mucociliary escalator.” The epithelium of the respiratory passages from the anterior third of the nose to the beginning of the respiratory bronchioles is ciliated. The cilia are bathed in a periciliary fluid where they typically beat at rates of 10–15 Hz. On top of the periciliary layer and the beating cilia rests a mucus layer, a complex mixture of proteins and polysaccharides secreted from specialized cells, glands, or both in the conducting airway. This combination allows for the trapping of foreign particles (in the mucus) and their transport out of the airway (powered by ciliary beat). The ciliary mechanism is capable of moving particles away from the lungs at a rate of at least 16 mm/min. When ciliary motility is defective, as can occur from smoking, other environmental conditions, or genetic deficiency, mucus transport is virtually absent. This can lead to chronic sinusitis, recurrent lung infections, and bronchiectasis. Some of these symptoms are evident in cystic fibrosis (Clinical Box 35–4). The pulmonary alveolar macrophages (PAMs) are another important component of the pulmonary defense system. Like other macrophages , these cells come originally from the bone marrow. Particles less than 2 μm in diameter can evade the mucociliary escalator and reach the alveoli. PAMs are actively phagocytic and ingest these small particles. They also help process inhaled antigens for immunologic attack, and they secrete substances that attract granulocytes to the lungs as
605
CLINICAL BOX 35–4 Cystic Fibrosis Among Caucasians, cystic fibrosis is one of the most common genetic disorders: 5% of the population carry a defective gene, and the disease occurs in 1 of every 2000 births. The gene that is abnormal in cystic fibrosis is located on the long arm of chromosome 7 and encodes the cystic fibrosis transmembrane conductance regulator (CFTR), a regulated Cl– channel located on the apical membrane of various secretary and reabsorptive epithelia. The number of reported mutations in the CFTR gene that cause cystic fibrosis is large, and the severity of the defect varies with the mutation; however, this is not surprising in a gene encoding such a complex protein. The most common mutation causing cystic fibrosis is loss of the phenylalanine residue at position 508 of the protein (ΔF508). This hinders proper folding of the molecule, leading to reduced membrane levels. One outcome of cystic fibrosis is repeated pulmonary infections, particularly with Pseudomonas aeruginosa, and progressive, eventually fatal destruction of the lungs. In this congenital recessive condition, the function of a Cl– channel, the CFTR channel, is depressed by loss-of-function mutations in the gene that encodes it. One would expect Na+ reabsorption to be depressed as well, and indeed in sweat glands it is. However, in the lungs, it is enhanced, so that the Na+ and water move out of airways, leaving their other secretions inspissated and sticky. This results in a reduced periciliary layer that inhibits function of the mucociliary escalator, and alters the local environment to reduce the effectiveness of antimicrobial secretions.
well as substances that stimulate granulocyte and monocyte formation in the bone marrow. When the PAMs ingest large amounts of the substances in cigarette smoke or other irritants, they may also release lysosomal products into the extracellular space to cause inflammation.
METABOLIC & ENDOCRINE FUNCTIONS OF THE LUNGS In addition to their functions in gas exchange, the lungs have a number of metabolic functions. They manufacture surfactant for local use, as noted above. They also contain a fibrinolytic system that lyses clots in the pulmonary vessels. They release a variety of substances that enter the systemic arterial blood (Table 35–5), and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. Prostaglandins are removed from the circulation, but they are also synthesized in the lungs and released into the blood when lung tissue is stretched.
606
SECTION VII Respiratory Physiology
TABLE 35–5 Biologically active substances
■
metabolized by the lungs. Synthesized and used in the lungs Surfactant Synthesized or stored and released into the blood
■
Prostaglandins Histamine Kallikrein Partially removed from the blood Prostaglandins
■
Bradykinin Adenine nucleotides Serotonin
■
Norepinephrine Acetylcholine
■
Activated in the lungs Angiotensin I → angiotensin II
The lungs also activate one hormone; the physiologically inactive decapeptide angiotensin I is converted to the pressor, aldosterone-stimulating octapeptide angiotensin II in the pulmonary circulation. The reaction occurs in other tissues as well, but it is particularly prominent in the lungs. Large amounts of the angiotensin-converting enzyme responsible for this activation are located on the surface of the endothelial cells of the pulmonary capillaries. The converting enzyme also inactivates bradykinin. Circulation time through the pulmonary capillaries is less than 1 s, yet 70% of the angiotensin I reaching the lungs is converted to angiotensin II in a single trip through the capillaries. Four other peptidases have been identified on the surface of the pulmonary endothelial cells, but their full physiologic role is unsettled. Removal of serotonin and norepinephrine reduces the amounts of these vasoactive substances reaching the systemic circulation. However, many other vasoactive hormones pass through the lungs without being metabolized. These include epinephrine, dopamine, oxytocin, vasopressin, and angiotensin II. In addition, various amines and polypeptides are secreted by neuroendocrine cells in the lungs.
CHAPTER SUMMARY ■
The pressure exerted by any one gas in a mixture of gases is defined as its partial pressure. Partial pressures (P) of gases in air at sea level are as follows: Po2 = 149 mm Hg; Pco2 = 0.3 mm Hg; PN2 (including other gases) = 564 mm Hg.
■
■
■
■ ■
Air enters the respiratory system in the upper airway, then proceeds to the conducting airway and on to the respiratory airway that ends in the alveoli. In the upper airway, air is humidified and warmed. The cross sectional area of the airway gradually increases through the conducting zone, then rapidly increases during the transition from conducting to respiratory zones The epithelium that line the conducting airway include ciliated cells that keep particulates from reaching the respiratory zone. The epithelium that lines the alveoli consist of two cell types: alveolar type I cells and alveolar type II cells. Type I cells are flattened epithelial cells that provide approximately 95% of the alveolar surface area and are the site of gas exchange. Type II cells are cuboidal epithelial cells that secrete surfactants that line the alveolar surface. There are several important measures of lung volume, including: tidal volume; inspiratory volume; expiratory reserve volume; forced vital capacity (FVC); the forced expiratory volume in one second (FEV1); respiratory minute volume and maximal voluntary ventilation. Lung compliance refers to the ability of lungs to stretch. However, many normal factors affect lung compliance and it is best represented by a whole pressure-volume curve. Surfactant is a lipid-protein mixture that is in the fluid lining the alveolar epithelium. A primary function of surfactant is to increase surface tension in the alveoli to keep them from deflating. Both ventilation and perfusion are greater at the base of the lung and lower at the apex of the lung. The ventilation/perfusion ratio is lower at the base compared to the apex of the lung. Not all air that enters the airway is available for gas exchange. The regions where gas is not exchanged in the airway are termed “dead space.” The conducting airway represents anatomical dead space. Increased dead space can occur in response to disease that affects air exchange in the respiratory zone. The pressure gradient in the pulmonary circulation system is much less than that in the systemic circulation. Because pulmonary capillary pressure is much lower than oncotic pressure in the plasma, fluid remains in the plasma as it traverses the lung. The mucociliary escalator in the conducting airway helps to keep particulates out of the respiratory zone. There are a variety of biologically activated substances that are metabolized in the lung. These include substances that are made and function in the lung (eg, surfactant), substances that are released or removed from the blood (eg, prostaglandins), and substances that are activated as they pass through the lung (eg, angiotensin II).
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. On the summit of Mt. Everest, where the barometric pressure is about 250 mm Hg, the partial pressure of O2 is about A) 0.1 mm Hg. B) 0.5 mm Hg. C) 5 mm Hg. D) 50 mm Hg. E) 100 mm Hg.
CHAPTER 35 Pulmonary Function 2. The forced vital capacity is A) the amount of air that normally moves into (or out of) the lung with each respiration. B) the amount of air that enters the lung but does not participate in gas exchange. C) the largest amount of air expired after maximal expiratory effort. D) the largest amount of gas that can be moved into and out of the lungs in 1 min. 3. The tidal volume is A) the amount of air that normally moves into (or out of) the lung with each respiration. B) the amount of air that enters the lung but does not participate in gas exchange. C) the largest amount of air expired after maximal expiratory effort. D) the largest amount of gas that can be moved into and out of the lungs in 1 min. 4. Which of the following is responsible for the movement of O2 from the alveoli into the blood in the pulmonary capillaries? A) active transport B) filtration C) secondary active transport D) facilitated diffusion E) passive diffusion 5. Which of the following causes relaxation of bronchial smooth muscle? A) leukotrienes B) vasoactive intestinal polypeptide C) acetylcholine D) cool air E) sulfur dioxide
607
6. Airway resistance A) is increased if the lungs are removed and inflated with saline. B) does not affect the work of breathing. C) is increased in paraplegic patients. D) is increased in asthma. E) makes up 80% of the work of breathing. 7. Surfactant lining the alveoli A) helps prevent alveolar collapse. B) is produced in alveolar type I cells and secreted into the alveolus. C) is increased in the lungs of heavy smokers. D) is a glycolipid complex.
CHAPTER RESOURCES Barnes PJ: Chronic obstructive pulmonary disease. N Engl J Med 2000;343:269. Budhiraja R, Tudor RM, Hassoun PM: Endothelial dysfunction in pulmonary hypertension. Circulation 2004;88:159. Crystal RG, West JB (editors): The Lung: Scientific Foundations, 2nd ed. Raven Press, 1997. Fishman AP, et al (editors): Fishman’s Pulmonary Diseases and Disorders, 4th ed. McGraw-Hill, 2008. Levitzky MG: Pulmonary Physiology, 7th ed. McGraw-Hill, 2007. Prisk GK, Paiva M, West JB (editors): Gravity and the Lung: Lessons from Micrography. Marcel Dekker, 2001. West JB: Pulmonary Pathophysiology, 5th ed. McGraw-Hill, 1995. Wright JR: Immunoregulatory functions of surfactant proteins. Nat Rev Immunol 2005;5:58.
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36 C
Gas Transport & pH in the Lung
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■ ■ ■
Describe the manner in which O2 flows “downhill” from the lungs to the tissues and CO2 flows “downhill” from the tissues to the lungs. Describe the reactions of O2 with hemoglobin and the oxygen–hemoglobin dissociation curve. List the important factors affecting the affinity of hemoglobin for O2 and the physiologic significance of each. List the reactions that increase the amount of CO2 in the blood, and draw the CO2 dissociation curve for arterial and venous blood. List the principal buffers in blood and, using the Henderson–Hasselbalch equation, describe what is unique about the bicarbonate buffer system. Define alkalosis and acidosis and outline respiratory and renal compensatory mechanisms in response to alkalosis and acidosis. Define hypoxia and describe its four principal forms. List and explain the effects of carbon monoxide on the body. Describe the effects of hypercapnia and hypocapnia, and give examples of conditions that can cause them.
INTRODUCTION The partial pressure gradients for O2 and CO2, plotted in graphic form in Figure 36–1, emphasize that they are the key to gas movement and that O2 “flows downhill” from the air through the alveoli and blood into the tissues, whereas CO2 “flows downhill” from the tissues to the alveoli. However, the amount of both of these gases transported to and from the tissues would be grossly inadequate if it were not that about 99% of the O2 that dissolves in the blood combines with the O2-
carrying protein hemoglobin and that about 94.5% of the CO2 that dissolves enters into a series of reversible chemical reactions that convert it into other compounds. Thus, the presence of hemoglobin increases the O2-carrying capacity of the blood 70-fold, and the reactions of CO2 increase the blood CO2 content 17-fold. In this chapter, physiologic details that underlie O2 and CO2 movement under various conditions are discussed.
OXYGEN TRANSPORT
depends on the amount of O2 entering the lungs, the adequacy of pulmonary gas exchange, the blood flow to the tissue, and the capacity of the blood to carry O2. The blood flow depends on the degree of constriction of the vascular bed in the tissue and the cardiac output. The amount of O2 in the blood is determined by the amount of dissolved O2, the amount of
OXYGEN DELIVERY TO THE TISSUES The O2 delivery system in the body consists of the lungs and the cardiovascular system. O2 delivery to a particular tissue
609
SECTION VII Respiratory Physiology
Partial pressure (mm Hg)
150
PO2
120
(Arterial)
90 (Venous)
60 30 0
PCO2 Air
Lungs
Blood
(Est) (Est) Tissues
FIGURE 36–1
PO2 and PCO2 values in air, lungs, blood, and tissues. Note that both O2 and CO2 diffuse “downhill” along gradients of decreasing partial pressure. (Redrawn and reproduced with permission
from Kinney JM: Transport of carbon dioxide in blood. Anesthesiology 1960;21:615.)
Percentage O2 saturation of hemoglobin
610
100 90 80
PO2 % Sat Dissolved (mm Hg) of Hb O2 (mL/dL)
70 60
10 20 30 40 50 60 70 80 90 100
50 40 30 20 10 0
13.5 35 57 75 83.5 89 92.7 94.5 96.5 97.5
0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27 0.30
10 20 30 40 50 60 70 80 90 100 110 PO2 (mm Hg)
FIGURE 36–2 hemoglobin in the blood, and the affinity of the hemoglobin for O2.
REACTION OF HEMOGLOBIN & OXYGEN The dynamics of the reaction of hemoglobin with O2 make it a particularly suitable O2 carrier. Hemoglobin is a protein made up of four subunits, each of which contains a heme moiety attached to a polypeptide chain. In normal adults, most of the hemoglobin molecules contain two α and two β chains. Heme (see Figure 32–7) is a porphyrin ring complex that includes one atom of ferrous iron. Each of the four iron atoms in hemoglobin can reversibly bind one O2 molecule. The iron stays in the ferrous state, so that the reaction is oxygenation, not oxidation. It has been customary to write the reaction of → HbO . Because it contains hemoglobin with O2 as Hb + O2 ← 2 four deoxyhemoglobin (Hb) units, the hemoglobin molecule can also be represented as Hb4, and it actually reacts with four molecules of O2 to form Hb4O8. Hb4 + O2
→ ←
Hb4O2
Hb4O2 + O2
→ ←
Hb4O4
Hb4O4 + O2
→ ←
Hb4O6
Hb4O6 + O2
→ ←
Hb4O8
The reaction is rapid, requiring less than 0.01 s. The deoxygenation (reduction) of Hb4O8 is also very rapid. The quaternary structure of hemoglobin determines its affinity for O2. In deoxyhemoglobin, the globin units are tightly bound in a tense (T) configuration, which reduces the affinity of the molecule for O2. When O2 is first bound, the bonds holding the globin units are released, producing a relaxed (R) configuration, which exposes more O2 binding sites. The net result is a 500-fold increase in O2 affinity. In tissue, these reactions are reversed, releasing O2. The transition from one state to another has been calculated to occur about 108 times in the life of a red blood cell. The oxygen–hemoglobin dissociation curve relates percentage saturation of the O2 carrying power of hemoglobin to
Oxygen–hemoglobin dissociation curve. pH 7.40, temperature 38 °C. Inset table notes the percentage of saturated hemoglobin to PO2 and dissolved O2. (Redrawn and reproduced with permission from Comroe JH Jr., et al: The Lung: Clinical Physiology and Pulmonary Function Tests, 2nd ed. Year Book, 1962.)
the PO2 (Figure 36–2). This curve has a characteristic sigmoid shape due to the T–R interconversion. Combination of the first heme in the Hb molecule with O2 increases the affinity of the second heme for O2, and oxygenation of the second increases the affinity of the third, and so on, so that the affinity of Hb for the fourth O2 molecule is many times that for the first. When blood is equilibrated with 100% O2 (PO2 = 760 mm Hg), the normal hemoglobin becomes 100% saturated. When fully saturated, each gram of normal hemoglobin contains 1.39 mL of O2. However, blood normally contains small amounts of inactive hemoglobin derivatives, and the measured value in vivo is lower. The traditional figure is 1.34 mL of O2. The hemoglobin concentration in normal blood is about 15 g/dL (14 g/dL in women and 16 g/dL in men). Therefore, 1 dL of blood contains 20.1 mL (1.34 mL × 15) of O2 bound to hemoglobin when the hemoglobin is 100% saturated. The amount of dissolved O2 is a linear function of the PO2 (0.003 mL/dL blood/mm Hg PO2). In vivo, the hemoglobin in the blood at the ends of the pulmonary capillaries is about 97.5% saturated with O2 (PO2 = 97 mm Hg). Because of a slight admixture with venous blood that bypasses the pulmonary capillaries (physiologic shunt), the hemoglobin in systemic arterial blood is only 97% saturated. The arterial blood therefore contains a total of about 19.8 mL of O2 per dL: 0.29 mL in solution and 19.5 mL bound to hemoglobin. In venous blood at rest, the hemoglobin is 75% saturated and the total O2 content is about 15.2 mL/dL: 0.12 mL in solution and 15.1 mL bound to hemoglobin. Thus, at rest the tissues remove about 4.6 mL of O2 from each deciliter of blood passing through them (Table 36–1); 0.17 mL of this total represents O2 that was in solution in the blood, and the remainder represents O2 that was liberated from hemoglobin. In this way, 250 mL of O2 per minute is transported from the blood to the tissues at rest.
CHAPTER 36 Gas Transport & pH in the Lung
TABLE 36–1 Gas content of blood.
2,3-BPG is very plentiful in red cells. It is formed from 3phosphoglyceraldehyde, which is a product of glycolysis via the Embden–Meyerhof pathway (Figure 36–4). It is a highly charged anion that binds to the β chains of deoxyhemoglobin. One mole of deoxyhemoglobin binds 1 mol of 2,3-BPG. In effect,
mL/dL of Blood Containing 15 g of Hemoglobin Arterial Blood (PO2 95 mm Hg; PCO2 40 mm Hg; Hb 97% Saturated)
Venous Blood (PO2 40 mm Hg; PCO2 46 mm Hg; Hb 75% Saturated)
Gas
Dissolved
Combined
Dissolved
Combined
O2
0.29
19.5
0.12
15.1
CO2
2.62
46.4
2.98
49.7
N2
0.98
0
0.98
0
→ Hb – 2,3-BPG + O HbO2 + 2,3-BPG ← 2
FACTORS AFFECTING THE AFFINITY OF HEMOGLOBIN FOR OXYGEN Three important conditions affect the oxygen–hemoglobin dissociation curve: the pH, the temperature, and the concentration of 2,3-biphosphoglycerate (BPG; 2,3-BPG). A rise in temperature or a fall in pH shifts the curve to the right (Figure 36–3). When the curve is shifted in this direction, a higher PO2 is required for hemoglobin to bind a given amount of O2. Conversely, a fall in temperature or a rise in pH shifts the curve to the left, and a lower PO2 is required to bind a given amount of O2. A convenient index for comparison of such shifts is the P50, the PO2 at which hemoglobin is half saturated with O2. The higher the P50, the lower the affinity of hemoglobin for O2. The decrease in O2 affinity of hemoglobin when the pH of blood falls is called the Bohr effect and is closely related to the fact that deoxygenated hemoglobin (deoxyhemoglobin) binds H+ more actively than does oxygenated hemoglobin (oxyhemoglobin). The pH of blood falls as its CO2 content increases, so that when the PCO2 rises, the curve shifts to the right and the P50 rises. Most of the unsaturation of hemoglobin that occurs in the tissues is secondary to the decline in the PO2, but an extra 1–2% unsaturation is due to the rise in PCO2 and consequent shift of the dissociation curve to the right.
100 20°
80
43°
80
60
60
40
40
20
0
Effect of temperature
20
40
60
In this equilibrium, an increase in the concentration of 2,3BPG shifts the reaction to the right, causing more O2 to be liberated. Because acidosis inhibits red cell glycolysis, the 2,3-BPG concentration falls when the pH is low. Conversely, thyroid hormones, growth hormones, and androgens can all increase the concentration of 2,3-BPG and the P50. Exercise has been reported to produce an increase in 2,3BPG within 60 min, although the rise may not occur in trained athletes. The P50 is also increased during exercise, because the temperature rises in active tissues and CO2 and metabolites accumulate, lowering the pH. In addition, much more O2 is removed from each unit of blood flowing through active tissues because the tissues’ PO2 declines. Finally, at low PO2 values, the oxygen–hemoglobin dissociation curve is steep, and large amounts of O2 are liberated per unit drop in PO2. Some clinical features of hemoglobin are discussed in Clinical Box 36–1.
MYOGLOBIN Myoglobin is an iron-containing pigment found in skeletal muscle. It resembles hemoglobin but binds 1 rather than 4 mol of O2 per mole. Its dissociation curve is a rectangular hyperbola rather than a sigmoid curve. Because its curve is to the left of the hemoglobin curve (Figure 36–5), it takes up O2 from hemoglobin in the blood. It releases O2 only at low PO2 values, but the PO2 in exercising muscle is close to zero. The myoglobin content is greatest in muscles specialized for sustained contraction. The muscle blood supply is compressed during such contractions, and myoglobin may provide O2 when blood flow is cut off.
100
38°
10°
611
80
7.6
pH = 6.10 + log
[HCO3−] 0.0301 PCO2
pH arterial blood ≅ 7.40 pH venous blood ≅ 7.36
20
0
7.4 7.2
Effect of pH
20
40
60
80
FIGURE 36–3 Effects of temperature and pH on the oxygen–hemoglobin dissociation curve. Both changes in temperature (left) and pH (right) can alter the affinity of hemoglobin for O 2. Plasma pH can be estimated using the modified Henderson–Hasselbalch equation, as shown. (Redrawn and reproduced with permission from Comroe JH Jr., et al: The Lung: Clinical Physiology and Pulmonary Function Tests, 2nd ed. Year Book, 1962.)
612
SECTION VII Respiratory Physiology
Glucose 6-PO4
CLINICAL BOX 36–1
3-Phosphoglyceraldehyde
Hemoglobin & O2 Binding In Vivo
1,3-Biphosphoglycerate
Cyanosis 2,3-BPG mutase COO−
H+ + HC
Phosphoglycerate kinase
— —
O
O
P
OH
OH — —
O
H2C
O
P
OH
OH 2,3-Biphosphoglycerate (2,3-BPG)
2,3-BPG phosphatase 3-Phosphoglycerate Pyruvate
FIGURE 36–4
Formation and catabolism of 2,3-BPG. Note that 2,3 BPG can be associated with the Embden–Meyerhoff pathway (see Chapter 1).
CARBON DIOXIDE TRANSPORT FATE OF CARBON DIOXIDE IN BLOOD The solubility of CO2 in blood is about 20 times that of O2; therefore, considerably more CO2 than O2 is present in simple solution at equal partial pressures. The CO2 that diffuses into red blood cells is rapidly hydrated to H2CO3 because of the presence of carbonic anhydrase. The H2CO3 dissociates to H+ and HCO3–, and the H+ is buffered, primarily by hemoglobin, while the HCO3– enters the plasma. Some of the CO2 in the red cells reacts with the amino groups of hemoglobin and other proteins (R), forming carbamino compounds: H
H
→ R—N CO2 + R—N ← H
COOH
Because deoxyhemoglobin binds more H+ than oxyhemoglobin does and forms carbamino compounds more readily, binding of O2 to hemoglobin reduces its affinity for CO2 (Haldane effect). Consequently, venous blood carries more CO2 than arterial blood, CO2 uptake is facilitated in the tissues, and CO2 release is facilitated in the lungs. About 11% of the CO2 added to the blood in the systemic capillaries is carried to the lungs as carbamino-CO2.
Reduced hemoglobin has a dark color, and a dusky bluish discoloration of the tissues, called cyanosis, appears when the reduced hemoglobin concentration of the blood in the capillaries is more than 5 g/dL. Its occurrence depends on the total amount of hemoglobin in the blood, the degree of hemoglobin unsaturation, and the state of the capillary circulation. Cyanosis is most easily seen in the nail beds and mucous membranes and in the earlobes, lips, and fingers, where the skin is thin.
Effects of 2,3-BPG on Fetal & Stored Blood The affinity of fetal hemoglobin (hemoglobin F) for O2, which is greater than that for adult hemoglobin (hemoglobin A), facilitates the movement of O2 from the mother to the fetus. The cause of this greater affinity is the poor binding of 2,3-BPG by the γ polypeptide chains that replace β chains in fetal hemoglobin. Some abnormal hemoglobins in adults have low P50 values, and the resulting high O2 affinity of the hemoglobin causes enough tissue hypoxia to stimulate increased red cell formation, with resulting polycythemia. It is interesting to speculate that these hemoglobins may not bind 2,3-BPG. Red cell 2,3-BPG concentration is increased in anemia and in a variety of diseases in which there is chronic hypoxia. This facilitates the delivery of O2 to the tissues by raising the PO2 at which O2 is released in peripheral capillaries. In banked blood that is stored, the 2,3-BPG level falls and the ability of this blood to release O2 to the tissues is reduced. This decrease, which obviously limits the benefit of the blood if it is transfused into a hypoxic patient, is less if the blood is stored in citrate–phosphate–dextrose solution rather than the usual acid–citrate–dextrose solution.
CHLORIDE SHIFT Because the rise in the HCO3– content of red cells is much greater than that in plasma as the blood passes through the capillaries, about 70% of the HCO3– formed in the red cells enters the plasma. The excess HCO3– leaves the red cells in exchange for Cl– (Figure 36–6). This process is mediated by anion exchanger 1 (AE1; formerly called Band 3), a major membrane protein in the red blood cell. Because of this chloride shift, the Cl– content of the red cells in venous blood is significantly greater than that in arterial blood. The chloride shift occurs rapidly and is essentially complete within 1 s. Note that for each CO2 molecule added to a red cell, there is an increase of one osmotically active particle in the cell— either an HCO3– or a Cl– in the red cell (Figure 36–6). Consequently, the red cells take up water and increase in size. For
CHAPTER 36 Gas Transport & pH in the Lung
TABLE 36–2 Fate of CO2 in blood.
100 B
80
In plasma 1. Dissolved A
2. Formation of carbamino compounds with plasma protein
40
3. Hydration, H+ buffered, HCO3– in plasma
A = Hemoglobin B = Myoglobin
20
0
40 80 PO2 (mm Hg)
In red blood cells 120
2. Formation of carbamino-Hb
FIGURE 36–5
Dissociation curve of hemoglobin and myoglobin. The myoglobin binding curve (B) lacks the sigmoidal shape of the hemoglobin binding curve (A) because of the single O 2 binding site in each molecule. Myoglobin also has greater affinity for O 2 than hemoglobin (curve shifted left) and thus can store O 2 in muscle.
this reason, plus the fact that a small amount of fluid in the arterial blood returns via the lymphatics rather than the veins, the hematocrit of venous blood is normally 3% greater than that of the arterial blood. In the lungs, the Cl– moves out of the cells and they shrink.
SUMMARY OF CARBON DIOXIDE TRANSPORT
3. Hydration, H+ buffered, 70% of HCO3– enters the plasma 4. Cl– shifts into cells; mOsm in cells increases
2.5 mL forms HCO3–. The pH of the blood drops from 7.40 to 7.36. In the lungs, the processes are reversed, and the 3.7 mL of CO2 is discharged into the alveoli. In this fashion, 200 mL of CO2 per minute at rest and much larger amounts during exercise are transported from the tissues to the lungs and excreted. It is worth noting that this amount of CO2 is equivalent in 24 hours to over 12,500 mEq of H+.
ACID–BASE BALANCE & GAS TRANSPORT
For convenience, the various fates of CO2 in the plasma and red cells are summarized in Table 36–2. The extent to which they increase the capacity of the blood to carry CO2 is indicated by the difference between the lines indicating the dissolved CO2 and the total CO2 in the dissociation curves for CO2 shown in Figure 36–7. Of the approximately 49 mL of CO2 in each deciliter of arterial blood (Table 36–1), 2.6 mL is dissolved, 2.6 mL is in carbamino compounds, and 43.8 mL is in HCO3–. In the tissues, 3.7 mL of CO2 per deciliter of blood is added; 0.4 mL stays in solution, 0.8 mL forms carbamino compounds, and
CI−
CO2
CO2 + H2O H2CO3 Carbonic anhydrase HHb
1. Dissolved
H+ + HCO3− H+ + Hb
The major source of acids in the blood under normal conditions is through cellular metabolism. The CO2 formed by metabolism in the tissues is in large part hydrated to H2CO3, and the total H+ load from this source is over 12,500 mEq/d. However, most of the CO2 is excreted in the lungs, and the small quantities of the remaining H+ are excreted by the kidneys.
70 60
v
50
25
a
20
40 Oxygenated blood 30
10 0
15 10
20
5
Dissolved CO2
−
FIGURE 36–6 Fate of CO2 in the red blood cell. Upon entering the red blood cell, CO2 is rapidly hydrated to H2CO3 by carbonic anhydrase. H2CO3 is in equilibrium with H+ and its conjugate base, HCO3–. H+ can interact with deoxyhemoglobin, whereas HCO 3– can be transported outside of the cell via AE1 (Band 3). In effect, for each CO 2 molecule that enters the red cell, there is an additional HCO 3– or Cl– in the cell.
30
Deoxygenated blood
10
20
30 40 50 PCO2 (mm Hg)
60
CO2 concentration (mmol/L)
60
CO2 concentration (mL/dL)
O2 saturation (%)
613
70
FIGURE 36–7 CO2 dissociation curves. The arterial point (a) and the venous point (v) indicate the total CO2 content found in arterial blood and venous blood of normal resting humans. Note the low amount of CO2 that is dissolved (orange trace) compared to that which can be carried by other means (Table 36–2). (Modified and reproduced with permission from Schmidt RF, Thews G [editors]: Human Physiology. Springer, 1983.)
614
SECTION VII Respiratory Physiology
Fruits are the main dietary source of alkali. They contain Na+ and K+ salts of weak organic acids, and the anions of these salts are metabolized to CO2, leaving NaHCO3 and KHCO3 in the body. Such ingestion contributes little to changes in pH and a more common cause of alkalosis is loss of acid from the body as a result of vomiting of gastric juice rich in HCl. This is, of course, equivalent to adding alkali to the body.
BUFFERING IN THE BLOOD Acid and base shifts in the blood are largely controlled by three main buffers in blood: (1) proteins, (2) hemoglobin, and (3) the carbonic acid–bicarbonate system. Plasma proteins are effective buffers because both their free carboxyl and their free amino groups dissociate: → RCOO− + H+ RCOOH ←
mmol +1.0
mmol of H+ added to 1 mmol of +0.5 HbO2 or Hb
Hb c
HbO2 a
0 mmol of H+ removed from 1 mmol of −0.5 HbO2 or Hb 7.30
b
7.40
7.50
7.60
7.70
pH
FIGURE 36–8
Titration curves for hemoglobin. Individual titration curves for deoxygenated hemoglobin (Hb) and oxygenated hemoglobin (HbO2) are shown. The arrow from a to c indicates the number of millimoles of H that can be added without pH shift. The arrow from a to b indicates the pH shift on deoxygenation.
− pH = pK´RCOOH + log [RCOO ] [RCOOH]
The pK for this system in an ideal solution is low (about 3), and the amount of H2CO3 is small and hard to measure accurately. However, in the body, H2CO3 is in equilibrium with CO2:
→ RNH2 + H+ RNH3+ ← pH = pK´RNH3 + log
[RNH2] [RNH3+]
The second buffer system is provided by the dissociation of the imidazole groups of the histidine residues in hemoglobin: H C NH
H C NH+
NH
C
+ H+ HC
R
R
→ H+ + HCO3− H2CO3 ←
The Henderson–Hasselbalch equation for this system is [HCO3−] [H2CO3]
pH = 6.10 + log
[HCO3−] [CO2]
The clinically relevant form of this equation is:
C
In the pH 7.0–7.7 range, the free carboxyl and amino groups of hemoglobin contribute relatively little to its buffering capacity. However, the hemoglobin molecule contains 38 histidine residues, and on this basis—plus the fact that hemoglobin is present in large amounts—the hemoglobin in blood has six times the buffering capacity of the plasma proteins. In addition, the action of hemoglobin is unique because the imidazole groups of deoxyhemoglobin (Hb) dissociate less than those of oxyhemoglobin (HbO2), making Hb a weaker acid and therefore a better buffer than HbO2. Titration curves for Hb and HbO2 are shown in Figure 36–8. The third and major buffer system in blood is the carbonic acid–bicarbonate system:
pH = pK + log
If the pK is changed to pK' (apparent ionization constant; distinguished from the true pK due to less than ideal conditions for the solution) and [CO2] is substituted for [H2CO3], the pK' is 6.1:
N
→ ← HC
→ CO2 + H2O H2CO3 ←
pH = 6.10 + log
[HCO3−] 0.0301 PCO2
since the amount of dissolved CO2 is proportional to the partial pressure of CO2 and the solubility coefficient of CO2 in mmol/L/mm Hg is 0.0301. [HCO3–] cannot be measured directly, but pH and PCO2 can be measured with suitable accuracy with pH and PCO2 glass electrodes, and [HCO3–] can then be calculated. The pK' of this system is still low relative to the pH of the blood, but the system is one of the most effective buffer systems in the body because the amount of dissolved CO2 is controlled by respiration. Additional control of the plasma concentration of HCO3– is provided by the kidneys. When H+ is added to the blood, HCO3– declines as more H2CO3 is formed. If the extra H2CO3 were not converted to CO2 and H2O and the CO2 excreted in the lungs, the H2CO3 concentration would rise. When enough H+ has been added to halve the plasma HCO3–, the pH would have dropped from 7.4 to 6.0. However, not only is all the extra H2CO3 that is formed removed, but also the H+ rise stimulates respiration and therefore produces a drop in
CHAPTER 36 Gas Transport & pH in the Lung
25
TABLE 36–3 Plasma pH, HCO3–, and PCO2 values in
Acid added
various typical disturbances of acid–base balance.a
20 meq/L 15 [HCO3−] 10
meq/L [H2CO3]
615
Arterial Plasma HCO3– (mEq/L)
PCO2 (mm Hg)
7.40
24.1
40
7.28
18.1
40
NH4 Cl ingestion
6.96
5.0
23
Diabetic acidosis
7.50
30.1
40
NaHCO3– ingestion
7.56
49.8
58
Prolonged vomiting
7.34
25.0
48
Breathing 7% CO2
7.34
33.5
64
Emphysema
7.53
22.0
27
Voluntary hyperventilation
7.48
18.7
26
Three-week residence at 4000-m altitude
5
Condition
pH
0
Normal
5
Metabolic acidosis
10
Cause
15 [HCO3−] [H2CO3]
ratio
20
0.9
10
16
pH
7.4
6.0
7.1
7.3
Metabolic alkalosis Respiratory acidosis
FIGURE 36–9
Buffering by the H2CO3–HCO3– system in blood. The bars are drawn as if buffering occurred in separate steps over time (left to right) in order to show the effect of the initial reaction, the reduction of H2CO3 to its previous value, and its further reduction by the increase in ventilation. In this case, [H2CO3] is actually the concentration of dissolved CO2, so that the mEq/L values for it are arbitrary.
PCO2, so that some additional H2CO3 is removed. The pH thus falls only to 7.2 or 7.3 (Figure 36–9). There are two additional factors that make the carbonicacid-bicarbonate system such a good biological buffer. First, → H CO proceeds slowly in either the reaction CO2 + H2O ← 2 3 direction unless the enzyme carbonic anhydrase is present. There is no carbonic anhydrase in plasma, but there is an abundant supply in red blood cells. Second, the presence of hemoglobin in the blood increases the buffering of the system by binding free H+ produced by the hydration of CO2 and allowing for movement of the HCO3– into the plasma.
ACIDOSIS & ALKALOSIS The pH of the arterial plasma is normally 7.40 and that of venous plasma slightly lower. A decrease in pH below the norm (acidosis) is technically present whenever the arterial pH is below 7.40 and an increase in pH (alkalosis) is technically present whenever pH is above 7.40. In practice, variations of up to 0.05 pH unit occur without untoward effects. Acid–base disorders are split into four categories: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. In addition, these disorders can occur in combination. Some examples of acid–base disturbances are shown in Table 36–3.
RESPIRATORY ACIDOSIS Any short-term rise in arterial PCO2 (ie, above 40 mm Hg) due to decreased ventilation results in respiratory acidosis. The CO2 that is retained is in equilibrium with H2CO3, which in turn is in equilibrium with HCO3–, so that the plasma HCO3– rises and a new equilibrium is reached at a lower pH. This can
Respiratory alkalosis
a
In the diabetic acidosis and prolonged vomiting examples, respiratory compensation for primary metabolic acidosis and alkalosis has occurred, and the Pco2 has shifted from 40 mm Hg. In the emphysema and high-altitude examples, renal compensation for primary respiratory acidosis and alkalosis has occurred and has made the deviations from normal of the plasma HCO3– larger than they would otherwise be.
be indicated graphically on a plot of plasma HCO3– concentration versus pH (Figure 36–10). The pH change observed at any increase in PCO2 during respiratory acidosis is dependent on the buffering capacity of the blood. The initial changes shown in Figure 36–10 are those that occur independently of any compensatory mechanism; that is, they are those of uncompensated respiratory acidosis.
RESPIRATORY ALKALOSIS Any short-term decrease in ventilation that lowers PCO2 below what is needed for proper CO2 exchange (ie, below 35 mm Hg) results in respiratory alkalosis. The decreased CO2 shifts the equilibrium of the carbonic acid–bicarbonate system to effectively lower the [H+] and increase the pH. As in respiratory acidosis, initial pH changes corresponding to respiratory alkalosis (Figure 36–10) are those that occur independently of any compensatory mechanism and are thus uncompensated respiratory alkalosis.
METABOLIC ACIDOSIS & ALKALOSIS Blood pH changes can also arise by nonrespiratory mechanism. Metabolic acidosis (or nonrespiratory acidosis) occurs when strong acids are added to blood. If, for example, a large amount of acid is ingested (eg, aspirin overdose), acids in the blood are
SECTION VII Respiratory Physiology
30
120 100 90 80 70
25 60
20 50
40
30
52 Metabolic alkalosis
48 44
32
30
Chronic respiratory acidosis
40 36
35
25
Acute respiratory acidosis
Acute 20 respiratory alkalosis 15
28 Normal
24 20 16
10
Metabolic acidosis
12
Chronic respiratory alkalosis
8 4
Plasma HCO3− (meq/L)
Ar terial plasma [HCO3−] (meq/L)
56
28 26 24 22 20
Uncompensated metabolic acidosis, PCO2 40 mm Hg
18 16
2
10 7.2
0 7.00
7.10
7.20
7.30
7.40
7.50
7.60
7.70
7.3
7.4
7.5
7.6
pH
7.80
Arterial blood pH
Normal
Compensated metabolic acidosis, PCO2 21 mm Hg
14 12
PCO (mm Hg)
Uncompensated metabolic alkalosis, PCO2 40 mm Hg
Hg
35
mm
40
40
50
CO 2
60
Compensated metabolic alkalosis, PCO2 48 mm Hg
34 32
Arterial blood [H+] (nmol/L) 100 90 80 70 60
P
616
FIGURE 36–11
Acid–base nomogram. Changes in the PCO2 (curved lines), plasma HCO3–, and pH (or [H+]) of arterial blood in respiratory and metabolic acidosis are shown. Note the shifts in HCO 3– and pH as acute respiratory acidosis and alkalosis are compensated, producing their chronic counterparts. (Reproduced with permission from
Acid–base paths during metabolic acidosis. Changes in true plasma pH, HCO3–, and PCO2 at rest, during metabolic acidosis and alkalosis, and following respiratory compensation are plotted. Metabolic acidosis or alkalosis causes changes in pH along the PCO2 isobar line. Respiratory compensation moves pH towards normal by altering PCO2. (This is called a Davenport diagram and is based on Davenport
Cogan MG, Rector FC Jr.: Acid–base disorders. In: The Kidney, 4th ed. Brenner BM,
HW: The ABC of Acid–Base Chemistry, 6th ed. University of Chicago Press, 1974.)
FIGURE 36–10
Rector FC Jr. [editors]. Saunders, 1991.)
–,
Prot–,
and quickly increased, lowering the available Hb HCO3– buffers. The H2CO3 that is formed is converted to H2O and CO2, and the CO2 is rapidly excreted via the lungs. This is the situation in uncompensated metabolic acidosis (Figure 36–10). Note that in contrast to respiratory acidosis, PCO2 is unchanged and the shift toward metabolic acidosis occurs along the isobar line (Figure 36–11). When the free [H+] level falls as a result of addition of alkali, or more commonly, the removal of large amounts of acid (eg, following vomiting), metabolic alkalosis results. In uncompensated metabolic alkalosis the pH rises along the isobar line (Figures 36–10 and 36–11).
RESPIRATORY & RENAL COMPENSATION Uncompensated acidosis and alkalosis as described above are seldom seen because of compensation systems. The two main compensatory systems are respiratory compensation and renal compensation. The respiratory system compensates for metabolic acidosis or alkalosis by altering ventilation, and consequently, the PCO2, which can directly change blood pH. Respiratory mechanisms tend to be fast. In response to metabolic acidosis, ventilation is increased, resulting in a decrease of PCO2 (eg, from 40 mm Hg to 20 mm Hg) and a subsequent increase in pH toward normal (Figure 36–11). In response to metabolic alkalosis, ventilation is decreased, PCO2 is increased, and a subsequent decrease in pH occurs. Because respiratory compensation is a quick response, the graphical representation in Figure 36–11 overstates the two-step adjustment in blood pH. In actuality, as
soon as metabolic acidosis begins, respiratory compensation is invoked and pH is kept from the large shifts depicted. For complete compensation from respiratory or metabolic acidosis/alkalosis, renal compensatory mechanisms are invoked. The kidney responds to acidosis by actively secreting fixed acids while retaining filtered HCO3–. In contrast, the kidney responds to alkalosis by decreasing H+ secretion and by decreasing the retention of filtered HCO3–. Renal tubule cells in the kidney have active carbonic anhydrase and thus can produce H+ and HCO3– from CO2. In response to acidosis, these cells secrete H+ into the tubular fluid in exchange for Na+ while the HCO3– is actively reabsorbed into the peritubular capillary; for each H+ secreted, one Na+ and one HCO3– are added to the blood. The result of this renal compensation for respiratory acidosis is shown graphically in the shift from acute to chronic respiratory acidosis in Figure 36–10. Conversely, in response to alkalosis, the kidney decreases H+ secretion and depresses HCO3– reabsorption. The kidney tends to reabsorb HCO3– until the level in plasma exceeds 26–28 mEq/L (normal is 24 mEq/L). Above this threshold, HCO3– appears in the urine. The result of this renal compensation for respiratory alkalosis is shown graphically in the shift from acute to chronic respiratory alkalosis in Figure 36–10. Clinical evaluations of acid–base status are discussed in Clinical Box 36–2.
HYPOXIA Hypoxia is O2 deficiency at the tissue level. It is a more correct term than anoxia, with there rarely being no O2 at all left in the tissues.
CHAPTER 36 Gas Transport & pH in the Lung
617
CLINICAL BOX 36–2
CLINICAL BOX 36–3
Clinical Evaluation of Acid–Base Status
Effects of Hypoxia on Cells and Selected Tissues
In evaluating disturbances of acid–base balance, it is important to know the pH and HCO3– content of arterial plasma. Reliable pH determinations can be made with a pH meter and a glass pH electrode. Using pH and a direct measurement of the PCO2 with a CO2 electrode, HCO3– concentration can be calculated. The PCO2 is 7 to 8 mm Hg higher and the pH 0.03 to 0.04 unit lower in venous than arterial plasma because venous blood contains the CO2 being carried from the tissues to the lungs. Therefore, the calculated HCO3– concentration is about 2 mmol/L higher. However, if this is kept in mind, free-flowing venous blood can be substituted for arterial blood in most clinical situations. A measurement that is of some value in the differential diagnosis of metabolic acidosis is the anion gap. This gap, which is something of a misnomer, refers to the difference between the concentration of cations other than Na+ and the concentration of anions other than Cl– and HCO3– in the plasma. It consists for the most part of proteins in the anionic form, HPO42–, SO42–, and organic acids, and a normal value is about 12 mEq/L. It is increased when the plasma concentration of K+, Ca2+, or Mg+ is decreased; when the concentration of or the charge on plasma proteins is increased; or when organic anions such as lactate or foreign anions accumulate in blood. It is decreased when cations are increased or when plasma albumin is decreased. The anion gap is increased in metabolic acidosis due to ketoacidosis, lactic acidosis, and other forms of acidosis in which organic anions are increased.
Effects on Cells
Traditionally, hypoxia has been divided into four types. Numerous other classifications have been used, but the fourtype system still has considerable utility if the definitions of the terms are kept clearly in mind. The four categories are (1) hypoxic hypoxia, in which the PO2 of the arterial blood is reduced; (2) anemic hypoxia, in which the arterial PO2 is normal but the amount of hemoglobin available to carry O2 is reduced; (3) stagnant or ischemic hypoxia, in which the blood flow to a tissue is so low that adequate O2 is not delivered to it despite a normal PO2 and hemoglobin concentration; and (4) histotoxic hypoxia, in which the amount of O2 delivered to a tissue is adequate but, because of the action of a toxic agent, the tissue cells cannot make use of the O2 supplied to them. Some specific effects of hypoxia on cells and tissues are discussed in Clinical Box 36–3.
HYPOXIC HYPOXIA By definition, hypoxic hypoxia is a condition of reduced arterial PO2. Hypoxic hypoxia is a problem in normal individuals
Hypoxia causes the production of transcription factors (hypoxia-inducible factors; HIFs). These are made up of α and β subunits. In normally oxygenated tissues, the α subunits are rapidly ubiquitinated and destroyed. However, in hypoxic cells, the α subunits dimerize with β subunits, and the dimers activate genes that produce angiogenic factors and erythropoietin.
Effects on the Brain In hypoxic hypoxia and the other generalized forms of hypoxia, the brain is affected first. A sudden drop in the inspired PO2 to less than 20 mm Hg, which occurs, for example, when cabin pressure is suddenly lost in a plane flying above 16,000 m, causes loss of consciousness in 10 to 20 s and death in 4 to 5 min. Less severe hypoxia causes a variety of mental aberrations not unlike those produced by alcohol: impaired judgment, drowsiness, dulled pain sensibility, excitement, disorientation, loss of time sense, and headache. Other symptoms include anorexia, nausea, vomiting, tachycardia, and, when the hypoxia is severe, hypertension. The rate of ventilation is increased in proportion to the severity of the hypoxia of the carotid chemoreceptor cells.
Respiratory Stimulation Dyspnea is by definition difficult or labored breathing in which the subject is conscious of shortness of breath; hyperpnea is the general term for an increase in the rate or depth of breathing regardless of the patient’s subjective sensations. Tachypnea is rapid, shallow breathing. In general, a normal individual is not conscious of respiration until ventilation is doubled, and breathing is not uncomfortable until ventilation is tripled or quadrupled. Whether or not a given level of ventilation is uncomfortable also appears to depend on a variety of other factors. Hypercapnia and, to a lesser extent, hypoxia cause dyspnea. An additional factor is the effort involved in moving the air in and out of the lungs (the work of breathing).
at high altitudes and is a complication of pneumonia and a variety of other diseases of the respiratory system.
EFFECTS OF DECREASED BAROMETRIC PRESSURE The composition of air stays the same, but the total barometric pressure falls with increasing altitude (Figure 36–12). Therefore, the PO2 also falls. At 3000 m (approximately 10,000 ft) above sea level, the alveolar PO2 is about 60 mm Hg and there is enough hypoxic stimulation of the chemoreceptors
618
SECTION VII Respiratory Physiology
Altitude (m) 0
3000
6000
9000
12,000 15,000
18,000 21,000
760 720 680
Pressure (mm Hg)
640 600 320
Highest permanent human habitations (5500 m) Loss of consciousness if unacclimatized breathing air Top of Mt. Everest (8854 m)
280 240
N2
Alveolar PO2 100 mm Hg (10,400 m) Alveolar PO2 40 mm Hg (13,700 m) Loss of consciousness breathing 100% O2
200 160 120 80 40
O2
Body fluids boil at 37° C
CO2
(19,200 m)
H 2O
0 Breathing air
Breathing 100% O2
Life impossible without pressurization
FIGURE 36–12
Composition of alveolar air in individuals breathing air (0–6100 m) and 100% O 2 (6100–13,700 m). The minimal alveolar PO2 that an unacclimatized subject can tolerate without loss of consciousness is about 35–40 mm Hg. Note that with increasing altitude, the alveolar PCO2 drops because of the hyperventilation due to hypoxic stimulation of the carotid and aortic chemoreceptors. The fall in barometric pressure with increasing altitude is not linear, because air is compressible.
to definitely increase ventilation. As one ascends higher, the alveolar PO2 falls less rapidly and the alveolar PCO2 declines somewhat because of the hyperventilation. The resulting fall in arterial PCO2 produces respiratory alkalosis.
HYPOXIC SYMPTOMS BREATHING AIR A number of compensatory mechanisms operate over a period of time to increase altitude tolerance (acclimatization), but in unacclimatized subjects, mental symptoms such as irritability appear at about 3700 m. At 5500 m, the hypoxic symptoms are severe; and at altitudes above 6100 m (20,000 ft), consciousness is usually lost.
HYPOXIC SYMPTOMS BREATHING OXYGEN The total atmospheric pressure becomes the limiting factor in altitude tolerance when breathing 100% O2. The partial pressure of water vapor in the alveolar air is constant at 47 mm Hg, and that of CO2 is normally 40 mm Hg, so that the lowest barometric pressure at which a normal alveolar PO2 of 100 mm Hg is possible is 187 mm Hg, the pressure at about 10,400 m (34,000 ft). At greater altitudes, the increased ventilation due to the decline in alveolar PO2 lowers the alveolar PCO2 somewhat, but the maximum alveolar PO2
that can be attained when breathing 100% O2 at the ambient barometric pressure of 100 mm Hg at 13,700 m is about 40 mm Hg. At about 14,000 m, consciousness is lost in spite of the administration of 100% O2. At 19,200 m, the barometric pressure is 47 mm Hg, and at or below this pressure the body fluids boil at body temperature. The point is largely academic, however, because any individual exposed to such a low pressure would be dead of hypoxia before the bubbles of steam could cause death. Of course, an artificial atmosphere can be created around an individual; in a pressurized suit or cabin supplied with O2 and a system to remove CO2, it is possible to ascend to any altitude and to live in the vacuum of interplanetary space. Some delayed effects of high altitude are discussed in Clinical Box 36–4.
ACCLIMATIZATION Acclimatization to altitude is due to the operation of a variety of compensatory mechanisms. The respiratory alkalosis produced by the hyperventilation shifts the oxygen–hemoglobin dissociation curve to the left, but a concomitant increase in red blood cell 2,3-BPG tends to decrease the O2 affinity of hemoglobin. The net effect is a small increase in P50. The decrease in O2 affinity makes more O2 available to the tissues. However, the value of the increase in P50 is limited because when the
CHAPTER 36 Gas Transport & pH in the Lung
CLINICAL BOX 36–4
arterial PO2 is markedly reduced, the decreased O2 affinity also interferes with O2 uptake by hemoglobin in the lungs. The initial ventilatory response to increased altitude is relatively small, because the alkalosis tends to counteract the stimulating effect of hypoxia. However, ventilation steadily increases over the next 4 d (Figure 36–13) because the active transport of H+ into cerebrospinal fluid (CSF), or possibly a developing lactic acidosis in the brain, causes a fall in CSF pH that increases the response to hypoxia. After 4 d, the ventila-
40 n
atio
atiz
’a
ays
30
4d
m ccli
xposure
Acute e
• •
When they first arrive at a high altitude, many individuals develop transient “mountain sickness.” This syndrome develops 8 to 24 h after arrival at altitude and lasts 4 to 8 d. It is characterized by headache, irritability, insomnia, breathlessness, and nausea and vomiting. Its cause is unsettled, but it appears to be associated with cerebral edema. The low PO2 at high altitude causes arteriolar dilation, and if cerebral autoregulation does not compensate, there is an increase in capillary pressure that favors increased transudation of fluid into brain tissue. Individuals who do not develop mountain sickness have a diuresis at high altitude, and urine volume is decreased in individuals who develop the condition. High-altitude illness includes not only mountain sickness but also two more serious syndromes that complicate it: high-altitude cerebral edema and high-altitude pulmonary edema. In high-altitude cerebral edema, the capillary leakage in mountain sickness progresses to frank brain swelling, with ataxia, disorientation, and in some cases coma and death due to herniation of the brain through the tentorium. High-altitude pulmonary edema is a patchy edema of the lungs that is related to the marked pulmonary hypertension that develops at high altitude. It has been argued that it occurs because not all pulmonary arteries have enough smooth muscle to constrict in response to hypoxia, and in the capillaries supplied by those arteries, the general rise in pulmonary arterial pressure causes a capillary pressure increase that disrupts their walls (stress failure). All forms of high-altitude illness are benefited by descent to lower altitude and by treatment with the diuretic acetazolamide. This drug inhibits carbonic anhydrase, producing increased HCO3– excretion in the urine, stimulating respiration, increasing PaCO2, and reducing the formation of CSF. When cerebral edema is marked, large doses of glucocorticoids are often administered as well. Their mechanism of action is unsettled. In high-altitude pulmonary edema, prompt treatment with O2 is essential—and, if available, use of a hyperbaric chamber. Portable hyperbaric chambers are now available in a number of mountain areas. Nifedipine, a Ca2+ channel blocker that lowers pulmonary artery pressure, is also useful.
50 VE /VO2 , mL min−1/mL min−1
Delayed Effects of High Altitude
619
20 0
1000
2000
3000
4000
5000 6000
Altitude (m)
FIGURE 36–13
Effect of acclimatization on the ventilatory • • response at various altitudes. VE VO2 is the ventilatory equivalent, • • the ratio of expired minute volume (VE) to the O2 consumption (VO2). (Reproduced with permission from Lenfant C, Sullivan K: Adaptation to high altitude. N Engl J Med 1971;284:1298.)
tory response begins to decline slowly, but it takes years of residence at higher altitudes for it to decline to the initial level. Associated with this decline is a gradual desensitization to the stimulatory effects of hypoxia. Erythropoietin secretion increases promptly on ascent to high altitude and then falls somewhat over the following 4 d as the ventilatory response increases and the arterial PO2 rises. The increase in circulating red blood cells triggered by the erythropoietin begins in 2 to 3 d and is sustained as long as the individual remains at high altitude. Compensatory changes also occur in the tissues. The mitochondria, which are the site of oxidative reactions, increase in number, and myoglobin increases, which facilitates the movement of O2 into the tissues. The tissue content of cytochrome oxidase also increases. The effectiveness of the acclimatization process is indicated by the fact that permanent human habitations exist in the Andes and Himalayas at elevations above 5500 m (18,000 ft). The natives who live in these villages are barrel-chested and markedly polycythemic. They have low alveolar PO2 values, but in most other ways they are remarkably normal.
DISEASES CAUSING HYPOXIC HYPOXIA Hypoxic hypoxia is the most common form of hypoxia seen clinically. The diseases that cause it can be roughly divided into those in which the gas exchange apparatus fails, those such as congenital heart disease in which large amounts of blood are shunted from the venous to the arterial side of the circulation, and those in which the respiratory pump fails. Lung failure occurs when conditions such as pulmonary fibrosis produce alveolar– capillary block, or there is ventilation–perfusion imbalance. Pump failure can be due to fatigue of the respiratory muscles in conditions in which the work of breathing is increased or to a
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variety of mechanical defects such as pneumothorax or bronchial obstruction that limit ventilation. It can also be caused by abnormalities of the neural mechanisms that control ventilation, such as depression of the respiratory neurons in the medulla by morphine and other drugs. Some specific causes of hypoxic hypoxia are discussed in the following text.
VENTILATION–PERFUSION IMBALANCE Patchy ventilation–perfusion imbalance is by far the most common cause of hypoxic hypoxia in clinical situations. In disease processes that prevent ventilation of some of the alveoli, the ventilation–blood flow ratios in different parts of the lung determine the extent to which systemic arterial PO2 declines. If nonventilated alveoli are perfused, the nonventilated but perfused portion of the lung is in effect a right-to-left shunt, dumping unoxygenated blood into the left side of the heart. Lesser degrees of ventilation–perfusion imbalance are more common. In the example illustrated in Figure 36–14, the underventilated alveoli (B) have a low alveolar PO2, whereas the overventilated alveoli (A) have a high alveolar PO2. However, the unsaturation of the hemoglobin of the blood coming from B is not completely compensated by the greater saturation of the blood coming
IDEAL
. VA = 4.0 L
MV = 6.0 L
Uniform ventilation
Uniform blood flow
from A, because hemoglobin is normally nearly saturated in the lungs and the higher alveolar PO2 adds only a little more O2 to the hemoglobin than it normally carries. Consequently, the arterial blood is unsaturated. On the other hand, the CO2 content of the arterial blood is generally normal in such situations, since extra loss of CO2 in overventilated regions can balance diminished loss in underventilated areas.
VENOUS-TO-ARTERIAL SHUNTS When a cardiovascular abnormality such as an interatrial septal defect permits large amounts of unoxygenated venous blood to bypass the pulmonary capillaries and dilute the oxygenated blood in the systemic arteries (“right-to-left shunt”), chronic hypoxic hypoxia and cyanosis (cyanotic congenital heart disease) result. Administration of 100% O2 raises the O2 content of alveolar air and improves the hypoxia due to hypoventilation, impaired diffusion, or ventilation–perfusion imbalance (short of perfusion of totally unventilated segments) by increasing the amount of O2 in the blood leaving the lungs. However, in patients with venous-to-arterial shunts and normal lungs, any beneficial effect of 100% O2 is slight and is due solely to an increase in the amount of dissolved O2 in the blood.
UNCOMPENSATED
. VA = 4.0 L
MV = 6.0 L
Nonuniform ventilation
Uniform blood flow
Mixed venous blood (A + B)
Mixed venous blood (A + B)
A
B
A
B
Arterial blood (A + B) A Alveolar ventilation (L/min) Pulmonary blood flow (L/min) Ventilation/blood flow ratio Mixed venous O2 saturation (%) Arterial O2 saturation (%) Mixed venous O2 tension (mm Hg) Alveolar O2 tension (mm Hg) Arterial O2 tension (mm Hg)
B
2.0 2.0 2.5 2.5 0.8 0.8 75.0 75.0 97.4 97.4 40.0 40.0 104.0 104.0 104.0 104.0
Arterial blood (A + B)
A+B 4.0 5.0 0.8 75.0 97.4 40.0 104.0 104.0
A Alveolar ventilation (L/min) Pulmonary blood flow (L/min) Ventilation/blood flow ratio Mixed venous O2 saturation (%) Arterial O2 saturation (%) Mixed venous O2 tension (mm Hg) Alveolar O2 tension (mm Hg) Arterial O2 tension (mm Hg)
3.2 2.5 1.3 75.0 98.2 40.0 116.0 116.0
FIGURE 36–14
B 0.8 2.5 0.3 75.0 91.7 40.0 66.0 66.0
A+B 4.0 5.0 0.8 75.0 95.0 40.0 106.0 84.0
Comparison of ventilation/blood flow relationships in health and disease. Left: “Ideal” ventilation/blood flow rela• tionship. Right: Nonuniform ventilation and uniform blood flow, uncompensated. VA, alveolar ventilation; MV, respiratory minute volume.
(Reproduced with permission from Comroe JH Jr., et al: The Lung: Clinical Physiology and Pulmonary Function Tests, 2nd ed. Year Book, 1962.)
CHAPTER 36 Gas Transport & pH in the Lung
OTHER FORMS OF HYPOXIA ANEMIC HYPOXIA Hypoxia due to anemia is not severe at rest unless the hemoglobin deficiency is marked, because red blood cell 2,3-BPG increases. However, anemic patients may have considerable difficulty during exercise because of limited ability to increase O2 delivery to the active tissues (Figure 36–15).
CARBON MONOXIDE POISONING
Blood oxygen content (mL/dL)
Small amounts of carbon monoxide (CO) are formed in the body, and this gas may function as a chemical messenger in the brain and elsewhere. In larger amounts, it is poisonous. Outside the body, it is formed by incomplete combustion of carbon. It was used by the Greeks and Romans to execute criminals, and today it causes more deaths than any other gas. CO poisoning has become less common in the United States, since natural gas, which does not contain CO, replaced artificial gases such as coal gas, which contains large amounts. However, the exhaust of gasoline engines is 6% or more CO. CO is toxic because it reacts with hemoglobin to form carbon monoxyhemoglobin (carboxyhemoglobin, COHb), and COHb cannot take up O2 (Figure 36–15). Carbon monoxide poisoning is often listed as a form of anemic hypoxia because the amount of hemoglobin that can carry O2 is reduced, but the total hemoglobin content of the blood is unaffected by CO. The affinity of hemoglobin for CO is 210 times its affinity for O2, and COHb liberates CO very slowly. An additional difficulty is that when COHb is present the dissociation curve of the remaining HbO2 shifts to the left, decreasing the amount of O2 released. This is why an anemic individual who has 50% of the normal amount of HbO2 may be able to perform moderate work,
Oxygen + hemoglobin (14 g/dL)
20
621
whereas an individual whose HbO2 is reduced to the same level because of the formation of COHb is seriously incapacitated. Because of the affinity of CO for hemoglobin, progressive COHb formation occurs when the alveolar PCO is greater than 0.4 mm Hg. However, the amount of COHb formed depends on the duration of exposure to CO as well as the concentration of CO in the inspired air and the alveolar ventilation. CO is also toxic to the cytochromes in the tissues, but the amount of CO required to poison the cytochromes is 1000 times the lethal dose; tissue toxicity thus plays no role in clinical CO poisoning. The symptoms of CO poisoning are those of any type of hypoxia, especially headache and nausea, but there is little stimulation of respiration, since in the arterial blood, PO2 remains normal and the carotid and aortic chemoreceptors are not stimulated. The cherry-red color of COHb is visible in the skin, nail beds, and mucous membranes. Death results when about 70– 80% of the circulating hemoglobin is converted to COHb. The symptoms produced by chronic exposure to sublethal concentrations of CO are those of progressive brain damage, including mental changes and, sometimes, a parkinsonism-like state. Treatment of CO poisoning consists of immediate termination of the exposure and adequate ventilation, by artificial respiration if necessary. Ventilation with O2 is preferable to ventilation with fresh air, since O2 hastens the dissociation of COHb. Hyperbaric oxygenation (see below) is useful in this condition.
HYPOPERFUSION HYPOXIA Hypoperfusion hypoxia, or stagnant hypoxia, is due to slow circulation and is a problem in organs such as the kidneys and heart during shock. The liver and possibly the brain are damaged by hypoperfusion hypoxia in congestive heart failure. The blood flow to the lung is normally very large, and it takes prolonged hypotension to produce significant damage. However, acute respiratory distress syndrome (ARDS) can develop when there is prolonged circulatory collapse.
HISTOTOXIC HYPOXIA
15
Oxygen + hemoglobin (14 g/dL) with 50% carboxyhemoglobin
10 5
Oxygen + hemoglobin (7 g/dL)
0 0
20 40 60 80 100 120 140 160 Oxygen partial pressure (mm Hg)
FIGURE 36–15
Effects of anemia and CO on hemoglobin binding of O2. Normal oxyhemoglobin (14g/dL hemoglobin) dissociation curve compared with anemia (7 g/dL hemoglobin) and with oxyhemoglobin dissociation curves in CO poisoning (50% carboxyhemoglobin). Note that the CO-poisoning curve is shifted to the left of the anemia curve. (Reproduced with permission from Leff AR, Schumacker
PT: Respiratory Physiology: Basics and Applications. Saunders, 1993.)
Hypoxia due to inhibition of tissue oxidative processes is most commonly the result of cyanide poisoning. Cyanide inhibits cytochrome oxidase and possibly other enzymes. Methylene blue or nitrites are used to treat cyanide poisoning. They act by forming methemoglobin, which then reacts with cyanide to form cyanmethemoglobin, a nontoxic compound. The extent of treatment with these compounds is, of course, limited by the amount of methemoglobin that can be safely formed. Hyperbaric oxygenation may also be useful.
OXYGEN TREATMENT OF HYPOXIA Administration of oxygen-rich gas mixtures is of very limited value in hypoperfusion, anemic, and histotoxic hypoxia
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SECTION VII Respiratory Physiology
CLINICAL BOX 36–5 Administration of Oxygen & Its Potential Toxicity It is interesting that while O2 is necessary for life in aerobic organisms, it is also toxic. Indeed, 100% O2 has been demonstrated to exert toxic effects not only in animals but also in bacteria, fungi, cultured animal cells, and plants. The toxicity seems to be due to the production of reactive oxygen species including superoxide anion (O2–) and H2O2. When 80–100% O2 is administered to humans for periods of 8 h or more, the respiratory passages become irritated, causing substernal distress, nasal congestion, sore throat, and coughing. Some infants treated with O2 for respiratory distress syndrome develop a chronic condition characterized by lung cysts and densities (bronchopulmonary dysplasia). This syndrome may be a manifestation of O2 toxicity. Another complication in these infants is retinopathy of prematurity (retrolental fibroplasia), the formation of opaque vascular tissue in the eyes, which can lead to serious visual defects. The retinal receptors mature from the center to the periphery of the retina, and they use considerable O2. This causes the retina to become vascularized in an orderly fashion. Oxygen treatment before maturation is complete provides the needed O2 to the photoreceptors, and consequently the normal vascular pattern fails to develop. Evidence indicates that this condition can be prevented or ameliorated by treatment with vitamin E, which exerts an antioxidant effect, and, in animals, by growth hormone inhibitors. Administration of 100% O2 at increased pressure accelerates the onset of O2 toxicity, with the production not only of tracheobronchial irritation but also of muscle twitching, ringing in the ears, dizziness, convulsions, and coma. The speed
because all that can be accomplished in this way is an increase in the amount of dissolved O2 in the arterial blood. This is also true in hypoxic hypoxia when it is due to shunting of unoxygenated venous blood past the lungs. In other forms of hypoxic hypoxia, O2 is of great benefit. Treatment regimens that deliver less than 100% O2 are of value both acutely and chronically, and administration of O2 24 h/d for 2 y in this fashion has been shown to significantly decrease the mortality of chronic obstructive pulmonary disease. O2 toxicity and therapy are discussed in Clinical Box 36–5.
HYPERCAPNIA & HYPOCAPNIA HYPERCAPNIA Retention of CO2 in the body (hypercapnia) initially stimulates respiration. Retention of larger amounts produces symptoms due to depression of the central nervous system: confusion, diminished sensory acuity, and, eventually, coma with respiratory depression and death. In patients with these
with which these symptoms develop is proportional to the pressure at which the O2 is administered; for example, at 4 atmospheres, symptoms develop in half the subjects in 30 min, whereas at 6 atmospheres, convulsions develop in a few minutes. On the other hand, exposure to 100% O2 at 2 to 3 atmospheres can increase dissolved O2 in arterial blood to the point that arterial O2 tension is greater than 2000 mm Hg and tissue O2 tension is 400 mm Hg. If exposure is limited to 5 h or less at these pressures, O2 toxicity is not a problem. Therefore, hyperbaric O2 therapy in closed tanks is used to treat diseases in which improved oxygenation of tissues cannot be achieved in other ways. It is of demonstrated value in carbon monoxide poisoning, radiation-induced tissue injury, gas gangrene, very severe blood loss anemia, diabetic leg ulcers and other wounds that are slow to heal, and rescue of skin flaps and grafts in which the circulation is marginal. It is also the primary treatment for decompression sickness and air embolism. In hypercapnic patients in severe pulmonary failure, the CO2 level may be so high that it depresses rather than stimulates respiration. Some of these patients keep breathing only because the carotid and aortic chemoreceptors drive the respiratory center. If the hypoxic drive is withdrawn by administering O2, breathing may stop. During the resultant apnea, the arterial PO2 drops but breathing may not start again, as PCO2 further depresses the respiratory center. Therefore, O2 therapy in this situation must be started with care.
symptoms, the PCO2 is markedly elevated, severe respiratory acidosis is present, and the plasma HCO3– may exceed 40 mEq/L. Large amounts of HCO3– are excreted, but more HCO3– is reabsorbed, raising the plasma HCO3– and partially compensating for the acidosis. CO2 is so much more soluble than O2 that hypercapnia is rarely a problem in patients with pulmonary fibrosis. However, it does occur in ventilation–perfusion inequality and when for any reason alveolar ventilation is inadequate in the various forms of pump failure. It is exacerbated when CO2 production is increased. For example, in febrile patients there is a 13% increase in CO2 production for each 1°C rise in temperature, and a high carbohydrate intake increases CO2 production because of the increase in the respiratory quotient. Normally, alveolar ventilation increases and the extra CO2 is expired, but it accumulates when ventilation is compromised.
CHAPTER 36 Gas Transport & pH in the Lung
HYPOCAPNIA Hypocapnia is the result of hyperventilation. During voluntary hyperventilation, the arterial PCO2 falls from 40 to as low as 15 mm Hg while the alveolar PO2 rises to 120 to 140 mm Hg. The more chronic effects of hypocapnia are seen in neurotic patients who chronically hyperventilate. Cerebral blood flow may be reduced 30% or more because of the direct constrictor effect of hypocapnia on the cerebral vessels. The cerebral ischemia causes light-headedness, dizziness, and paresthesias. Hypocapnia also increases cardiac output. It has a direct constrictor effect on many peripheral vessels, but it depresses the vasomotor center, so that the blood pressure is usually unchanged or only slightly elevated. Other consequences of hypocapnia are due to the associated respiratory alkalosis, the blood pH being increased to 7.5 or 7.6. The plasma HCO3– level is low, but HCO3– reabsorption is decreased because of the inhibition of renal acid secretion by the low PCO2. The plasma total calcium level does not change, but the plasma Ca2+ level falls and hypocapnic individuals develop carpopedal spasm, a positive Chvostek sign, and other signs of tetany.
CHAPTER SUMMARY ■
■
■
■
■
■
Partial pressure differences between air and blood for O2 and CO2 dictate a net flow of O2 into the blood and CO2 out of the blood in the pulmonary system. However, this flow is greatly enhanced by the ability for hemoglobin to bind O2 and chemical reactions that increase CO2 in the blood (eg, carbonic anhydrase). The amount of O2 in the blood is determined by the amount dissolved (minor) and the amount bound (major) to hemoglobin. Each hemoglobin molecule contains four subunits that each can bind O2. Binding of the first O2 to hemoglobin increases the affinity for the second O2, and this pattern is continued until four O2 are bound. Hemoglobin O2 binding is also affected by pH, temperature, and the concentration of 2,3-biphospholycerate (2,3-BPG). CO2 in blood is rapidly converted into H2CO3 due to the activity of carbonic anhydrase. CO2 also readily forms carbamino compounds with blood proteins (including hemoglobin). The rapid net loss of CO2 allows more CO2 to dissolve in blood. The pH of plasma is 7.4. A decrease in plasma pH is termed acidosis and an increase of plasma pH is termed alkalosis. Acid and base shifts in the blood are controlled by proteins, including hemoglobin, and principally by the carbonic acidbicarbonate buffering system. The carbonic acid-bicarbonate buffering system is effective because dissolved CO2 can be controlled by respiration. A short-term change in arterial PCO2 due to decreased ventilation results in respiratory acidosis. A short-term change in arterial PCO2 due to increased ventilation results in respiratory alkalosis. Metabolic acidosis occurs when strong acids are added to the blood, and metabolic alkalosis occurs when strong bases are added to (or strong acids are removed from) the blood. Respiratory compensation to acidosis or alkalosis involves quick changes in ventilation. Such changes effectively change the PCO2
■
623
in the blood plasma. Renal compensation mechanisms are much slower and involve H+ secretion or HCO3– reabsorption. Hypoxia is a deficiency of O2 at the tissue level. Hypoxia has powerful consequences at the cellular, tissue, and organ level: It can alter cellular transcription factors and thus protein expression; it can quickly alter brain function and produce symptoms similar to alcohol (eg, dizziness, impaired mental function, drowsiness, headache); and it can affect ventilation. Long-term hypoxia results in cell and tissue death.
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. Most of the CO2 transported in the blood is A) dissolved in plasma. B) in carbamino compounds formed from plasma proteins. C) in carbamino compounds formed from hemoglobin. D) bound to Cl–. E) in HCO3–. 2. Which of the following has the greatest effect on the ability of blood to transport oxygen? A) capacity of the blood to dissolve oxygen B) amount of hemoglobin in the blood C) pH of plasma D) CO2 content of red blood cells E) temperature of the blood 3. Which of the following is not true of the system? →1 H CO →2 H+ + HCO – CO2 + H2O ← 2 3← 3 A) Reaction 1 is catalyzed by carbonic anhydrase. B) Because of reaction 2, the pH of blood declines during breath holding. C) Reaction 1 occurs in the kidneys. D) Reaction 1 occurs primarily in plasma. E) The reactions move to the left when there is excess H+ in the tissues. 4. Uncompensated respiratory acidosis differs from uncompensated metabolic acidosis in that A) plasma pH change is always greater in uncompensated respiratory acidosis compared to uncompensated metabolic acidosis. B) there are no compensation mechanisms for respiratory acidosis, whereas there is respiratory compensation for metabolic acidosis. C) uncompensated respiratory acidosis involves changes in plasma [HCO3–], whereas plasma [HCO3–] is unchanged in uncompensated metabolic acidosis. D) uncompensated respiratory acidosis is associated with a change in PCO2, whereas uncompensated metabolic acidosis occurs along the isobar line for PCO2. 5. O2 delivery to the tissues would be reduced to the greatest extent in A) a normal subject breathing 100% O2 on top of Mt. Everest. B) a normal subject running a marathon at sea level. C) a patient with carbon monoxide poisoning. D) a patient who has ingested cyanide. E) a patient with moderately severe metabolic acidosis.
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CHAPTER RESOURCES Crystal RG, West JB (editors): The Lung: Scientific Foundations, 2nd ed. Raven Press, 1997. Fishman AP, et al (editors): Fishman’s Pulmonary Diseases and Disorders, 4th ed. McGraw-Hill, 2008. Hackett PH, Roach RC: High-altitude illness. N Engl J Med 2001;345:107.
Laffey JG, Kavanagh BP: Hypocapnia. N Engl J Med 2002;347:43. Levitzky, MG: Pulmonary Physiology, 7th ed. McGraw-Hill, 2007. Prisk GK, Paiva M, West JB (editors): Gravity and the Lung: Lessons from Micrography. Marcel Dekker, 2001. Voelkel NF: High-altitude pulmonary edema. N Engl J Med 2002;346:1607. West JB: Pulmonary Pathophysiology, 5th ed. McGraw-Hill, 1995.
37 C
Regulation of Respiration
H A
P
T
E
R
O B JE C TIVE S After studying this chapter, you should be able to: ■ ■ ■
■ ■ ■ ■ ■
Locate the pre-Bötzinger complex and describe its role in producing spontaneous respiration. Identify the location and probable functions of the dorsal and ventral groups of respiratory neurons, the pneumotaxic center, and the apneustic center in the brain stem. List the specific respiratory functions of the vagus nerves and the respiratory receptors in the carotid body, the aortic body, and the ventral surface of the medulla oblongata. Describe and explain the ventilatory responses to increased CO2 concentrations in the inspired air. Describe and explain the ventilatory responses to decreased O2 concentrations in the inspired air. Describe the effects of each of the main non-chemical factors that influence respiration. Describe the effects of exercise on ventilation and O2 exchange in the tissues. Define periodic breathing and explain its occurrence in various disease states.
INTRODUCTION Spontaneous respiration is produced by rhythmic discharge of motor neurons that innervate the respiratory muscles. This discharge is totally dependent on nerve impulses from the brain; breathing stops if the spinal cord is transected above the origin of the phrenic nerves. The rhythmic discharges
from the brain that produce spontaneous respiration are regulated by alterations in arterial PO2, PCO2, and H+ concentration, and this chemical control of breathing is supplemented by a number of non-chemical influences. The physiological bases for these phenomena are discussed in this chapter.
NEURAL CONTROL OF BREATHING
that innervate inspiratory muscles. Those in the cervical cord activate the diaphragm via the phrenic nerves, and those in the thoracic spinal cord activate the external intercostal muscles. However, the impulses also reach the innervation of the internal intercostal muscles and other expiratory muscles. The motor neurons to the expiratory muscles are inhibited when those supplying the inspiratory muscles are active, and vice versa. Although spinal reflexes contribute to this reciprocal innervation, it is due primarily to activity in descending pathways. Impulses in these descending pathways excite agonists and inhibit antagonists. The one exception to
CONTROL SYSTEMS Two separate neural mechanisms regulate respiration. One is responsible for voluntary control and the other for automatic control. The voluntary system is located in the cerebral cortex and sends impulses to the respiratory motor neurons via the corticospinal tracts. The automatic system is driven by a group of pacemaker cells in the medulla. Impulses from these cells activate motor neurons in the cervical and thoracic spinal cord
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SECTION VII Respiratory Physiology
MEDULLARY SYSTEMS The main components of the respiratory control pattern generator responsible for automatic respiration are located in the medulla. Rhythmic respiration is initiated by a small group of synaptically coupled pacemaker cells in the pre-Bötzinger complex (pre-BÖTC) on either side of the medulla between the nucleus ambiguus and the lateral reticular nucleus (Figure 37–1). These neurons discharge rhythmically, and they produce rhythmic discharges in phrenic motor neurons that are abolished by sections between the pre-Bötzinger complex and these motor neurons. They also contact the hypoglossal nuclei, and the tongue is involved in the regulation of airway resistance. Neurons in the pre-Bötzinger complex discharge rhythmically in brain slice preparations in vitro, and if the slices become hypoxic, discharge changes to one associated with gasping. Addition of cadmium to the slices causes occasional sigh-like discharge patterns. There are NK1 receptors and μ-opioid receptors on these neurons, and, in vivo, substance P stimulates and opioids inhibit respiration. Depression of respiration is a side effect that limits the use of opioids in the treatment of pain. However, it is now known that 5HT4 receptors are present in the pre-Bötzinger complex and treatment with 5HT4 agonists blocks the inhibitory effect of opiates on respiration in experimental animals, without inhibiting their analgesic effect. In addition, dorsal and ventral groups of respiratory neurons are present in the medulla (Figure 37–2). However, lesions of these neurons do not abolish respiratory activity, and they apparently project to the pre-Bötzinger pacemaker neurons.
XII 5SP
NA
IO ••
Pre-BOTC
LRN
20 mV
−60 mV 5s
FIGURE 37–1
Pacemaker cells in the pre-Bötzinger complex (pre-BÖTC). Top: Anatomical diagram of the pre-BÖTC from a neonatal rat. Bottom: Sample rhythmic discharge tracing of neurons in the pre-BÖTC complex from a brain slice of a neonatal rat. IO, inferior olive; LRN, lateral reticular nucleus; NA, nucleus ambiguus; XII, nucleus of 12th cranial nerve; 5SP, spinal nucleus of trigeminal nerve. (Modified from Feldman JC, Gray PA: Sighs and gasps in a dish. Nat Neurosci 2000;3:531.)
the reciprocal inhibition is a small amount of activity in phrenic axons for a short period after inspiration. The function of this post-inspiratory output appears to be to brake the lung’s elastic recoil and make respiration smooth. A IC
NPBL
B CP 4th vent
IX VRG
C
X XI XII
DRG D Vagi intact
FIGURE 37–2
Vagi cut
Respiratory neurons in the brain stem. Dorsal view of brain stem; cerebellum removed. The effects of various lesions and brain stem transections are shown; the spirometer tracings at the right indicate the depth and rate of breathing. If a lesion is introduced at D, breathing ceases. The effects of higher transections, with and without vagus nerves transection, are shown (see text for details). DRG, dorsal group of respiratory neurons; VRG, ventral group of respiratory neurons; NPBL, nucleus parabrachialis (pneumotaxic center); 4th vent, fourth ventricle; IC, inferior colliculus; CP, middle cerebellar peduncle. The roman numerals identify cranial nerves. (Modified from Mitchell RA, Berger A: State of the art:
Review of neural regulation of respiration. Am Rev Respir Dis 1975;111:206.)
CHAPTER 37 Regulation of Respiration
PONTINE & VAGAL INFLUENCES
Summed vagal afferent activity
Although the rhythmic discharge of medullary neurons concerned with respiration is spontaneous, it is modified by neurons in the pons and afferents in the vagus from receptors in the airways and lungs. An area known as the pneumotaxic center in the medial parabrachial and Kölliker–Fuse nuclei of the dorsolateral pons contains neurons active during inspiration and neurons active during expiration. When this area is damaged, respiration becomes slower and tidal volume greater, and when the vagi are also cut in anesthetized animals, there are prolonged inspiratory spasms that resemble breath holding (apneusis; section B in Figure 37–2). The normal function of the pneumotaxic center is unknown, but it may play a role in switching between inspiration and expiration. Stretching of the lungs during inspiration initiates impulses in afferent pulmonary vagal fibers. These impulses inhibit inspiratory discharge. This is why the depth of inspiration is increased after vagotomy (Figure 37–2) and apneusis develops if the vagi are cut after damage to the pneumotaxic center. Vagal feedback activity does not alter the rate of rise of the neural activity in respiratory motor neurons (Figure 37–3). When the activity of the inspiratory neurons is increased in intact animals, the rate and the depth of breathing are increased. The depth of respiration is increased because the lungs are stretched to a greater degree before the amount of vagal and pneumotaxic center inhibitory activity is sufficient to overcome the more intense inspiratory neuron discharge. The respiratory rate is increased because the after-discharge in the vagal and possibly the pneumotaxic afferents to the medulla is rapidly overcome.
A
627
REGULATION OF RESPIRATORY ACTIVITY A rise in the PCO2 or H+ concentration of arterial blood or a drop in its PO2 increases the level of respiratory neuron activity in the medulla, and changes in the opposite direction have a slight inhibitory effect. The effects of variations in blood chemistry on ventilation are mediated via respiratory chemoreceptors—the carotid and aortic bodies and collections of cells in the medulla and elsewhere that are sensitive to changes in the chemistry of the blood. They initiate impulses that stimulate the respiratory center. Superimposed on this basic chemical control of respiration, other afferents provide non-chemical controls that affect breathing in particular situations (Table 37–1).
CHEMICAL CONTROL OF BREATHING The chemical regulatory mechanisms adjust ventilation in such a way that the alveolar PCO2 is normally held constant, the effects of excess H+ in the blood are combated, and the PO2 is raised when it falls to a potentially dangerous level. The respiratory minute volume is proportional to the metabolic rate, but the link between metabolism and ventilation is CO2, not O2. The receptors in the carotid and aortic bodies are stimulated by a rise in the PCO2 or H+ concentration of arterial blood or a decline in its PO2. After denervation of the carotid chemoreceptors, the response to a drop in PO2 is abolished; the predominant effect of hypoxia after denervation of the carotid bodies is a direct depression of the respiratory center. The response to changes in arterial blood H+ concentration in the pH 7.3–7.5 range is also abolished, although larger changes exert some effect. The response to changes in arterial PCO2, on the other hand, is affected only slightly; it is reduced no more than 30–35%.
Summed phrenic efferent activity
B
TABLE 37–1 Stimuli affecting the respiratory center.
B
Chemical control
A
CO2 (via CSF and brain interstitial fluid H+ concentration) 0
1
2
O2 H+
} (via carotid and aortic bodies)
Time (s)
FIGURE 37–3
Afferent vagal fibers inhibit inspiratory discharge. Superimposed records of two breaths: (A) with and (B) without feedback vagal afferent activity from stretch receptors in the lungs. Note that the rate of rise in phrenic nerve activity to the diaphragm is unaffected but the discharge is prolonged in the absence of vagal input.
Non-chemical control Vagal afferents from receptors in the airways and lungs Afferents from the pons, hypothalamus, and limbic system Afferents from proprioceptors Afferents from baroreceptors: arterial, atrial, ventricular, pulmonary
628
SECTION VII Respiratory Physiology
Carotid body
Glossopharyngeal afferent axons
Type II cell
Carotid sinus Type I (glomus) cell Common carotid arteries
Aortic bodies
Aortic arch
FIGURE 37–5
FIGURE 37–4 Location of carotid and aortic bodies. Carotid bodies are positioned near a major arterial baroreceptor, the carotid sinus. Two aortic bodies are shown near the aortic arch.
CAROTID & AORTIC BODIES There is a carotid body near the carotid bifurcation on each side, and there are usually two or more aortic bodies near the arch of the aorta (Figure 37–4). Each carotid and aortic body (glomus) contains islands of two types of cells, type I and type II cells, surrounded by fenestrated sinusoidal capillaries. The type I or glomus cells are closely associated with cuplike endings of the afferent nerves (Figure 37–5). The glomus cells resemble adrenal chromaffin cells and have dense-core granules containing catecholamines that are released upon exposure to hypoxia and cyanide. The cells are excited by hypoxia, and the principal transmitter appears to be dopamine, which excites the nerve endings by way of D2 receptors. The type II cells are glia-like, and each surrounds four to six type I cells. Their function is probably sustentacular. Outside the capsule of each body, the nerve fibers acquire a myelin sheath; however, they are only 2 to 5 μm in diameter and conduct at the relatively low rate of 7 to 12 m/s. Afferents from the carotid bodies ascend to the medulla via the carotid sinus and glossopharyngeal nerves, and fibers from the aortic bodies ascend in the vagi. Studies in which one carotid body has been isolated and perfused while recordings are being taken from its
afferent nerve fibers show that there is a graded increase in impulse traffic in these afferent fibers as the PO2 of the perfusing blood is lowered (Figure 37–6) or the PCO2 is raised. Type I glomus cells have O2-sensitive K+ channels, whose conductance is reduced in proportion to the degree of hypoxia to which they are exposed. This reduces the K+ efflux, depolarizing the cell and causing Ca2+ influx, primarily via L-type Ca2+ channels. The Ca2+ influx triggers action potentials and transmitter release, with consequent excitation
8 6 Impulses/s
Heart
Organization of the carotid body. Type I (glomus) cells contain catecholamines. When exposed to hypoxia, they release their catecholamines, which stimulate the cuplike endings of the carotid sinus nerve fibers in the glossopharyngeal nerve. The glia-like type II cells surround the type I cells and probably have a sustentacular function.
4 2
0
100
200
400
600
Arterial PO2 (mm Hg)
FIGURE 37–6 Effect of PCO2 on afferent nerve firing. The rate of discharge of a single afferent fiber from the carotid body is plotted at several PO2 (circles) and fitted to a line. A sharp increase in firing rate is observed as PO2 falls below normal resting levels (ie, near 100 mm Hg). (Courtesy of S Sampson.)
CHAPTER 37 Regulation of Respiration
CHEMORECEPTORS IN THE BRAIN STEM The chemoreceptors that mediate the hyperventilation produced by increases in arterial PCO2 after the carotid and aortic bodies are denervated are located in the medulla oblongata and consequently are called medullary chemoreceptors. They are separate from the dorsal and ventral respiratory neurons and are located on the ventral surface of the medulla (Figure 37–7). Recent evidence indicates that additional chemoreceptors are located in the vicinity of the solitary tract nuclei, the locus ceruleus, and the hypothalamus.
Pons
VI V VII VIII
R
R
C
Pyramid
of the afferent nerve endings. The smooth muscle of pulmonary arteries contains similar O2-sensitive K+ channels, which mediate the vasoconstriction caused by hypoxia. This is in contrast to systemic arteries, which contain adenosine triphosphate (ATP) dependent K+ channels that permit more K+ efflux with hypoxia and consequently cause vasodilation instead of vasoconstriction. The blood flow in each 2-mg carotid body is about 0.04 mL/min, or 2000 mL/100 g of tissue/min compared with a blood flow 54 mL or 420 mL per 100 g/min in the brain and kidneys, respectively. Because the blood flow per unit of tissue is so enormous, the O2 needs of the cells can be met largely by dissolved O2 alone. Therefore, the receptors are not stimulated in conditions such as anemia or carbon monoxide poisoning, in which the amount of dissolved O2 in the blood reaching the receptors is generally normal, even though the combined O2 in the blood is markedly decreased. The receptors are stimulated when the arterial PO2 is low or when, because of vascular stasis, the amount of O2 delivered to the receptors per unit time is decreased. Powerful stimulation is also produced by cyanide, which prevents O2 utilization at the tissue level. In sufficient doses, nicotine and lobeline activate the chemoreceptors. It has also been reported that infusion of K+ increases the discharge rate in chemoreceptor afferents, and because the plasma K+ level is increased during exercise, the increase may contribute to exercise-induced hyperpnea. Because of their anatomic location, the aortic bodies have not been studied in as great detail as the carotid bodies. Their responses are probably similar but of lesser magnitude. In humans in whom both carotid bodies have been removed but the aortic bodies left intact, the responses are essentially the same as those following denervation of both carotid and aortic bodies in animals: little change in ventilation at rest, but the ventilatory response to hypoxia is lost and the ventilatory response to CO2 is reduced by 30%. Neuroepithelial bodies composed of innervated clusters of amine-containing cells are found in the airways. These cells have an outward K+ current that is reduced by hypoxia, and this would be expected to produce depolarization. However, the function of these hypoxia-sensitive cells is uncertain because, as noted above, removal of the carotid bodies alone abolishes the respiratory response to hypoxia.
629
C
IX X XI XII
FIGURE 37–7
Rostral (R) and caudal (C) chemosensitive areas on the ventral surface of the medulla.
The chemoreceptors monitor the H+ concentration of cerebrospinal fluid (CSF), including the brain interstitial fluid. CO2 readily penetrates membranes, including the blood– brain barrier, whereas H+ and HCO3– penetrate slowly. The CO2 that enters the brain and CSF is promptly hydrated. The H2CO3 dissociates, so that the local H+ concentration rises. The H+ concentration in brain interstitial fluid parallels the arterial PCO2. Experimentally produced changes in the PCO2 of CSF have minor, variable effects on respiration as long as the H+ concentration is held constant, but any increase in spinal fluid H+ concentration stimulates respiration. The magnitude of the stimulation is proportional to the rise in H+ concentration. Thus, the effects of CO2 on respiration are mainly due to its movement into the CSF and brain interstitial fluid, where it increases the H+ concentration and stimulates receptors sensitive to H+.
VENTILATORY RESPONSES TO CHANGES IN ACID–BASE BALANCE In metabolic acidosis due, for example, to the accumulation of acid ketone bodies in the circulation in diabetes mellitus, there is pronounced respiratory stimulation (Kussmaul breathing). The hyperventilation decreases alveolar PCO2 (“blows off CO2”) and thus produces a compensatory fall in blood H+ concentration. Conversely, in metabolic alkalosis due, for example, to protracted vomiting with loss of HCl from the body, ventilation is depressed and the arterial PCO2 rises, raising the H+ concentration toward normal. If there is an increase in ventilation that is not secondary to a rise in arterial H+ concentration, the drop in PCO2 lowers the H+ concentration below normal (respiratory alkalosis); conversely, hypoventilation that is not secondary to a fall in plasma H+ concentration causes respiratory acidosis.
630
SECTION VII Respiratory Physiology
VENTILATORY RESPONSES TO CO2 The arterial PCO2 is normally maintained at 40 mm Hg. When arterial PCO2 rises as a result of increased tissue metabolism, ventilation is stimulated and the rate of pulmonary excretion of CO2 increases until the arterial PCO2 falls to normal, shutting off the stimulus. The operation of this feedback mechanism keeps CO2 excretion and production in balance. When a gas mixture containing CO2 is inhaled, the alveolar PCO2 rises, elevating the arterial PCO2 and stimulating ventilation as soon as the blood that contains more CO2 reaches the medulla. CO2 elimination is increased, and the alveolar PCO2 drops toward normal. This is why relatively large increments in the PCO2 of inspired air (eg, 15 mm Hg) produce relatively slight increments in alveolar PCO2 (eg, 3 mm Hg). However, the PCO2 does not drop to normal, and a new equilibrium is reached at which the alveolar PCO2 is slightly elevated and the hyperventilation persists as long as CO2 is inhaled. The essentially linear relationship between respiratory minute volume and the alveolar PCO2 is shown in Figure 37–8. Of course, this linearity has an upper limit. When the PCO2 of the inspired gas is close to the alveolar PCO2, elimination of CO2 becomes difficult. When the CO2 content of the inspired gas is more than 7%, the alveolar and arterial PCO2 begin to rise
abruptly in spite of hyperventilation. The resultant accumulation of CO2 in the body (hypercapnia) depresses the central nervous system, including the respiratory center, and produces headache, confusion, and eventually coma (CO2 narcosis).
VENTILATORY RESPONSE TO OXYGEN LACK When the O2 content of the inspired air is decreased, respiratory minute volume is increased. The stimulation is slight when the PO2 of the inspired air is more than 60 mm Hg, and marked stimulation of respiration occurs only at lower PO2 values (Figure 37–9). However, any decline in arterial PO2 below 100 mm Hg produces increased discharge in the nerves from the carotid and aortic chemoreceptors. There are two reasons why this increase in impulse traffic does not increase ventilation to any extent in normal individuals until the PO2 is less than 60 mm Hg. Because Hb is a weaker acid than HbO2, there is a slight decrease in the H+ concentration of arterial blood when the arterial PO2 falls and hemoglobin becomes less saturated with O2. The fall in H+ concentration tends to inhibit respiration. In addition, any increase in ventilation that does
32
Ventilation (L /min)
40
± 1 SE
20 10
24
0 %O2 in insp gas 21 20 PO2 in insp gas 160 152
20
Pressure (mm Hg)
Respiratory minute volume (L / min)
28
30
16
12
8
10 76
5 38
120 100 80 60 40 20 0
Alveolar PO2 Alveolar PCO2
760 700
600
500
400
300
200
Barometric pressure (mm Hg)
4
38
40
42 44 46 48 Alveolar PCO2 (mm Hg)
50
FIGURE 37–8
Responses of normal subjects to inhaling O2 and approximately 2, 4, and 6% CO2. The relatively linear increase in respiratory minute volume in response to increased CO 2 is due to an increase in both the depth and rate of respiration. (Reproduced with permission from Lambertsen CJ in: Medical Physiology, 13th ed. Mountcastle VB [editor]. Mosby, 1974.)
15 114
FIGURE 37–9
Top: Average respiratory minute volume during the first half hour of exposure to gases containing various amounts of O2. Marked changes in ventilation occur at P O2 values lower than 60 mm Hg. The horizontal line in each case indicates the mean; the vertical bar indicates one standard deviation. Bottom: Alveolar PO2 and PCO2 values when breathing air at various barometric pressures. The two graphs are aligned so that the P O2 of the inspired gas mixtures in the upper graph correspond to the P O2 at the various barometric pressures in the lower graph. (Courtesy of RH Kellogg.)
CHAPTER 37 Regulation of Respiration
60
100
PAO255
40
Ventilation (L/min BTPS)
Ventilation (L/min, BTPS)
50
PACO249
30 PACO244 20 PA CO237
10 0
631
20
40
60
80
100
120
140
75 PAO240
50
25
0
PAO2(mm Hg)
FIGURE 37–10 Ventilation at various alveolar PO2 values when PCO2 is held constant at 49, 44, or 37 mm Hg. Note the dramatic effect on the ventilatory response to P O2 when PCO is increased above normal. (Data from Loeschke HH and Gertz KH.) occur lowers the alveolar PCO2, and this also tends to inhibit respiration. Therefore, the stimulatory effects of hypoxia on ventilation are not clearly manifest until they become strong enough to override the counterbalancing inhibitory effects of a decline in arterial H+ concentration and PCO2. The effects on ventilation of decreasing the alveolar PO2 while holding the alveolar PCO2 constant are shown in Figure 37–10. When the alveolar PCO2 is stabilized at a level 2 to 3 mm Hg above normal, there is an inverse relationship between ventilation and the alveolar PO2 even in the 90 to 110 mm Hg range; but when the alveolar PCO2 is fixed at lower than normal values, there is no stimulation of ventilation by hypoxia until the alveolar PO2 falls below 60 mm Hg.
EFFECTS OF HYPOXIA ON THE CO2 RESPONSE CURVE When the converse experiment is performed—that is, when the alveolar PO2 is held constant while the response to varying amounts of inspired CO2 is tested—a linear response is obtained (Figure 37–11). When the CO2 response is tested at different fixed PO2 values, the slope of the response curve changes, with the slope increased when alveolar PO2 is decreased. In other words, hypoxia makes the individual more sensitive to increases in arterial PCO2. However, the alveolar PCO2 level at which the curves in Figure 37–11 intersect is unaffected. In the normal individual, this threshold value is just below the normal alveolar PCO2, indicating that normally there is a very slight but definite “CO2 drive” of the respiratory area.
PAO2100
40
50 PACO2 (mm Hg)
FIGURE 37–11
Fan of lines showing CO2 response curves at various fixed values of alveolar PO2. Decreased PAO2 results in a more sensitive response to PACO2.
EFFECT OF H+ ON THE CO2 RESPONSE The stimulatory effects of H+ and CO2 on respiration appear to be additive and not, like those of CO2 and O2, complexly interrelated. In metabolic acidosis, the CO2 response curves are similar to those in Figure 37–11, except that they are shifted to the left. In other words, the same amount of respiratory stimulation is produced by lower arterial PCO2 levels. It has been calculated that the CO2 response curve shifts 0.8 mm Hg to the left for each nanomole rise in arterial H+. About 40% of the ventilatory response to CO2 is removed if the increase in arterial H+ produced by CO2 is prevented. As noted above, the remaining 60% is probably due to the effect of CO2 on spinal fluid or brain interstitial fluid H+ concentration.
BREATH HOLDING Respiration can be voluntarily inhibited for some time, but eventually the voluntary control is overridden. The point at which breathing can no longer be voluntarily inhibited is called the breaking point. Breaking is due to the rise in arterial PCO2 and the fall in PO2. Individuals can hold their breath longer after removal of the carotid bodies. Breathing 100% oxygen before breath holding raises alveolar PO2 initially, so that the breaking point is delayed. The same is true of hyperventilating room air, because CO2 is blown off and arterial PCO2 is lower at the start. Reflex or mechanical factors appear to influence the breaking point, since subjects who hold their breath as long as possible and then breathe a gas mixture low in O2 and high in CO2 can hold their breath for an additional 20 s or more. Psychological factors also play a role, and subjects can hold their breath longer when they are told their performance is very good than when they are not.
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SECTION VII Respiratory Physiology
NON-CHEMICAL INFLUENCES ON RESPIRATION RESPONSES MEDIATED BY RECEPTORS IN THE AIRWAYS & LUNGS Receptors in the airways and lungs are innervated by myelinated and unmyelinated vagal fibers. The unmyelinated fibers are C fibers. The receptors innervated by myelinated fibers are commonly divided into slowly adapting receptors and rapidly adapting receptors on the basis of whether sustained stimulation leads to prolonged or transient discharge in their afferent nerve fibers (Table 37–2). The other group of receptors presumably consists of the endings of C fibers, and they are divided into pulmonary and bronchial subgroups on the basis of their location. The shortening of inspiration produced by vagal afferent activity (Figure 37–3) is mediated by slowly adapting receptors, as are the Hering–Breuer reflexes. The Hering–Breuer inflation reflex is an increase in the duration of expiration produced by steady lung inflation, and the Hering–Breuer deflation reflex is a decrease in the duration of expiration produced by marked deflation of the lung. Because the rapidly adapting receptors are stimulated by chemicals such as histamine, they have been called irritant receptors. Activation of rapidly adapting receptors in the trachea causes coughing, bronchoconstriction, and mucus secretion, and activation of rapidly adapting receptors in the lung may produce hyperpnea.
Because the C fiber endings are close to pulmonary vessels, they have been called J (juxtacapillary) receptors. They are stimulated by hyperinflation of the lung, but they respond as well to intravenous or intracardiac administration of chemicals such as capsaicin. The reflex response that is produced is apnea followed by rapid breathing, bradycardia, and hypotension (pulmonary chemoreflex). A similar response is produced by receptors in the heart (Bezold–Jarisch reflex or the coronary chemoreflex). The physiologic role of this reflex is uncertain, but it probably occurs in pathologic states such as pulmonary congestion or embolization, in which it is produced by endogenously released substances.
COUGHING & SNEEZING Coughing begins with a deep inspiration followed by forced expiration against a closed glottis. This increases the intrapleural pressure to 100 mm Hg or more. The glottis is then suddenly opened, producing an explosive outflow of air at velocities up to 965 km (600 mi) per hour. Sneezing is a similar expiratory effort with a continuously open glottis. These reflexes help expel irritants and keep airways clear. Other aspects of innervation are considered in a special case (Clinical Box 37–1).
AFFERENTS FROM PROPRIOCEPTORS Carefully controlled experiments have shown that active and passive movements of joints stimulate respiration, presumably
TABLE 37–2 Airway and lung receptors. Vagal Innervation Myelinated
Type Slowly adapting
Location in Interstitium Among airway smooth muscle cells (?)
Stimulus
Response
Lung inflation
Inspiratory time shortening Hering–Breuer inflation and deflation reflexes Bronchodilation Tachycardia Hyperpnea
Rapidly adapting
Among airway epithelial cells
Lung hyperinflation
Cough
Exogenous and endogenous substances (eg, histamine, prostaglandins)
Bronchoconstriction Mucus secretion
Unmyelinated C fibers
Pulmonary C fibers Bronchial C fibers
Close to blood vessels
Lung hyperinflation
Apnea followed by rapid breathing
Exogenous and endogenous substances (eg, capsaicin, bradykinin, serotonin)
Bronchoconstriction Bradycardia Hypotension Mucus secretion
Modified and reproduced with permission from Berger AJ, Hornbein TF: Control of respiration. In: Textbook of Physiology, 21st ed. Vol. 2. Patton HD, et al (editors). Saunders, 1989.
CHAPTER 37 Regulation of Respiration
CLINICAL BOX 37–1
CLINICAL BOX 37–2
Lung Innervation & Patients with Heart–Lung Transplants
Afferents from “Higher Centers”
Transplantation of the heart and lungs is now an established treatment for severe pulmonary disease and other conditions. In individuals with transplants, the recipient’s right atrium is sutured to the donor heart, and the donor heart does not reinnervate, so the resting heart rate is elevated. The donor trachea is sutured to the recipient’s just above the carina, and afferent fibers from the lungs do not regrow. Consequently, healthy patients with heart–lung transplants provide an opportunity to evaluate the role of lung innervation in normal physiology. Their cough responses to stimulation of the trachea are normal because the trachea remains innervated, but their cough responses to stimulation of the smaller airways are absent. Their bronchi tend to be dilated to a greater degree than normal. In addition, they have the normal number of yawns and sighs, indicating that these do not depend on innervation of the lungs. Finally, they lack Hering– Breuer reflexes, but their pattern of breathing at rest is normal, indicating that these reflexes do not play an important role in the regulation of resting respiration in humans.
because impulses in afferent pathways from proprioceptors in muscles, tendons, and joints stimulate the inspiratory neurons. This effect probably helps increase ventilation during exercise. Other afferents are considered in Clinical Box 37–2.
RESPIRATORY COMPONENTS OF VISCERAL REFLEXES Inhibition of respiration and closure of the glottis during vomiting, swallowing, and sneezing not only prevent the aspiration of food or vomitus into the trachea but, in the case of vomiting, fix the chest so that contraction of the abdominal muscles increases the intra-abdominal pressure. Similar glottic closure and inhibition of respiration occur during voluntary and involuntary straining. Hiccup is a spasmodic contraction of the diaphragm and other inspiratory muscles that produces an inspiration during which the glottis suddenly closes. The glottic closure is responsible for the characteristic sensation and sound. Hiccups occur in the fetus in utero as well as throughout extrauterine life. Their function is unknown. Most attacks of hiccups are usually of short duration, and they often respond to breath holding or other measures that increase arterial PCO2. Intractable hiccups, which can be debilitating, sometimes respond to dopamine antagonists and perhaps to some centrally acting analgesic compounds. Yawning is a peculiar “infectious” respiratory act whose physiologic basis and significance are uncertain. Like hiccup-
633
Pain and emotional stimuli affect respiration, so there must also be afferents from the limbic system and hypothalamus to the respiratory neurons in the brain stem. In addition, even though breathing is not usually a conscious event, both inspiration and expiration are under voluntary control. The pathways for voluntary control pass from the neocortex to the motor neurons innervating the respiratory muscles, bypassing the medullary neurons. Because voluntary and automatic control of respiration are separate, automatic control is sometimes disrupted without loss of voluntary control. The clinical condition that results has been called Ondine’s curse. In German legend, Ondine was a water nymph who had an unfaithful mortal lover. The king of the water nymphs punished the lover by casting a curse on him that took away all his automatic functions. In this state, he could stay alive only by staying awake and remembering to breathe. He eventually fell asleep from sheer exhaustion, and his respiration stopped. Patients with this intriguing condition generally have bulbar poliomyelitis or disease processes that compress the medulla.
ing, it occurs in utero, and it occurs in fish and tortoises as well as mammals. The view that it is needed to increase O2 intake has been discredited. Underventilated alveoli have a tendency to collapse, and it has been suggested that the deep inspiration and stretching them open prevents the development of atelectasis. However, in actual experiments, no atelectasis-preventing effect of yawning could be demonstrated. Yawning increases venous return to the heart, which may benefit the circulation. It has been suggested that yawning is a nonverbal signal used for communication between monkeys in a group, and one could argue that on a different level, the same thing is true in humans.
RESPIRATORY EFFECTS OF BARORECEPTOR STIMULATION Afferent fibers from the baroreceptors in the carotid sinuses, aortic arch, atria, and ventricles relay to the respiratory neurons, as well as the vasomotor and cardioinhibitory neurons in the medulla. Impulses in them inhibit respiration, but the inhibitory effect is slight and of little physiologic importance. The hyperventilation in shock is due to chemoreceptor stimulation caused by acidosis and hypoxia secondary to local stagnation of blood flow, and is not baroreceptor-mediated. The activity of inspiratory neurons affects blood pressure and heart rate, and activity in the vasomotor and cardiac areas in the medulla may have minor effects on respiration.
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SECTION VII Respiratory Physiology
EFFECTS OF SLEEP Partial pressure (mm Hg)
Respiration is less rigorously controlled during sleep than in the waking state, and brief periods of apnea occur in normal sleeping adults. Changes in the ventilatory response to hypoxia vary. If the PCO2 falls during the waking state, various stimuli from proprioceptors and the environment maintain respiration, but during sleep, these stimuli are decreased and a decrease in PCO2 can cause apnea. During rapid eye movement (REM) sleep, breathing is irregular and the CO2 response is highly variable.
160
120 Alveolar PO2 80
40 Alveolar PCO2
RESPIRATORY ABNORMALITIES ASPHYXIA In asphyxia produced by occlusion of the airway, acute hypercapnia and hypoxia develop together. Stimulation of respiration is pronounced, with violent respiratory efforts. Blood pressure and heart rate rise sharply, catecholamine secretion is increased, and blood pH drops. Eventually the respiratory efforts cease, the blood pressure falls, and the heart slows. Asphyxiated animals can still be revived at this point by artificial respiration, although they are prone to ventricular fibrillation, probably because of the combination of hypoxic myocardial damage and high circulating catecholamine levels. If artificial respiration is not started, cardiac arrest occurs in 4 to 5 min.
DROWNING Drowning is asphyxia caused by immersion, usually in water. In about 10% of drownings, the first gasp of water after the losing struggle not to breathe triggers laryngospasm, and death results from asphyxia without any water in the lungs. In the remaining cases, the glottic muscles eventually relax and fluid enters the lungs. Fresh water is rapidly absorbed, diluting the plasma and causing intravascular hemolysis. Ocean water is markedly hypertonic and draws fluid from the vascular system into the lungs, decreasing plasma volume. The immediate goal in the treatment of drowning is, of course, resuscitation, but long-term treatment must also take into account the circulatory effects of the water in the lungs.
PERIODIC BREATHING The acute effects of voluntary hyperventilation demonstrate the interaction of the chemical mechanisms regulating respiration. When a normal individual hyperventilates for 2 to 3 min, then stops and permits respiration to continue without exerting any voluntary control over it, a period of apnea occurs. This is followed by a few shallow breaths and then by another period of apnea, followed again by a few breaths (periodic breathing). The cycles may last for some time before normal breathing is resumed (Figure 37–12). The apnea
Breathing pattern
0
0
1
2
3
4
5
6
Time after stopping hyperventilation (min)
FIGURE 37–12
Changes in breathing and composition of alveolar air after forced hyperventilation for 2 min. Bars in bottom indicate breathing, whereas blank spaces are indicative of apnea.
apparently is due to a lack of CO2 because it does not occur following hyperventilation with gas mixtures containing 5% CO2. During the apnea, the alveolar PO2 falls and the PCO2 rises. Breathing resumes because of hypoxic stimulation of the carotid and aortic chemoreceptors before the CO2 level has returned to normal. A few breaths eliminate the hypoxic stimulus, and breathing stops until the alveolar PO2 falls again. Gradually, however, the PCO2 returns to normal, and normal breathing resumes. Changes in breathing patterns can be symptomatic of disease (Clinical Box 37–3).
EFFECTS OF EXERCISE Exercise provides a physiological example to explore many of the control systems discussed above. Of course, many cardiovascular and respiratory mechanisms must operate in an integrated fashion if the O2 needs of the active tissue are to be met and the extra CO2 and heat removed from the body during exercise. Circulatory changes increase muscle blood flow while maintaining adequate circulation in the rest of the body. In addition, there is an increase in the extraction of O2 from the blood in exercising muscles and an increase in ventilation. This provides extra O2, eliminates some of the heat, and excretes extra CO2. A focus on regulation of ventilation and tissue O2 is presented below, as many other aspects of regulation have been presented in previous chapters.
CHANGES IN VENTILATION During exercise, the amount of O2 entering the blood in the lungs is increased because the amount of O2 added to each
CHAPTER 37 Regulation of Respiration
CLINICAL BOX 37–3
Sleep Apnea Episodes of apnea during sleep can be central in origin; that is, due to failure of discharge in the nerves producing respiration, or they can be due to airway obstruction (obstructive sleep apnea). This can occur at any age and is produced when the pharyngeal muscles relax during sleep. In some cases, failure of the genioglossus muscles to contract during inspiration contributes to the blockage; these muscles pull the tongue forward, and when they do not contract the tongue falls back and obstructs the airway. After several increasingly strong respiratory efforts, the patient wakes up, takes a few normal breaths, and falls back to sleep. Not surprisingly, the apneic episodes are most common during REM sleep, when the muscles are most hypotonic. The symptoms are loud snoring, morning headaches, fatigue, and daytime sleepiness. When severe and prolonged, the condition apparently causes hypertension and its complications. In addition, the incidence of motor vehicle accidents in sleep apnea patients is 7 times greater than it is in the general driving population.
2
9 O2 uptake
6 Blood lactate
1
3
0
Blood lactate (meq/L)
3
Su w bm or a k xim lo ad al s
O2 uptake (L /min)
Periodic breathing occurs in various disease states and is often called Cheyne–Stokes respiration. It is seen most commonly in patients with congestive heart failure and uremia, but it occurs also in patients with brain disease and during sleep in some normal individuals. Some of the patients with Cheyne–Stokes respiration have increased sensitivity to CO2. The increased response is apparently due to disruption of neural pathways that normally inhibit respiration. In these individuals, CO2 causes relative hyperventilation, lowering the arterial PCO2. During the resultant apnea, the arterial PCO2 again rises to normal, but the respiratory mechanism again overresponds to CO2. Breathing ceases, and the cycle repeats. Another cause of periodic breathing in patients with cardiac disease is prolongation of the lung-to-brain circulation time, so that it takes longer for changes in arterial gas tensions to affect the respiratory area in the medulla. When individuals with a slower circulation hyperventilate, they lower the PCO2 of the blood in their lungs, but it takes longer than normal for the blood with a low PCO2 to reach the brain. During this time, the PCO2 in the pulmonary capillary blood continues to be lowered, and when this blood reaches the brain, the low PCO2 inhibits the respiratory area, producing apnea. In other words, the respiratory control system oscillates because the negative feedback loop from lungs to brain is abnormally long.
12
0 Rest
I
II III Work load
IV
V
VI
FIGURE 37–13
Relation between work load, blood lactate level, and O2 uptake. I–VI, increasing work loads produced by increasing the speed and grade of a treadmill on which the subjects worked. (Reproduced with permission from Mitchell JH, Blomqvist G: Maximal oxygen uptake. N Engl J Med 1971;284:1018.)
unit of blood and the pulmonary blood flow per minute are increased. The PO2 of blood flowing into the pulmonary capillaries falls from 40 to 25 mm Hg or less, so that the alveolar– capillary PO2 gradient is increased and more O2 enters the blood. Blood flow per minute is increased from 5.5 L/min to as much as 20 to 35 L/min. The total amount of O2 entering the blood therefore increases from 250 mL/min at rest to values as high as 4000 mL/min. The amount of CO2 removed from each unit of blood is increased, and CO2 excretion increases from 200 mL/min to as much as 8000 mL/min. The increase in O2 uptake is proportional to work load, up to a maximum. Above this maximum, O2 consumption levels off and the blood lactate level continues to rise (Figure 37–13). The lactate comes from muscles in which aerobic resynthesis of energy stores cannot keep pace with their utilization, and an oxygen debt is being incurred. Ventilation increases abruptly with the onset of exercise, which is followed after a brief pause by a further, more gradual increase (Figure 37–14). With moderate exercise, the increase is due mostly to an increase in the depth of respiration; this is accompanied by an increase in the respiratory rate when the exercise is more strenuous. Ventilation abruptly decreases when exercise ceases, which is followed after a brief pause by a
Ventilation (L/min)
Cheyne–Stokes Respiration
Maximal work loads
4
Periodic Breathing in Disease
635
Rest
Exercise
Recovery
Time
FIGURE 37–14
Diagrammatic representation of changes in ventilation during exercise. See text for details.
SECTION VII Respiratory Physiology
PACO
50 2
(mm Hg)
Isocapnic Resp buffering comp
0
150
•
70
VE 120 (L/minBTPS) 3
PAO 2 (mm Hg)
•
VE •
VCO2 (L/minSTPD)
•
2
VCO2
0
•
VO2 (L/minSTPD)
•
VO2 0 −
HCO3
2 min 25
HCO3−
0
7.40 pH
15
pH
7.30 15 30 45 60 75 90 105 120 135 150 165 180
more gradual decline to pre-exercise values. The abrupt increase at the start of exercise is presumably due to psychic stimuli and afferent impulses from proprioceptors in muscles, tendons, and joints. The more gradual increase is presumably humoral, even though arterial pH, PCO2, and PO2 remain constant during moderate exercise. The increase in ventilation is proportional to the increase in O2 consumption, but the mechanisms responsible for the stimulation of respiration are still the subject of much debate. The increase in body temperature may play a role. Exercise increases the plasma K+ level, and this increase may stimulate the peripheral chemoreceptors. In addition, it may be that the sensitivity of the neurons controlling the response to CO2 is increased or that the respiratory fluctuations in arterial PCO2 increase so that, even though the mean arterial PCO2 does not rise, it is CO2 that is responsible for the increase in ventilation. O2 also seems to play some role, despite the lack of a decrease in arterial PO2, since during the performance of a given amount of work, the increase in ventilation while breathing 100% O2 is 10–20% less than the increase while breathing air. Thus, it currently appears that a number of different factors combine to produce the increase in ventilation seen during moderate exercise. When exercise becomes more vigorous, buffering of the increased amounts of lactic acid that are produced liberates more CO2, and this further increases ventilation. The response to graded exercise is shown in Figure 37–15. With increased production of acid, the increases in ventilation and CO2 production remain proportional, so alveolar and arterial CO2 change relatively little (isocapnic buffering). Because of the hyperventilation, alveolar PO2 increases. With further accumulation of lactic acid, the increase in ventilation outstrips CO2 production and alveolar PCO2 falls, as does arterial PCO2. The decline in arterial PCO2 provides respiratory compensation for the metabolic acidosis produced by the additional lactic acid. The additional increase in ventilation produced by the acidosis is dependent on the carotid bodies and does not occur if they are removed. The respiratory rate after exercise does not reach basal levels until the O2 debt is repaid. This may take as long as 90 min. The stimulus to ventilation after exercise is not the arterial PCO2, which is normal or low, or the arterial PO2, which is normal or high, but the elevated arterial H+ concentration due to the lactic acidemia. The magnitude of the O2 debt is the amount by which O2 consumption exceeds basal consumption from the end of exertion until the O2 consumption has returned to pre-exercise basal levels. During repayment of the O2 debt, the O2 concentration in muscle myoglobin rises slightly. ATP and phosphorylcreatine are resynthesized, and lactic acid is removed. Eighty percent of the lactic acid is converted to glycogen and 20% is metabolized to CO2 and H2O. Because of the extra CO2 produced by the buffering of lactic acid during strenuous exercise, the ratio of CO2 to O2 (respiratory exchange ratio; R) rises, reaching 1.5 to 2.0. After exertion, while the O2 debt is being repaid, the R falls to 0.5 or less.
0
636
Work rate (watts)
FIGURE 37–15
Physiologic responses to work rate during exercise. Changes in alveolar PCO2, alveolar PO, ventilation (V˙E), CO2 production (V˙CO2), O2 consumption (V˙O2), arterial HCO3–, and arterial pH with graded increases in work by an adult male on a bicycle ergometer. Resp comp, respiratory compensation. See text for details. (Reproduced with permission from Wasserman K, Whipp BJ, Casaburi R: Respiratory control during exercise. In: Handbook of Physiology. Section 3, The Respiratory System.Vol II, part 2. Fishman AP [editor]. American Physiological Society, 1986.]
CHANGES IN THE TISSUES Maximum O2 uptake during exercise is limited by the maximum rate at which O2 is transported to the mitochondria in the exercising muscle. However, this limitation is not normally due to deficient O2 uptake in the lungs, and hemoglobin in arterial blood is saturated even during the most severe exercise. During exercise, the contracting muscles use more O2, and the tissue PO2 and the PO2 in venous blood from exercising muscle fall nearly to zero. More O2 diffuses from the blood, the blood PO2 of the blood in the muscles drops, and more O2 is removed from hemoglobin. Because the capillary bed of contracting muscle is dilated and many previously closed capillaries are open, the mean distance from the blood to the tissue cells is greatly decreased; this facilitates the movement of O2 from blood to cells. The oxygen–hemoglobin dissociation curve is steep in the PO2 range below 60 mm Hg, and a relatively large amount of O2 is supplied for each drop of 1 mm Hg in PO2 (see Figure 36–2). Additional O2 is supplied because, as a result of the accumulation of CO2 and the rise in temperature in active tissues—and perhaps because of a rise in red blood cell 2,3-biphosphoglycerate (2,3-BPG)—the dissociation curve shifts to the right. The net effect is a threefold
CHAPTER 37 Regulation of Respiration increase in O2 extraction from each unit of blood (see Figure 36–3). Because this increase is accompanied by a 30-fold or greater increase in blood flow, it permits the metabolic rate of muscle to rise as much as 100-fold during exercise.
EXERCISE TOLERANCE & FATIGUE What determines the maximum amount of exercise that can be performed by an individual? Obviously, exercise tolerance has a time as well as an intensity dimension. For example, a fit young man can produce a power output on a bicycle of about 700 watts for 1 min, 300 watts for 5 min, and 200 watts for 40 min. It used to be argued that the limiting factors in exercise performance were the rate at which O2 could be delivered to the tissues or the rate at which O2 could enter the body in the lungs. These factors play a role, but it is clear that other factors also contribute and that exercise stops when the sensation of fatigue progresses to the sensation of exhaustion. Fatigue is produced in part by bombardment of the brain by neural impulses from muscles, and the decline in blood pH produced by lactic acidosis also makes one feel tired, as do the rise in body temperature, dyspnea, and, perhaps, the uncomfortable sensations produced by activation of the J receptors in the lungs.
CHAPTER SUMMARY ■
■
■
■
■
Breathing is under both voluntary control (located in the cerebral cortex) and automatic control (driven by pacemaker cells in the medulla). There is a reciprocal innervation to expiratory and inspiratory muscles in that motor neurons supplying expiratory muscles are inactive when motor neurons supplying inspiratory muscles are active, and vice versa. The pre-Bötzinger complex on either side of the medulla contains synaptically coupled pacemaker cells that allow for rhythmic generation of breathing. The spontaneous activity of these neurons can be altered by neurons in the pneumotaxic center, although the full regulatory function of these neurons on normal breathing is not understood. Breathing patterns are sensitive to chemicals in the blood through activation of respiratory chemoreceptors. There are chemoreceptors in the carotid and aortic bodies and in collections of cells in the medulla. These chemoreceptors respond to changes in PO2 and PCO2 as well as H+ to regulate breathing. Receptors in the airway are additionally innervated by slowly adapting and rapidly adapting myelinated vagal fibers. Slowly adapting receptors can be activated by lung inflation. Rapidly adapting receptors, or irritant receptors, can be activated by chemicals such as histamine and result in cough or even hyperpnea. Receptors in the airway are also innervated by unmyelinated vagal fibers (C fibers) that are typically found next to pulmonary vessels. They are stimulated by hyperinflation (or exogenous substances including capsaicin) and lead to the pulmonary chemoreflex. The physiologic role for this response is not fully understood.
637
MULTIPLE-CHOICE QUESTIONS For all questions, select the single best answer unless otherwise directed. 1. The main respiratory control neurons A) send out regular bursts of impulses to expiratory muscles during quiet respiration. B) are unaffected by stimulation of pain receptors. C) are located in the pons. D) send out regular bursts of impulses to inspiratory muscles during quiet respiration. E) are unaffected by impulses from the cerebral cortex. 2. Intravenous lactic acid increases ventilation. The receptors responsible for this effect are located in the A) medulla oblongata. B) carotid bodies. C) lung parenchyma. D) aortic baroreceptors. E) trachea and large bronchi. 3. Spontaneous respiration ceases after A) transection of the brain stem above the pons. B) transection of the brain stem at the caudal end of the medulla. C) bilateral vagotomy. D) bilateral vagotomy combined with transection of the brain stem at the superior border of the pons. E) transection of the spinal cord at the level of the first thoracic segment. 4. The following physiologic events that occur in vivo are listed in random order: (1) decreased CSF pH; (2) increased arterial PCO2; (3) increased CSF PCO2; (4) stimulation of medullary chemoreceptors; (5) increased alveolar PCO2. What is the usual sequence in which they occur when they affect respiration? A) 1, 2, 3, 4, 5 B) 4, 1, 3, 2, 5 C) 3, 4, 5, 1, 2 D) 5, 2, 3, 1, 4 E) 5, 3, 2, 4, 1 5. The following events that occur in the carotid bodies when they are exposed to hypoxia are listed in random order: (1) depolarization of type I glomus cells; (2) excitation of afferent nerve endings; (3) reduced conductance of hypoxia-sensitive K+ channels in type I glomus cells; (4) Ca2+ entry into type I glomus cells; (5) decreased K+ efflux. What is the usual sequence in which they occur on exposure to hypoxia? A) 1, 3, 4, 5, 2 B) 1, 4, 2, 5, 3 C) 3, 4, 5, 1, 2 D) 3, 1, 4, 5, 2 E) 3, 5, 1, 4, 2 6. Stimulation of the central (proximal) end of a cut vagus nerve would be expected to A) increase heart rate. B) stimulate inspiration. C) inhibit coughing. D) raise blood pressure. E) cause apnea.
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SECTION VII Respiratory Physiology
7. Injection of a drug that stimulates the carotid bodies would be expected to cause A) a decrease in the pH of arterial blood. B) a decrease in the PCO2 of arterial blood. C) an increase in the HCO3– concentration of arterial blood. D) an increase in urinary Na+ excretion. E) an increase in plasma Cl–. 8. Variations in which of the following components of blood or CSF do not affect respiration? A) arterial HCO3– concentration B) arterial H+ concentration C) arterial Na+ concentration D) CSF CO2 concentration E) CSF H+ concentration
CHAPTER RESOURCES Barnes PJ: Chronic obstructive pulmonary disease. N Engl J Med 2000;343:269. Crystal RG, West JB (editors): The Lung: Scientific Foundations, 2nd ed. Lippincott-Raven, 1997.
Fishman AP, et al (editors): Fishman’s Pulmonary Diseases and Disorders, 4th ed. McGraw-Hill, 2008. Hackett PH, Roach RC: High-altitude illness. N Engl J Med 2001;345:107. Jones NL, Killian KJ: Exercise limitation in health and disease. N Engl J Med 2000;343:632. Laffey JG, Kavanagh BP: Hypocapnia. N Engl J Med 2002;347:43. Levitzky, MG: Pulmonary Physiology, 7th ed. McGraw Hill, 2007. Prisk GK, Paiva M, West JB (editors): Gravity and the Lung: Lessons from Micrography. Marcel Dekker, 2001. Putnam RW, Dean JB, Ballantyne D (editors): Central chemosensitivity. Respir Physiol 2001;129:1. Rekling JC, Feldman JL: Pre-Bötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation. Annu Rev Physiol 1998;60:385. Tobin MJ: Advances in mechanical ventilation. N Engl J Med 2001;344:1986. Voelkel NF: High-altitude pulmonary edema. N Engl J Med 2002;346:1607. Ware LB, Matthay MA: The acute respiratory distress syndrome. N Engl J Med 2000;342:1334. West JB: Pulmonary Pathophysiology, 5th ed. McGraw-Hill, 1995.
SECTION VIII RENAL PHYSIOLOGY
38 C
Renal Function & Micturition
H A
P
T
E
R
O B JE C TIVE S After reading this chapter, you should be able to: ■ ■ ■ ■ ■ ■ ■ ■
Describe the morphology of a typical nephron and its blood supply. Define autoregulation and list the major theories advanced to explain autoregulation in the kidneys. Define glomerular filtration rate, describe how it can be measured, and list the major factors affecting it. Outline tubular handling of Na+ and water. Discuss tubular reabsorption and secretion of glucose and K+. Describe how the countercurrent mechanism in the kidney operates to produce hypertonic or hypotonic urine. List the major classes of diuretics and how each operates to increase urine flow. Describe the voiding reflex and draw a cystometrogram.
INTRODUCTION In the kidneys, a fluid that resembles plasma is filtered through the glomerular capillaries into the renal tubules (glomerular filtration). As this glomerular filtrate passes down the tubules, its volume is reduced and its composition altered by the processes of tubular reabsorption (removal of water and solutes from the tubular fluid) and tubular secretion (secretion of solutes into the tubular fluid) to form the urine that enters the renal pelvis. A comparison of the composition of the plasma and an average urine specimen illustrates the magnitude of some of these changes (Table 38–1). It emphasizes the manner by which water and important electrolytes
and metabolites are conserved while wastes are eliminated in the urine. Furthermore, the composition of the urine can be varied to maintain whole body fluid homeostasis (extracellular fluid [ECF]). This is achieved via many homeostatic regulatory mechanisms that function to change the amount of water and solutes in the urine. From the renal pelvis, the urine passes to the bladder and is expelled to the exterior by the process of urination, or micturition. The kidneys are also endocrine organs, making kinins (see Chapter 33) and 1, 25dihydroxycholecalciferol (see Chapter 23), and making and secreting renin (see Chapter 39). 639
640
SECTION VIII Renal Physiology
TABLE 38–1 Typical urinary and plasma concentrations of some physiologically important substances. Concentration in Substance
Urine (U)
Plasma (P)
U/P Ratio
0
100
0
Na+ (mEq/L)
90
140
0.6
Urea (mg/dL)
900
15
60
Creatinine (mg/dL)
150
1
150
Glucose (mg/dL)
FUNCTIONAL ANATOMY THE NEPHRON Each individual renal tubule and its glomerulus is a unit (nephron). The size of the kidneys between species varies, as does the number of nephrons they contain. Each human kidney has approximately 1.3 million nephrons. The specific structures of the nephron are shown in diagrammatic fashion in Figure 38–1.
Distal convoluted tubule Proximal convoluted tubule
Collecting duct
Glomerulus
Cortex
Outer medulla
Loop of Henle, thick ascending limb Inner medulla
Loop of Henle, thin descending limb
FIGURE 38–1 Diagram of a juxtamedullary nephron. The main histologic features of the cells that make up each portion of the tubule are also shown.
The glomerulus, which is about 200 μm in diameter, is formed by the invagination of a tuft of capillaries into the dilated, blind end of the nephron (Bowman’s capsule). The capillaries are supplied by an afferent arteriole and drained by a slightly smaller efferent arteriole (Figure 38–2), and it is from the glomerulus that the filtrate is formed. Two cellular layers separate the blood from the glomerular filtrate in Bowman’s capsule: the capillary endothelium and the specialized epithelium of the capsule. The endothelium of the glomerular capillaries is fenestrated, with pores that are 70 to 90 nm in diameter. The endothelium of the glomerular capillaries is completely surrounded by the glomerular basement membrane along with specialized cells called podocytes. Podocytes have numerous pseudopodia that interdigitate (Figure 38–2) to form filtration slits along the capillary wall. The slits are approximately 25 nm wide, and each is closed by a thin membrane. The glomerular basement membrane, the basal lamina, does not contain visible gaps or pores. Stellate cells called mesangial cells are located between the basal lamina and the endothelium. They are similar to cells called pericytes, which are found in the walls of capillaries elsewhere in the body. Mesangial cells are especially common between two neighboring capillaries, and in these locations the basal membrane forms a sheath shared by both capillaries (Figure 38–2). The mesangial cells are contractile and play a role in the regulation of glomerular filtration. Mesangial cells secrete the extracellular matrix, take up immune complexes, and are involved in the progression of glomerular disease. Functionally, the glomerular membrane permits the free passage of neutral substances up to 4 nm in diameter and almost totally excludes those with diameters greater than 8 nm. However, the charges on molecules as well as their diameters affect their passage into Bowman’s capsule. The total area of glomerular capillary endothelium across which filtration occurs in humans is about 0.8 m2. The general features of the cells that make up the walls of the tubules are shown in Figure 38–1; however, there are cell subtypes in all segments, and the anatomic differences between them correlate with differences in function. The human proximal convoluted tubule is about 15 mm long and 55 μm in diameter. Its wall is made up of a single layer of cells that interdigitate with one another and are united by apical tight junctions. Between the bases of the cells are extensions of the extracellular space called the lateral intercellular spaces. The luminal edges of the cells have a striate brush border due to the presence of many microvilli. The convoluted proximal tubule straightens and the next portion of each nephron is the loop of Henle. The descending portion of the loop and the proximal portion of the ascending limb are made up of thin, permeable cells. On the other hand, the thick portion of the ascending limb (Figure 38–1) is made up of thick cells containing many mitochondria. The nephrons with glomeruli in the outer portions of the renal cortex have short loops of Henle (cortical nephrons), whereas those with glomeruli in the juxtamedullary region of the cortex (juxtamedullary nephrons) have long loops extending down
CHAPTER 38 Renal Function & Micturition
641
Podocyte
B Proximal tubule Capsule Red blood cells
A
Glomerular basal lamina
Mesangial cell
Capillary Capillary
Bowman's space
Juxtaglomerular cells Podocyte process
Podocyte processes
Nerve fibers Capillary
Capillary Efferent arteriole
Basal lamina
Afferent arteriole Mesangial cell
Smooth muscle Distal tubule Macula densa
Basal lamina
C
Endothelium
Cytoplasm of endothelial cell
D
Basal lamina Endothelium
Foot processes of podocytes
Podocyte
Filtration slit
Fenestrations
Bowman's space
Capillary lumen Basal lamina
FIGURE 38–2 Structural details of glomerulus. A) Section through vascular pole, showing capillary loops. B) Relation of mesangial cells and podocytes to glomerular capillaries. C) Detail of the way podocytes form filtration slits on the basal lamina, and the relation of the lamina to the capillary endothelium. D) Enlargement of the rectangle in C to show the podocyte processes. The fuzzy material on their surfaces is glomerular polyanion. into the medullary pyramids. In humans, only 15% of the nephrons have long loops. The thick end of the ascending limb of the loop of Henle reaches the glomerulus of the nephron from which the tubule arose and nestles between its afferent and efferent arterioles. Specialized cells at the end form the macula densa, which is close to the efferent and particularly the afferent arteriole (Figure 38–2). The macula, the neighboring lacis cells, and the renin-secreting juxtaglomerular cells in the afferent arteriole form the juxtaglomerular apparatus (see Figure 39–9). The distal convoluted tubule, which starts at the macula densa, is about 5 mm long. Its epithelium is lower than that of
the proximal tubule, and although a few microvilli are present, there is no distinct brush border. The distal tubules coalesce to form collecting ducts that are about 20 mm long and pass through the renal cortex and medulla to empty into the pelvis of the kidney at the apexes of the medullary pyramids. The epithelium of the collecting ducts is made up of principal cells (P cells) and intercalated cells (I cells). The P cells, which predominate, are relatively tall and have few organelles. They are involved in Na+ reabsorption and vasopressin-stimulated water reabsorption. The I cells, which are present in smaller numbers and are also found in the distal tubules, have more microvilli, cytoplasmic vesicles, and
642
SECTION VIII Renal Physiology
mitochondria. They are concerned with acid secretion and HCO3– transport. The total length of the nephrons, including the collecting ducts, ranges from 45 to 65 mm. Cells in the kidneys that appear to have a secretory function include not only the juxtaglomerular cells but also some of the cells in the interstitial tissue of the medulla. These cells are called type I medullary interstitial cells. They contain lipid droplets and probably secrete prostaglandins, predominantly PGE2. PGE2 is also secreted by the cells in the collecting ducts; prostacyclin (PGI2) and other prostaglandins are secreted by the arterioles and glomeruli.
BLOOD VESSELS The renal circulation is diagrammed in Figure 38–3. The afferent arterioles are short, straight branches of the interlobu-
Efferent arteriole Afferent arteriole Interlobular artery Juxtamedullary glomerulus
lar arteries. Each divides into multiple capillary branches to form the tuft of vessels in the glomerulus. The capillaries coalesce to form the efferent arteriole, which in turn breaks up into capillaries that supply the tubules (peritubular capillaries) before draining into the interlobular veins. The arterial segments between glomeruli and tubules are thus technically a portal system, and the glomerular capillaries are the only capillaries in the body that drain into arterioles. However, there is relatively little smooth muscle in the efferent arterioles. The capillaries draining the tubules of the cortical nephrons form a peritubular network, whereas the efferent arterioles from the juxtamedullary glomeruli drain not only into a peritubular network, but also into vessels that form hairpin loops (the vasa recta). These loops dip into the medullary pyramids alongside the loops of Henle (Figure 38–3). The descending vasa recta have a nonfenestrated endothelium that contains a
Renal cortex Superficial glomeruli Interlobular vein Peritubular capillary bed Arcuate Arcuate vein artery
Loop of Henle Ascending vasa recta Descending vasa recta
Interlobar vein Interlobar artery
Renal medulla (pyramid)
FIGURE 38–3 Renal circulation. Interlobar arteries divide into arcuate arteries, which give off interlobular arteries in the cortex. The interlobular arteries provide an afferent arteriole to each glomerulus. The efferent arteriole from each glomerulus breaks up into capillaries that supply blood to the renal tubules. Venous blood enters interlobular veins, which in turn flow via arcuate veins to the interlobar veins. (Modified from Boron WF, Boulpaep EL: Medical Physiology. Saunders, 2003.)
CHAPTER 38 Renal Function & Micturition facilitated transporter for urea, and the ascending vasa recta have a fenestrated endothelium, consistent with their function in conserving solutes. The efferent arteriole from each glomerulus breaks up into capillaries that supply a number of different nephrons. Thus, the tubule of each nephron does not necessarily receive blood solely from the efferent arteriole of the same nephron. In humans, the total surface of the renal capillaries is approximately equal to the total surface area of the tubules, both being about 12 m2. The volume of blood in the renal capillaries at any given time is 30 to 40 mL.
LYMPHATICS The kidneys have an abundant lymphatic supply that drains via the thoracic duct into the venous circulation in the thorax.
CAPSULE The renal capsule is thin but tough. If the kidney becomes edematous, the capsule limits the swelling, and the tissue pressure (renal interstitial pressure) rises. This decreases the glomerular filtration rate and is claimed to enhance and prolong anuria in acute renal failure.
INNERVATION OF THE RENAL VESSELS The renal nerves travel along the renal blood vessels as they enter the kidney. They contain many postganglionic sympathetic efferent fibers and a few afferent fibers. There also appears to be a cholinergic innervation via the vagus nerve, but its function is uncertain. The sympathetic preganglionic innervation comes primarily from the lower thoracic and upper lumbar segments of the spinal cord, and the cell bodies of the postganglionic neurons are in the sympathetic ganglion chain, in the superior mesenteric ganglion, and along the renal artery. The sympathetic fibers are distributed primarily to the afferent and efferent arterioles, the proximal and distal tubules, and the juxtaglomerular cells (see Chapter 39). In addition, there is a dense noradrenergic innervation of the thick ascending limb of the loop of Henle. Nociceptive afferents that mediate pain in kidney disease parallel the sympathetic efferents and enter the spinal cord in the thoracic and upper lumbar dorsal roots. Other renal afferents presumably mediate a renorenal reflex by which an increase in ureteral pressure in one kidney leads to a decrease in efferent nerve activity to the contralateral kidney, and this decrease permits an increase in its excretion of Na+ and water.
RENAL CIRCULATION BLOOD FLOW In a resting adult, the kidneys receive 1.2 to 1.3 L of blood per minute, or just under 25% of the cardiac output. Renal blood
643
flow can be measured with electromagnetic or other types of flow meters, or it can be determined by applying the Fick principle (see Chapter 33) to the kidney; that is, by measuring the amount of a given substance taken up per unit of time and dividing this value by the arteriovenous difference for the substance across the kidney. Because the kidney filters plasma, the renal plasma flow equals the amount of a substance excreted per unit of time divided by the renal arteriovenous difference as long as the amount in the red cells is unaltered during passage through the kidney. Any excreted substance can be used if its concentration in arterial and renal venous plasma can be measured and if it is not metabolized, stored, or produced by the kidney and does not itself affect blood flow. Renal plasma flow can be measured by infusing p-aminohippuric acid (PAH) and determining its urine and plasma concentrations. PAH is filtered by the glomeruli and secreted by the tubular cells, so that its extraction ratio (arterial concentration minus renal venous concentration divided by arterial concentration) is high. For example, when PAH is infused at low doses, 90% of the PAH in arterial blood is removed in a single circulation through the kidney. It has therefore become commonplace to calculate the “renal plasma flow” by dividing the amount of PAH in the urine by the plasma PAH level, ignoring the level in renal venous blood. Peripheral venous plasma can be used because its PAH concentration is essentially identical to that in the arterial plasma reaching the kidney. The value obtained should be called the effective renal plasma flow (ERPF) to indicate that the level in renal venous plasma was not measured. In humans, ERPF averages about 625 mL/min. ˙ U PAH V ERPF = ------------------ = Clearance of PAH ( C PAH ) P PAH Example: Concentration of PAH in urine (UPAH): 14 mg/mL • Urine flow (V): 0.9 mL/min Concentration of PAH in plasma (PPAH): 0.02 mg/mL × 0.9 ERPF = 14 ------------------0.02 = 630 mL/min It should be noted that the ERPF determined in this way is the clearance of PAH. The concept of clearance is discussed in detail below. ERPF can be converted to actual renal plasma flow (RPF): Average PAH extraction ratio: 0.9 ERP 630 ----------------------------------------- = -------- = Actual RPF = 700 mL/min Extraction ration 0.9
644
SECTION VIII Renal Physiology
From the renal plasma flow, the renal blood flow can be calculated by dividing by 1 minus the hematocrit:
TABLE 38–2 Renal responses to graded renal nerve stimulation.
Hematocrit (Hct): 45% Renal blood flow = RPF × = 700 ×
Renal Nerve Stimulation Frequency (Hz)
1 1–Hct 1 0.55
The pressure in the glomerular capillaries has been measured directly in rats and has been found to be considerably lower than predicted on the basis of indirect measurements. When the mean systemic arterial pressure is 100 mm Hg, the glomerular capillary pressure is about 45 mm Hg. The pressure drop across the glomerulus is only 1 to 3 mm Hg, but a further drop occurs in the efferent arteriole so that the pressure in the peritubular capillaries is about 8 mm Hg. The pressure in the renal vein is about 4 mm Hg. Pressure gradients are similar in squirrel monkeys and presumably in humans, with a glomerular capillary pressure that is about 40% of systemic arterial pressure.
REGULATION OF THE RENAL BLOOD FLOW Norepinephrine (noradrenaline) constricts the renal vessels, with the greatest effect of injected norepinephrine being exerted on the interlobular arteries and the afferent arterioles. Dopamine is made in the kidney and causes renal vasodilation and natriuresis. Angiotensin II exerts a constrictor effect on both the afferent and efferent arterioles. Prostaglandins increase blood flow in the renal cortex and decrease blood flow in the renal medulla. Acetylcholine also produces renal vasodilation. A high-protein diet raises glomerular capillary pressure and increases renal blood flow.
FUNCTIONS OF THE RENAL NERVES Stimulation of the renal nerves increases renin secretion by a direct action of released norepinephrine on β1-adrenergic receptors on the juxtaglomerular cells (see Chapter 39) and it increases Na+ reabsorption, probably by a direct action of norepinephrine on renal tubular cells. The proximal and distal tubules and the thick ascending limb of the loop of Henle are richly innervated. When the renal nerves are stimulated to increasing extents in experimental animals, the first response is an increase in the sensitivity of the juxtaglomerular cells (Table 38–2), followed by increased renin secretion, then increased Na+ reabsorption, and finally, at the highest threshold, renal vasoconstriction with decreased glomerular filtration and renal blood flow. It is still unsettled whether the effect on
a
UNAV
GFR
RBFa
0.25
No effect on basal values; augments RSR mediated by nonneural stimuli.
0
0
0
0.50
Increased without changing UNAV, GFR, or RBF.
0
0
0
1.0
Increased with decreased without changing GFR or RBF.
↓
0
0
2.50
Increased with decreased UNAV, GFR, and RBF.
↓
↓
↓
= 1273 mL/min
PRESSURE IN RENAL VESSELS
RSRa
RSR, renin secretion rate; , urinary sodium excretion; RBF, renal blood flow.
Reproduced from DiBona GF: Neural control of renal function: Cardiovascular implications. Hypertension 1989;13:539. By permission of the American Heart Association.
Na+ reabsorption is mediated via α- or β-adrenergic receptors, and it may be mediated by both. The physiologic role of the renal nerves in Na+ metabolism is also unsettled, in part because most renal functions appear to be normal in patients with transplanted kidneys, and it takes some time for transplanted kidneys to acquire a functional innervation. Strong stimulation of the sympathetic noradrenergic nerves to the kidneys causes a marked decrease in renal blood flow. This effect is mediated by α1-adrenergic receptors and to a lesser extent by postsynaptic α2-adrenergic receptors. Some tonic discharge takes place in the renal nerves at rest in animals and humans. When systemic blood pressure falls, the vasoconstrictor response produced by decreased discharge in the baroreceptor nerves includes renal vasoconstriction. Renal blood flow is decreased during exercise and, to a lesser extent, on rising from the supine position.
AUTOREGULATION OF RENAL BLOOD FLOW When the kidney is perfused at moderate pressures (90–220 mm Hg in the dog), the renal vascular resistance varies with the pressure so that renal blood flow is relatively constant (Figure 38–4). Autoregulation of this type occurs in other organs, and several factors contribute to it (see Chapter 33). Renal autoregulation is present in denervated and in isolated, perfused kidneys, but is prevented by the administration of drugs that paralyze vascular smooth muscle. It is probably produced in part by a direct contractile response to stretch of the smooth muscle of the afferent arteriole. NO may also be involved. At low perfusion pressures, angiotensin II also appears to play a role by constricting the efferent arterioles, thus
CHAPTER 38 Renal Function & Micturition
800 Renal blood flow
mL/min
600
400
200
0
Glomerular filtration
70
140
210
Arterial pressure (mm Hg)
FIGURE 38–4
Autoregulation in the kidneys.
maintaining the glomerular filtration rate. This is believed to be the explanation of the renal failure that sometimes develops in patients with poor renal perfusion who are treated with drugs that inhibit angiotensin-converting enzyme.
REGIONAL BLOOD FLOW & OXYGEN CONSUMPTION The main function of the renal cortex is filtration of large volumes of blood through the glomeruli, so it is not surprising that the renal cortical blood flow is relatively great and little oxygen is extracted from the blood. Cortical blood flow is about 5 mL/g of kidney tissue/min (compared with 0.5 mL/g/min in the brain), and the arteriovenous oxygen difference for the whole kidney is only 14 mL/L of blood, compared with 62 mL/L for the brain and 114 mL/L for the heart (see Table 34–1). The PO2 of the cortex is about 50 mm Hg. On the other hand, maintenance of the osmotic gradient in the medulla requires a relatively low blood flow. It is not surprising, therefore, that the blood flow is about 2.5 mL/g/min in the outer medulla and 0.6 mL/g/min in the inner medulla. However, metabolic work is being done, particularly to reabsorb Na+ in the thick ascending limb of Henle, so relatively large amounts of O2 are extracted from the blood in the medulla. The PO2 of the medulla is about 15 mm Hg. This makes the medulla vulnerable to hypoxia if flow is reduced further. NO, prostaglandins, and many cardiovascular peptides in this region function in a paracrine fashion to maintain the balance between low blood flow and metabolic needs.
GLOMERULAR FILTRATION MEASURING GFR The glomerular filtration rate (GFR) can be measured in intact experimental animals and humans by measuring the excretion and plasma level of a substance that is freely filtered through the glomeruli and neither secreted nor reabsorbed by
645
the tubules. The amount of such a substance in the urine per unit of time must have been provided by filtering exactly the number of milliliters of plasma that contained this amount. Therefore, if the substance is designated by the letter X, the GFR is equal to the concentration of X in urine (UX) times the • urine flow per unit of time (V ) divided by the arterial plasma • level of X (PX), or UXV/PX. This value is called the clearance of X (CX). PX is, of course, the same in all parts of the arterial circulation, and if X is not metabolized to any extent in the tissues, the level of X in peripheral venous plasma can be substituted for the arterial plasma level.
SUBSTANCES USED TO MEASURE GFR In addition to the requirement that it be freely filtered and neither reabsorbed nor secreted in the tubules, a substance suitable for measuring the GFR should be nontoxic and not metabolized by the body. Inulin, a polymer of fructose with a molecular weight of 5200 that is found in Jerusalem artichokes (Helianthus tuberosus), meets these criteria in humans and most animals and is extensively used to measure GFR. In practice, a loading dose of inulin is administered intravenously, followed by a sustaining infusion to keep the arterial plasma level constant. After the inulin has equilibrated with body fluids, an accurately timed urine specimen is collected and a plasma sample obtained halfway through the collection. Plasma and urinary inulin concentrations are determined and the clearance calculated: UIN = 35 mg/mL • V = 0.9 mL/min PIN = 0.25 mg/mL • U V 35 × 0.9 CIN = IN = PIN 0.25 CIN = 126 mL/min In dogs, cats, rabbits, and a number of other mammalian species, clearance of creatinine (CCr) can also be used to determine the precise GFR, but in primates, including humans, some creatinine is secreted by the tubules and some may be reabsorbed. In addition, plasma creatinine determinations are inaccurate at low creatinine levels because the method for determining creatinine measures small amounts of other plasma constituents. In spite of this, the clearance of endogenous creatinine is frequently measured in patients. The values agree quite well with the GFR val-• ues measured with inulin because, although the value for UCrV is high as a result of tubular secretion, the value for PCr is also high as a result of nonspecific chromogens, and the errors thus tend to cancel. Endogenous creatinine clearance is easy to measure and is a worthwhile index of renal function, but when precise measurements of GFR are needed it seems unwise to rely on a method that owes what accuracy it has to compensating errors.
646
SECTION VIII Renal Physiology
NORMAL GFR
1.0 0.9
CONTROL OF GFR The factors governing filtration across the glomerular capillaries are the same as those governing filtration across all other capillaries (see Chapter 32), that is, the size of the capillary bed, the permeability of the capillaries, and the hydrostatic and osmotic pressure gradients across the capillary wall. For each nephron: GFR = Kf [(PGC – PT) – (πGC – πT)] Kf, the glomerular ultrafiltration coefficient, is the product of the glomerular capillary wall hydraulic conductivity (ie, its permeability) and the effective filtration surface area. PGC is the mean hydrostatic pressure in the glomerular capillaries, PT the mean hydrostatic pressure in the tubule (Bowman’s space), πGC the oncotic pressure of the plasma in the glomerular capillaries, and πT the oncotic pressure of the filtrate in the tubule (Bowman’s space).
PERMEABILITY The permeability of the glomerular capillaries is about 50 times that of the capillaries in skeletal muscle. Neutral substances with effective molecular diameters of less than 4 nm are freely filtered, and the filtration of neutral substances with diameters of more than 8 nm approaches zero (Figure 38–5). Between these values, filtration is inversely proportionate to diameter. However, sialoproteins in the glomerular capillary wall are negatively charged, and studies with anionically charged and cationically charged dextrans indicate that the negative charges repel negatively charged substances in blood, with the result that filtration of anionic substances 4 nm in diameter is less than half that of neutral substances of the same size. This probably explains why albumin, with an effective molecular diameter of approximately 7 nm, normally has a glomerular concentration only 0.2% of its plasma concentration rather than the higher concentration that would be expected on the basis of diameter alone; circulating albumin is negatively charged. Filtration of cationic substances is greater than that of neutral substances. The amount of protein in the urine is normally less than 100 mg/d, and most of this is not filtered but comes from shed tubular cells. The presence of significant amounts of albumin in the urine is called albuminuria. In nephritis, the negative charges in the glomerular wall are dissipated, and albumin-
Fractional clearance
The GFR in a healthy person of average size is approximately 125 mL/min. Its magnitude correlates fairly well with surface area, but values in women are 10% lower than those in men even after correction for surface area. A rate of 125 mL/min is 7.5 L/h, or 180 L/d, whereas the normal urine volume is about 1 L/d. Thus, 99% or more of the filtrate is normally reabsorbed. At the rate of 125 mL/min, in 1 day the kidneys filter an amount of fluid equal to 4 times the total body water, 15 times the ECF volume, and 60 times the plasma volume.
0.8 0.7 0.6 0.5
Cationic
0.4
Neutral
0.3
Anionic
0.2 0.1 0
6.0
4.0
8.0
Effective molecular diameter (nm)
FIGURE 38–5
Effect of electric charge on the fractional clearance of dextran molecules of various sizes in rats. The negative charges in the glomerular membrane retard the passage of negatively charged molecules (anionic dextran) and facilitate the passage of positively charged molecules (cationic dextran). (Reproduced with permission from Brenner BM, Beeuwkes R: The renal circulations. Hosp Pract [July] 1978;13:35.)
uria can occur for this reason without an increase in the size of the “pores” in the membrane.
SIZE OF THE CAPILLARY BED Kf can be altered by the mesangial cells, with contraction of these cells producing a decrease in Kf that is largely due to a reduction in the area available for filtration. Contraction of points where the capillary loops bifurcate probably shifts flow away from some of the loops, and elsewhere, contracted mesangial cells distort and encroach on the capillary lumen. Agents that have been shown to affect the mesangial cells are listed in Table 38–3. Angiotensin II is an important regulator of mesangial contraction, and there are angiotensin II receptors in the glomeruli. In addition, some evidence suggests that mesangial cells make renin.
TABLE 38–3 Agents causing contraction or relaxation of mesangial cells. Contraction
Relaxation
Endothelins
ANP
Angiotensin II
Dopamine
Vasopressin
PGE2
Norepinephrine
cAMP
Platelet-activating factor Platelet-derived growth factor Thromboxane A2 PGF2 Leukotrienes C4 and D4 Histamine
CHAPTER 38 Renal Function & Micturition
HYDROSTATIC & OSMOTIC PRESSURE The pressure in the glomerular capillaries is higher than that in other capillary beds because the afferent arterioles are short, straight branches of the interlobular arteries. Furthermore, the vessels “downstream” from the glomeruli, the efferent arterioles, have a relatively high resistance. The capillary hydrostatic pressure is opposed by the hydrostatic pressure in Bowman’s capsule. It is also opposed by the oncotic pressure gradient across the glomerular capillaries (πGC – πT). πT is normally negligible, and the gradient is essentially equal to the oncotic pressure of the plasma proteins. The actual pressures in one strain of rats are shown in Figure 38–6. The net filtration pressure (PUF) is 15 mm Hg at the afferent end of the glomerular capillaries, but it falls to zero— that is, filtration equilibrium is reached—proximal to the efferent end of the glomerular capillaries. This is because fluid leaves the plasma and the oncotic pressure rises as blood passes through the glomerular capillaries. The calculated change in Δπ along an idealized glomerular capillary is also shown in Figure 38–6. It is apparent that portions of the glomerular capillaries do not normally contribute to the formation of the glomerular ultrafiltrate; that is, exchange across the glomerular capillaries is flow-limited rather than diffusion-limited. It is also apparent that a decrease in the rate of rise of the Δ curve produced by an increase in renal plasma flow would increase filtration because it would increase the distance along the capillary in which filtration was taking place. There is considerable species variation in whether filtration equilibrium is reached, and some uncertainties are inherent in
(mm Hg) Afferent end 45 10 20 15
PGC PT πGC PUF
Efferent end 45 10 35 0
Pressure (mm Hg)
PUF = PGC − PT − πGC 60 40 20
ΔP
647
TABLE 38–4 Factors affecting the GFR. Changes in renal blood flow Changes in glomerular capillary hydrostatic pressure Changes in systemic blood pressure Afferent or efferent arteriolar constriction Changes in hydrostatic pressure in Bowman’s capsule Ureteral obstruction Edema of kidney inside tight renal capsule Changes in concentration of plasma proteins: dehydration, hypoproteinemia, etc (minor factors) Changes in Kf Changes in glomerular capillary permeability Changes in effective filtration surface area
the measurement of Kf. It is uncertain whether filtration equilibrium is reached in humans.
CHANGES IN GFR Variations in the factors discussed in the preceding paragraphs and listed in Table 38–4 have predictable effects on the GFR. Changes in renal vascular resistance as a result of autoregulation tend to stabilize filtration pressure, but when the mean systemic arterial pressure drops below the autoregulatory range (Figure 38–4), GFR drops sharply. The GFR tends to be maintained when efferent arteriolar constriction is greater than afferent constriction, but either type of constriction decreases blood flow to the tubules.
FILTRATION FRACTION The ratio of the GFR to the RPF, the filtration fraction, is normally 0.16 to 0.20. The GFR varies less than the RPF. When there is a fall in systemic blood pressure, the GFR falls less than the RPF because of efferent arteriolar constriction, and consequently the filtration fraction rises.
Δπ
0 0 1 Dimensionless distance along idealized glomerular capillary
FIGURE 38–6
Hydrostatic pressure (PGC) and osmotic pressure (πGC) in a glomerular capillary in the rat. PT, pressure in Bowman’s capsule; PUF, net filtration pressure. πT is normally negligible, so Δπ = πGC. ΔP = PGC – PT. (Reproduced with permission from Mercer PF, Maddox DA, Brenner BM: Current concepts of sodium chloride and water transport by the mammalian nephron. West J Med 1974;120:33.)
TUBULAR FUNCTION GENERAL CONSIDERATIONS The amount of any substance (X) that is filtered is the product of the GFR and the plasma level of the substance (ClnPX). The tubular cells may add more of the substance to the filtrate (tubular secretion), may remove some or all of the substance from the filtrate (tubular reabsorption), or may do both. The • amount of the substance excreted per unit of time (UXV)
648
SECTION VIII Renal Physiology
˙ GFR x PX + TX = UXV Filtered = GFR x PX Secreted Reabsorbed
Excreted ˙ = UXV
TX = 0 ˙ GFR x PX = UXV Example: Inulin
FIGURE 38–7
TX = Negative ˙ GFR x PX > UXV
TX = Positive ˙ GFR x PX < UXV
Example: Glucose
Example: PAH
Tubular function. For explanation of symbols,
see text.
equals the amount filtered plus the net amount transferred by the tubules. This latter quantity is conveniently indicated by the symbol TX (Figure 38–7). The clearance of the substance equals the GFR if there is no net tubular secretion or reabsorption, exceeds the GFR if there is net tubular secretion, and is less than the GFR if there is net tubular reabsorption. Much of our knowledge about glomerular filtration and tubular function has been obtained by using micropuncture techniques. Micropipettes can be inserted into the tubules of the living kidney and the composition of aspirated tubular fluid determined by the use of microchemical techniques. In addition, two pipettes can be inserted in a tubule and the tubule perfused in vivo. Alternatively, isolated perfused segments of tubules can be studied in vitro, and tubular cells can be grown and studied in culture.
MECHANISMS OF TUBULAR REABSORPTION & SECRETION Small proteins and some peptide hormones are reabsorbed in the proximal tubules by endocytosis. Other substances are secreted or reabsorbed in the tubules by passive diffusion between cells and through cells by facilitated diffusion down chemical or electrical gradients or active transport against such gradients. Movement is by way of ion channels, exchangers, cotransporters, and pumps. Many of these have now been cloned, and their regulation is being studied. It is important to note that the pumps and other units in the luminal membrane are different from those in the basolateral membrane. It is this different distribution that makes possible net movement of solutes across the epithelia. Like transport systems elsewhere, renal active transport systems have a maximal rate, or transport maximum (Tm), at
which they can transport a particular solute. Thus, the amount of a particular solute transported is proportional to the amount present up to the Tm for the solute, but at higher concentrations, the transport mechanism is saturated and there is no appreciable increment in the amount transported. However, the Tms for some systems are high, and it is difficult to saturate them. It should also be noted that the tubular epithelium, like that of the small intestine, is a leaky epithelium in that the tight junctions between cells permit the passage of some water and electrolytes. The degree to which leakage by this paracellular pathway contributes to the net flux of fluid and solute into and out of the tubules is controversial since it is difficult to measure, but current evidence seems to suggest that it is a significant factor in the proximal tubule. One indication of this is that paracellin-1, a protein localized to tight junctions, is related to Mg2+ reabsorption, and a loss-of-function mutation of its gene causes severe Mg2+ and Ca2+ loss in the urine. The effects of tubular reabsorption and secretion on substances of major physiologic interest are summarized in Table 38–5.
Na+ REABSORPTION The reabsorption of Na+ and Cl– plays a major role in body electrolyte and water homeostasis. In addition, Na+ transport is coupled to the movement of H+, glucose, amino acids, organic acids, phosphate, and other electrolytes and substances across the tubule walls. The principal cotransporters and exchangers in the various parts of the nephron are listed in Table 38–6. In the proximal tubules, the thick portion of the ascending limb of the loop of Henle, the distal tubules, and the collecting ducts, Na+ moves by cotransport or exchange from the tubular lumen into the tubular epithelial cells down its concentration and electrical gradients, and is then actively pumped from these cells into the interstitial space. Na+ is pumped into the interstitium by Na, K ATPase in the basolateral membrane. Thus, Na+ is actively transported out of all parts of the renal tubule except the thin portions of the loop of Henle. The operation of the ubiquitous Na+ pump is considered in detail in Chapter 2. It extrudes three Na+ in exchange for two K+ that are pumped into the cell. The tubular cells along the nephron are connected by tight junctions at their luminal edges, but there is space between the cells along the rest of their lateral borders. Much of the Na+ is actively transported into these extensions of the interstitial space, the lateral intercellular spaces (Figure 38–8). Normally about 60% of the filtered Na+ is reabsorbed in the proximal tubule, primarily by Na–H exchange. Another 30% is absorbed via the Na–2Cl–K cotransporter in the thick ascending limb of the loop of Henle, and about 7% is absorbed by Na–Cl cotransporter in the distal convoluted tubule. The remainder of the filtered Na+, about 3%, is absorbed via the ENaC channels in the collecting ducts, and this is the portion that is regulated by aldosterone in the production of homeostatic adjustments in Na+ balance.
CHAPTER 38 Renal Function & Micturition
649
TABLE 38–5 Renal handling of various plasma constituents in a normal adult human on an average diet. Per 24 Hours Substance
Filtered
Na+ (mEq)
26,000
K+ (mEq)
600
Cl– (mEq)
Reabsorbed
Secreted
Excreted
Percentage Reabsorbed
150
99.4
90
93.3 99.2
25,850 560a
502
18,000
17,850
150
– (mEq)
4,900
4,900
0
100
Urea (mmol)
870
460b
410
53
Creatinine (mmol)
12
1c
Uric acid (mmol)
50
49
Glucose (mmol)
800
800
54,000
53,400
180,000
179,000
HCO3
Total solute (mOsm) Water (mL)
1c
12
4
5
98
0
100
100
700
98.9
1000
99.4
a +
K is both reabsorbed and secreted.
b c
Urea moves into as well as out of some portions of the nephron.
Variable secretion and probable reabsorption of creatinine in humans.
TABLE 38–6 Transport proteins involved
in the movement of Na+ and Cl– across the apical membranes of renal tubular cells.a Site
Apical Transporter
Function
Proximal tubule
Na/glucose CT
Na+ uptake, glucose uptake
Na+/Pi CT
Na+ uptake, Pi uptake
Na+ amino acid CT
Na+ uptake, amino acid uptake
Na/lactate CT
Na+ uptake, lactate uptake
Na/H exchanger
Na+ uptake, H+ extrusion
Cl/base exchanger
Cl– uptake
Na–K–2Cl CT
Na+ uptake, Cl– uptake, K+
Thick ascending limb
Tight junction
Na+ uptake, H+ extrusion
K+ channels
K+ extrusion (recycling)
Distal convoluted tubule
NaCl CT
Na+ uptake, Cl– uptake
Collecting duct
Na+ channel (ENaC)
Na+ uptake
a
Uptake indicates movement from tubular lumen to cell interior, extrusion is movement from cell interior to tubular lumen. CT, cotransporter; Pi, inorganic phosphate.
Modified with permission from Schnermann JB, Sayegh EI: Kidney Physiology. Lippincott-Raven, 1998.
K+ Na+
Tubular lumen Na+
Na+ Na+ K+ Na+ Interstitial fluid K+
uptake Na/H exchanger
Lateral intercellular space
Na+, etc
FIGURE 38–8
Mechanism of Na+ reabsorption in the proximal tubule. Na+ moves out of the tubular lumen by cotransport and exchange mechanism through the apical membrane of the tubule (dashed line). The Na+ is then actively transported into the interstitial fluid by Na, K ATPase in the basolateral membrane (solid lines). K + enters the interstitial fluid via K+ channels. A small amount of Na +, other solutes, and H2O re-enter the tubular lumen by passive transport through the tight junctions (dotted lines).
650
SECTION VIII Renal Physiology
GLUCOSE REABSORPTION
Inulin
UV
P TmG Splay "Ideal" Actual Plasma glucose (PG)
FIGURE 38–10
Top: Relation between the plasma level (P) and excretion (UV) of glucose and inulin. Bottom: Relation between the plasma glucose level (PG) and amount of glucose reabsorbed (T G).
ates considerably from the “ideal” curve. This deviation is called splay. The magnitude of the splay is inversely proportionate to the avidity with which the transport mechanism binds the substance it transports.
GLUCOSE TRANSPORT MECHANISM
2.6
Glucose reabsorption in the kidneys is similar to glucose reabsorption in the intestine (see Chapter 27). Glucose and Na+ bind to the sodium-dependent glucose transporter (SGLT) 2 in the apical membrane, and glucose is carried into the cell as Na+ moves down its electrical and chemical gradient. The Na+ is then pumped out of the cell into the interstitium, and the glucose is transported by glucose transporter (GLUT) 2 into the interstitial fluid. At least in the rat, there is some transport by SGLT 1 and GLUT 1 as well. SGLT 2 specifically binds the d isomer of glucose, and the rate of transport of d-glucose is many times greater than that of l-glucose. Glucose transport in the kidneys is inhibited, as it is in the intestine, by the plant glucoside phlorhizin, which competes with d-glucose for binding to the carrier.
2.4 2.2 Inulin 2.0 1.8 1.6 Cl−
1.4
Na+
K+
1.2 TF P
Glucose
.
Glucose reabsorbed (TG)
Glucose, amino acids, and bicarbonate are reabsorbed along with Na+ in the early portion of the proximal tubule (Figure 38–9). Farther along the tubule, Na+ is reabsorbed with Cl–. Glucose is typical of substances removed from the urine by secondary active transport. It is filtered at a rate of approximately 100 mg/min (80 mg/dL of plasma × 125 mL/min). Essentially all of the glucose is reabsorbed, and no more than a few milligrams appear in the urine per 24 h. The amount reabsorbed is proportional to the amount filtered and hence to the plasma glucose level (PG) times the GFR up to the transport maximum (TmG). When the TmG is exceeded, the amount of glucose in the urine rises (Figure 38–10). The TmG is about 375 mg/min in men and 300 mg/min in women. The renal threshold for glucose is the plasma level at which the glucose first appears in the urine in more than the normal minute amounts. One would predict that the renal threshold would be about 300 mg/dL, that is, 375 mg/min (TmG) divided by 125 mL/min (GFR). However, the actual renal threshold is about 200 mg/dL of arterial plasma, which corresponds to a venous level of about 180 mg/dL. Figure 38–10 shows why the actual renal threshold is less than the predicted threshold. The "ideal" curve shown in this diagram would be obtained if the TmG in all the tubules was identical and if all the glucose were removed from each tubule when the amount filtered was below the TmG. This is not the case, and in humans, for example, the actual curve is rounded and devi-
1.0
osm
0.8 0.6
HCO3
0.4
−
ADDITIONAL EXAMPLES OF SECONDARY ACTIVE TRANSPORT
Amino acids
0.2 Glucose 0
25
30
75
100
% Proximal tubule length
FIGURE 38–9
Reabsorption of various solutes in the proximal tubule. TF/P, tubular fluid:plasma concentration ratio.
(Courtesy of FC Rector Jr.)
Like glucose reabsorption, amino acid reabsorption is most marked in the early portion of the proximal convoluted tubule. Absorption in this location resembles absorption in the intestine (see Chapter 27). The main carriers in the apical membrane cotransport Na+, whereas the carriers in the basolateral membranes are not Na+-dependent. Na+ is pumped
CHAPTER 38 Renal Function & Micturition
651
CLINICAL BOX 38–1 PAH
Other Substances Secreted by the Tubules UV
.
Splay
Derivatives of hippuric acid in addition to PAH, phenol red and other sulfonphthalein dyes, penicillin, and a variety of iodinated dyes are actively secreted into the tubular fluid. Substances that are normally produced in the body and secreted by the tubules include various ethereal sulfates, steroid and other glucuronides, and 5-hydroxyindoleacetic acid, the principal metabolite of serotonin.
Inulin
P
FIGURE 38–11
Relation between plasma levels (P) and excretion (UV) of PAH and inulin.
out of the cells by Na, K ATPase and the amino acids leave by passive or facilitated diffusion to the interstitial fluid. Some Cl– is reabsorbed with Na+ and K+ in the thick ascending limb of the loop of Henle. In addition, two members of the family of Cl channels have been identified in the kidney. Mutations in the gene for one of the renal channels is associated with Ca2+-containing kidney stones and hypercalciuria (Dent disease), but how tubular transport of Ca2+ and Cl– are linked is still unsettled.
PAH TRANSPORT The dynamics of PAH transport illustrate the operation of the active transport mechanisms that secrete substances into the tubular fluid (see Clinical Box 38–1). The filtered load of PAH is a linear function of the plasma level, but PAH secretion increases as PPAH rises only until a maximal secretion rate (TmPAH) is reached (Figure 38–11). When PPAH is low, CPAH is high; but as PPAH rises above TmPAH, CPAH falls progressively. It eventually approaches the clearance of inulin (CIn) (Figure 38–12), because the amount of PAH secreted becomes a smaller and smaller fraction of the total amount excreted.
Conversely, the clearance of glucose is essentially zero at PG levels below the renal threshold; but above the threshold, CG rises to approach CIn as PG is raised. The use of CPAH to measure ERPF is discussed above.
TUBULOGLOMERULAR FEEDBACK & GLOMERULOTUBULAR BALANCE Signals from the renal tubule in each nephron feed back to affect filtration in its glomerulus. As the rate of flow through the ascending limb of the loop of Henle and first part of the distal tubule increases, glomerular filtration in the same nephron decreases, and, conversely, a decrease in flow increases the GFR (Figure 38–13). This process, which is called
Renal arteriolar pressure
Glomerular capillary pressure
Glucose, mg/dL 200
Clearance (mL/min)
600
20
400 40
GFR
600 60 80 PAH, mg/dL
500
Glomerulotubular balance
Solute reabsorption in proximal tubule
Tubuloglomerular feedback
400 Solute reabsorption in thick ascending limb
300 PAH 200
Inulin
100 0
FIGURE 38–12
Salt and fluid delivery to the distal tubule
Glucose Plasma level (P)
Clearance of inulin, glucose, and PAH at various plasma levels of each substance in humans.
FIGURE 38–13
Mechanisms of glomerulotubular balance and tubuloglomerular feedback.
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SECTION VIII Renal Physiology
tubuloglomerular feedback, tends to maintain the constancy of the load delivered to the distal tubule. The sensor for this response is the macula densa. The amount of fluid entering the distal tubule at the end of the thick ascending limb of the loop of Henle depends on the amount of Na+ and Cl– in it. The Na+ and Cl– enter the macula densa cells via the Na–K–2Cl cotransporter in their apical membranes. The increased Na+ causes increased Na, K ATPase activity and the resultant increased ATP hydrolysis causes more adenosine to be formed. Presumably, adenosine is secreted from the basal membrane of the cells. It acts via adenosine A1 receptors on the macula densa cells to increase their release of Ca2+ to the vascular smooth muscle in the afferent arterioles. This causes afferent vasoconstriction and a resultant decrease in GFR. Presumably, a similar mechanism generates a signal that decreases renin secretion by the adjacent juxtaglomerular cells in the afferent arteriole (see Chapter 39), but this remains unsettled. Conversely, an increase in GFR causes an increase in the reabsorption of solutes, and consequently of water, primarily in the proximal tubule, so that in general the percentage of the solute reabsorbed is held constant. This process is called glomerulotubular balance, and it is particularly prominent for Na+. The change in Na+ reabsorption occurs within seconds after a change in filtration, so it seems unlikely that an extrarenal humoral factor is involved. One factor is the oncotic pressure in the peritubular capillaries. When the GFR is high, there is a relatively large increase in the oncotic pressure of the plasma leaving the glomeruli via the efferent arterioles and hence in their capillary branches. This increases the reabsorption of Na+ from the tubule. However, other as yet unidentified intrarenal mechanisms are also involved.
WATER TRANSPORT Normally, 180 L of fluid is filtered through the glomeruli each day, while the average daily urine volume is about 1 L. The same load of solute can be excreted per 24 h in a urine volume of 500 mL with a concentration of 1400 mOsm/kg or in a volume of 23.3 L with a concentration of 30 mOsm/kg (Table 38–7). These figures demonstrate two important facts: First, at least 87% of the filtered water is reabsorbed, even when the urine volume is
23 L; and second, the reabsorption of the remainder of the filtered water can be varied without affecting total solute excretion. Therefore, when the urine is concentrated, water is retained in excess of solute; and when it is dilute, water is lost from the body in excess of solute. Both facts have great importance in the regulation of the osmolality of the body fluids. A key regulator of water output is vasopressin acting on the collecting ducts.
AQUAPORINS Rapid diffusion of water across cell membranes depends on the presence of water channels, integral membrane proteins called aquaporins. To date, 13 aquaporins have been cloned; however, only 4 aquaporins (aquaporin-1, aquaporin-2, aquaporin-3, and aquaporin-4) play a key role in the kidney. The roles played by aquaporin-1 and aquaporin-2 in renal water transport are discussed below.
PROXIMAL TUBULE Active transport of many substances occurs from the fluid in the proximal tubule, but micropuncture studies have shown that the fluid remains essentially iso-osmotic to the end of the proximal tubule (Figure 38–9). Aquaporin-1 is localized to both the basolateral and apical membrane of the proximal tubules and its presence allows water to move rapidly out of the tubule along the osmotic gradients set up by active transport of solutes, and isotonicity is maintained. Because the ratio of the concentration in tubular fluid to the concentration in plasma (TF/P) of the nonreabsorbable substance inulin is 2.5 to 3.3 at the end of the proximal tubule, it follows that 60–70% of the filtered solute and 60–70% of the filtered water have been removed by the time the filtrate reaches this point (Figure 38–14). When aquaporin-1 was knocked out in mice, proximal tubular water permeability was reduced by 80%. When the mice were subjected to dehydration, their urine osmolality did not increase (
