Medicina fisiologia. atlas de fisiologia


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Medicina fisiologia. atlas de fisiologia

  1. 1. At a Glance 2 1 Fundamentals and Cell Physiology 42 2 Nerve and Muscle, Physical Work 78 3 Autonomic Nervous System (ANS) 88 4 Blood 106 5 Respiration 138 6 Acid–Base Homeostasis 148 7 Kidneys, Salt, and Water Balance 186 8 Cardiovascular System 222 9 Thermal Balance and Thermoregulation 226 10 Nutrition and Digestion 266 11 Hormones and Reproduction 310 12 Central Nervous System and Senses 372 13 Appendix 391 Further Reading 394 IndexDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  2. 2. II Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  3. 3. Color Atlas of Physiology 5th edition, completely revised and expanded Agamemnon Despopoulos, M.D. Professor Formerly: Ciba Geigy Basel Stefan Silbernagl, M.D. Professor Head of Department Institute of Physiology University of Wuerzburg Wuerzburg, Germany 186 color plates by Ruediger Gay and Astried Rothenburger Thieme Stuttgart · New YorkDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  4. 4. Library of Congress Cataloging-in-Publication Important Note: Medicine is an ever-changing Data science undergoing continual development. is available from the publisher Research and clinical experience are continu- ally expanding our knowledge, in particular our knowledge of proper treatment and drug 1st German edition 1979 1st Czech edition 1984 therapy. Insofar as this book mentions any do- 2nd German edition 1983 2nd Czech edition 1994 sage or application, readers may rest assured 3rd German edition 1988 1st French edition 1985 that the authors, editors, and publishers have 4th German edition 1991 2nd French edition 1992 5th German edition 2001 3rd French edition 2001 made every effort to ensure that such refe- 1st English edition 1981 rences are in accordance with the state of 1st Turkish edition 1986 2nd English edition 1984 2nd Turkish edition 1997 knowledge at the time of production of the 3rd English edition 1986 book. 4th English edition 1991 1st Greek edition 1989 Nevertheless, this does not involve, imply, 1st Dutch edition 1981 1st Chinese edition 1991 or express any guarantee or responsibility on 2nd Dutch edition 2001 1st Polish edition 1994 the part of the publishers in respect to any do- 1st Italian edition 1981 1st Hungarian edition 1994 sage instructions and forms of applications 2nd Italian edition 2001 2nd Hungarian edition 1996 stated in the book. Every user is requested to 1st Japanese edition 1982 1st Indonesion edition 2000 examine carefully the manufacturers’ leaflets 2nd Japanese edition 1992 accompanying each drug and to check, if 1st Spanish edition 1982 2nd Spanish edition 1985 necessary in consultation with a physician or 3rd Spanish edition 1994 specialist, whether the dosage schedules men- 4th Spanish edition 2001 tioned therein or the contraindications stated by the manufacturers differ from the state- ments made in the present book. Such exami- This book is an authorized translation of the nation is particularly important with drugs 5th German edition published and copy- that are either rarely used or have been newly righted 2001 by Georg Thieme Verlag, Stutt- released on the market. Every dosage schedule gart, Germany. or every form of application used is entirely at Title of the German edition: the user’s own risk and responsibility. The au- Taschenatlas der Physiologie thors and publishers request every user to re- port to the publishers any discrepancies or Translated by Suzyon O’Neal Wandrey, Berlin, inaccuracies noticed. Germany Some of the product names, patents, and Illustrated by Atelier Gay + Rothenburger, Ster- registered designs referred to in this book are nenfels, Germany in fact registered trademarks or proprietary names even though specific reference to this 1981, 2003 Georg Thieme Verlag fact is not always made in the text. Therefore, Rüdigerstraße 14, D-70469 Stuttgart, Germany the appearance of a name without designation as proprietary is not to be construed as a repre- Thieme New York, 333 Seventh Avenue, sentation by the publisher that it is in the New York, N.Y. 10001, U.S.A. public domain. This book, including all parts thereof, is le- gally protected by copyright. Any use, exploita- Cover design: Cyclus, Stuttgart tion, or commercialization outside the narrow Typesetting by: Druckhaus Götz GmbH, limits set by copyright legislation, without the Ludwigsburg, Germany publisher’s consent, is illegal and liable to pro- Printed in Germany by: Appl Druck secution. This applies in particular to photostat GmbH & Co. KG, Wemding, Germany reproduction, copying, mimeographing or duplication of any kind, translating, prepara-IV ISBN 3-13-545005-8 (GTV) tion of microfilms, and electronic data pro- ISBN 1-58890-061-4 (TNY) 1 2 3 4 5 cessing and storage. Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  5. 5. Preface to the Fifth EditionThe base of knowledge in many sectors of phy- Prof. C. von Campenhausen, Mainz, Dr. M. Fi-siology has grown considerably in magnitude scher, Mainz, Prof. K.H. Plattig, Erlangen, andand in depth since the last edition of this book Dr. C. Walther, Marburg, and from my collea-was published. Many advances, especially the gues and staff at the Institute in Würzburg. Itrapid progress in sequencing the human ge- was again a great pleasure to work with Rüdi-nome and its gene products, have brought ger Gay and Astried Rothenburger, to whom Icompletely new insight into cell function and am deeply indebted for revising practically allcommunication. This made it necessary to edit the illustrations in the book and for designing aand, in some cases, enlarge many parts of the number of new color plates. Their extraordina-book, especially the chapter on the fundamen- ry enthusiasm and professionalism played atals of cell physiology and the sections on decisive role in the materialization of this newneurotransmission, mechanisms of intracellu- edition. To them I extend my sincere thanks. Ilar signal transmission, immune defense, and would also like to thank Suzyon O’Neal Wan-the processing of sensory stimuli. A list of phy- drey for her outstanding translation. I greatlysiological reference values and important for- appreciate her capable and careful work. I ammulas were added to the appendix for quick also indebted to the publishing staff, especiallyreference. The extensive index now also serves Marianne Mauch, an extremely competent andas a key to abbreviations used in the text. motivated editor, and Gert Krüger for invalu- Some of the comments explaining the con- able production assistance. I would also like tonections between pathophysiological princi- thank Katharina Völker for her ever observantples and clinical dysfunctions had to be slight- and conscientious assistance in preparing thely truncated and set in smaller print. However, index.this base of knowledge has also grown consi- I hope that the 5th Edition of the Color Atlasderably for the reasons mentioned above. To of Physiology will prove to be a valuable tool formake allowances for this, a similarly designed helping students better understand physiolog-book, the Color Atlas of Pathophysiology ical correlates, and that it will be a valuable re-(S. Silbernagl and F. Lang, Thieme), has now ference for practicing physicians and scien-been introduced to supplement the well- tists, to help them recall previously learned in-established Color Atlas of Physiology. formation and gain new insights in physiology. I am very grateful for the many helpful com-ments from attentive readers (including myson Jakob) and for the welcome feedback from Würzburg, December 2002my peers, especially Prof. H. Antoni, Freiburg, Stefan Silbernagl* V* e-mail: Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  6. 6. Preface to the First Edition In the modern world, visual pathways have A book of this nature is inevitably deriva- outdistanced other avenues for informational tive, but many of the representations are new input. This book takes advantage of the econo- and, we hope, innovative. A number of people my of visual representation to indicate the si- have contributed directly and indirectly to the multaneity and multiplicity of physiological completion of this volume, but none more phenomena. Although some subjects lend than Sarah Jones, who gave much more than themselves more readily than others to this editorial assistance. Acknowledgement of treatment, inclusive rather than selective helpful criticism and advice is due also to Drs. coverage of the key elements of physiology has R. Greger, A. Ratner, J. Weiss, and S. Wood, and been attempted. Prof. H. Seller. We are grateful to Joy Wieser for Clearly, this book of little more than 300 her help in checking the proofs. Wolf-Rüdiger pages, only half of which are textual, cannot be and Barbara Gay are especially recognized, not considered as a primary source for the serious only for their art work, but for their conceptual student of physiology. Nevertheless, it does contributions as well. The publishers, Georg contain most of the basic principles and facts Thieme Verlag and Deutscher Taschenbuch taught in a medical school introductory Verlag, contributed valuable assistance based course. Each unit of text and illustration can on extensive experience; an author could wish serve initially as an overview for introduction for no better relationship. Finally, special to the subject and subsequently as a concise recognition to Dr. Walter Kumpmann for in- review of the material. The contents are as cur- spiring the project and for his unquestioning rent as the publishing art permits and include confidence in the authors. both classical information for the beginning students as well as recent details and trends Basel and Innsbruck, Summer 1979 for the advanced student. Agamemnon Despopoulos Stefan SilbernaglVI Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  7. 7. From the Preface to the Third EditionThe first German edition of this book was al-ready in press when, on November 2nd, 1979,Agamennon Despopoulos and his wife, SarahJones-Despopoulos put to sea from Bizerta, Tu-nisia. Their intention was to cross the Atlanticin their sailing boat. This was the last that wasever heard of them and we have had to aban-don all hope of seeing them again. Without the creative enthusiasm of Aga-mennon Despopoulos, it is doubtful whetherthis book would have been possible; withouthis personal support it has not been easy tocontinue with the project. Whilst keeping inmind our original aims, I have completely re-vised the book, incorporating the latest advan-ces in the field of physiology as well as the wel-come suggestions provided by readers of the Dr. Agamemnon Despopoulosearlier edition, to whom I extend my thanks fortheir active interest. Born 1924 in New York; Professor of Physiology at the University of New Mexico. Albuquerque, USA, until 1971; Würzburg, Fall 1985 thereafter scientific adviser to CIBA-GEIGY, Basel. Stefan Silbernagl VII Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  8. 8. VIII Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  9. 9. Table of Contents Fundamentals and Cell Physiology 2 1 The Body: an Open System with an Internal Environment · · · 2 Control and Regulation · · · 4 The Cell · · · 8 Transport In, Through, and Between Cells · · · 16 Passive Transport by Means of Diffusion · · · 20 Osmosis, Filtration, and Convection · · · 24 Active Transport · · · 26 Cell Migration · · · 30 Electrical Membrane Potentials and Ion Channels · · · 32 Role of Ca2+ in Cell Regulation · · · 36 Energy Production and Metabolism · · · 38 Nerve and Muscle, Physical Work 42 2 Neuron Structure and Function · · · 42 Resting Membrane Potential · · · 44 Action Potential · · · 46 Propagation of Action Potentials in Nerve Fiber · · · 48 Artificial Stimulation of Nerve Cells · · · 50 Synaptic Transmission · · · 50 Motor End-plate · · · 56 Motility and Muscle Types · · · 58 Motor Unit of Skeletal Muscle · · · 58 Contractile Apparatus of Striated Muscle · · · 60 Contraction of Striated Muscle · · · 62 Mechanical Features of Skeletal Muscle · · · 66 Smooth Muscle · · · 70 Energy Supply for Muscle Contraction · · · 72 Physical Work · · · 74 Physical Fitness and Training · · · 76 Autonomic Nervous System (ANS) 78 3 Organization of the Autonomic Nervous System · · · 78 Acetylcholine and Cholinergic Transmission · · · 82 Catecholamine, Adrenergic Transmission and Adrenoceptors · · · 84 Adrenal Medulla · · · 86 Non-cholinergic, Non-adrenergic Transmitters · · · 86 IXDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  10. 10. Blood 88 4 Composition and Function of Blood · · · 88 Iron Metabolism and Erythropoiesis · · · 90 Flow Properties of Blood · · · 92 Plasma, Ion Distribution · · · 92 Immune System · · · 94 Hypersensitivity Reactions (Allergies) · · · 100 Blood Groups · · · 100 Hemostasis · · · 102 Fibrinolysis and Thromboprotection · · · 104 Respiration 106 5 Lung Function, Respiration · · · 106 Mechanics of Breathing · · · 108 Purification of Respiratory Air · · · 110 Artificial Respiration · · · 110 Pneumothorax · · · 110 Lung Volumes and their Measurement · · · 112 Dead Space, Residual Volume, and Airway Resistance · · · 114 Lung–Chest Pressure—Volume Curve, Respiratory Work · · · 116 Surface Tension, Surfactant · · · 118 Dynamic Lung Function Tests · · · 118 Pulmonary Gas Exchange · · · 120 Pulmonary Blood Flow, Ventilation–Perfusion Ratio · · · 122 CO2 Transport in Blood · · · 124 CO2 Binding in Blood · · · 126 CO2 in Cerebrospinal Fluid · · · 126 Binding and Transport of O2 in Blood · · · 128 Internal (Tissue) Respiration, Hypoxia · · · 130 Respiratory Control and Stimulation · · · 132 Effects of Diving on Respiration · · · 134 Effects of High Altitude on Respiration · · · 136 Oxygen Toxicity · · · 136 Acid–Base Homeostasis 138 6 pH, pH Buffers, Acid–Base Balance · · · 138 Bicarbonate/Carbon Dioxide Buffer · · · 140 Acidosis and Alkalosis · · · 142 Assessment of Acid–Base Status · · · 146 Kidneys, Salt, and Water Balance 148 7 Kidney Structure and Function · · · 148 Renal Circulation · · · 150 Glomerular Filtration and Clearance · · · 152X Transport Processes at the Nephron · · · 154 Reabsorption of Organic Substances · · · 158 Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  11. 11. Excretion of Organic Substances · · · 160 Reabsorption of Na+ and Cl– · · · 162 Reabsorption of Water, Formation of Concentrated Urine · · · 164 Body Fluid Homeostasis · · · 168 Salt and Water Regulation · · · 170 Diuresis and Diuretics · · · 172 Disturbances of Salt and Water Homeostasis · · · 172 The Kidney and Acid–Base Balance · · · 174 Reabsorption and Excretion of Phosphate, Ca2+ and Mg2+ · · · 178 Potassium Balance · · · 180 Tubuloglomerular Feedback, Renin–Angiotensin System · · · 184 Cardiovascular System 186 8 Overview · · · 186 Blood Vessels and Blood Flow · · · 188 Cardiac Cycle · · · 190 Cardiac Impulse Generation and Conduction · · · 192 Electrocardiogram (ECG) · · · 196 Excitation in Electrolyte Disturbances · · · 198 Cardiac Arrhythmias · · · 200 Ventricular Pressure–Volume Relationships · · · 202 Cardiac Work and Cardiac Power · · · 202 Regulation of Stroke Volume · · · 204 Venous Return · · · 204 Arterial Blood Pressure · · · 206 Endothelial Exchange Processes · · · 208 Myocardial Oxygen Supply · · · 210 Regulation of the Circulation · · · 212 Circulatory Shock · · · 218 Fetal and Neonatal Circulation · · · 220 Thermal Balance and Thermoregulation 222 9 Thermal Balance · · · 222 Thermoregulation · · · 224 Nutrition and Digestion 226 10 Nutrition · · · 226 Energy Metabolism and Calorimetry · · · 228 Energy Homeostasis and Body Weight · · · 230 Gastrointestinal (GI) Tract: Overview, Immune Defense and Blood Flow · · · 232 Neural and Hormonal Integration · · · 234 Saliva · · · 236 Deglutition · · · 238 Vomiting · · · 238 Stomach Structure and Motility · · · 240 Gastric Juice · · · 242 XI Small Intestinal Function · · · 244Despopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  12. 12. Pancreas · · · 246 Bile · · · 248 Excretory Liver Function—Bilirubin · · · 250 Lipid Digestion · · · 252 Lipid Distribution and Storage · · · 254 Digestion and Absorption of Carbohydrates and Protein · · · 258 Vitamin Absorption · · · 260 Water and Mineral Absorption · · · 262 Large Intestine, Defecation, Feces · · · 264 Hormones and Reproduction 266 11 Integrative Systems of the Body · · · 266 Hormones · · · 268 Humoral Signals: Control and Effects · · · 272 Cellular Transmission of Signals from Extracellular Messengers · · · 274 Hypothalamic–Pituitary System · · · 280 Carbohydrate Metabolism and Pancreatic Hormones · · · 282 Thyroid Hormones · · · 286 Calcium and Phosphate Metabolism · · · 290 Biosynthesis of Steroid Hormones · · · 294 Adrenal Cortex and Glucocorticoid Synthesis · · · 296 Oogenesis and the Menstrual Cycle · · · 298 Hormonal Control of the Menstrual Cycle · · · 300 Estrogens · · · 302 Progesterone · · · 302 Prolactin and Oxytocin · · · 303 Hormonal Control of Pregnancy and Birth · · · 304 Androgens and Testicular Function · · · 306 Sexual Response, Intercourse and Fertilization · · · 308 Central Nervous System and Senses 310 12 Central Nervous System · · · 310 Cerebrospinal Fluid · · · 310 Stimulus Reception and Processing · · · 312 Sensory Functions of the Skin · · · 314 Proprioception, Stretch Reflex · · · 316 Nociception and Pain · · · 318 Polysynaptic Reflexes · · · 320 Synaptic Inhibition · · · 320 Central Conduction of Sensory Input · · · 322 Motor System · · · 324 Hypothalamus, Limbic System · · · 330 Cerebral Cortex, Electroencephalogram (EEG) · · · 332 Sleep–Wake Cycle, Circadian Rhythms · · · 334 Consciousness, Memory, Language · · · 336 Glia · · · 338 Sense of Taste · · · 338XII Sense of Smell · · · 340 Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  13. 13. Sense of Balance · · · 342 Eye Structure, Tear Fluid, Aqueous Humor · · · 344 Optical Apparatus of the Eye · · · 346 Visual Acuity, Photosensors · · · 348 Adaptation of the Eye to Different Light Intensities · · · 352 Retinal Processing of Visual Stimuli · · · 354 Color Vision · · · 356 Visual Field, Visual Pathway, Central Processing of Visual Stimuli · · · 358 Eye Movements, Stereoscopic Vision, Depth Perception · · · 360 Physical Principles of Sound—Sound Stimulus and Perception · · · 362 Conduction of Sound, Sound Sensors · · · 364 Central Processing of Acoustic Information · · · 368 Voice and Speech · · · 370 Appendix 372 13 Dimensions and Units · · · 372 Powers and Logarithms · · · 380 Graphic Representation of Data · · · 381 The Greek Alphabet · · · 384 Reference Values in Physiology · · · 384 Important Equations in Physiology · · · 388 Further Reading 391 Index 394 XIIIDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  14. 14. 1 Fundamentals and Cell Physiology “. . . If we break up a living organism by isolating its different parts, it is only for the sake of ease in analysis and by no means in order to conceive them separately. Indeed, when we wish to ascribe to a physiological quality its value and true significance, we must always refer it to the whole and draw our final conclusions only in relation to its effects on the whole.” Claude Bernard (1865) ganism is capable of eliciting motor responses The Body: an Open System with an to signals from the environment. This is Internal Environment achieved by moving its pseudopodia or The existence of unicellular organisms is the flagella, for example, in response to changes in epitome of life in its simplest form. Even the food concentration. simple protists must meet two basic but essen- The evolution from unicellular organisms to tially conflicting demands in order to survive. multicellular organisms, the transition from A unicellular organism must, on the one hand, specialized cell groups to organs, the emer- isolate itself from the seeming disorder of its gence of the two sexes, the coexistence of in- inanimate surroundings, yet, as an “open sys- dividuals in social groups, and the transition tem” ( p. 40), it is dependent on its environ- from water to land have tremendously in- ment for the exchange of heat, oxygen, creased the efficiency, survival, radius of ac- nutrients, waste materials, and information. tion, and independence of living organisms. “Isolation” is mainly ensured by the cell This process required the simultaneous devel- membrane, the hydrophobic properties of opment of a complex infrastructure within the which prevent the potentially fatal mixing of organism. Nonetheless, the individual cells of hydrophilic components in watery solutions the body still need a milieu like that of the inside and outside the cell. Protein molecules primordial sea for life and survival. Today, the within the cell membrane ensure the perme- extracellular fluid is responsible for providing ability of the membrane barrier. They may constant environmental conditions ( B), but exist in the form of pores (channels) or as more the volume of the fluid is no longer infinite. In complex transport proteins known as carriers fact, it is even smaller than the intracellular ( p. 26 ff.). Both types are selective for cer- volume ( p. 168). Because of their metabolic tain substances, and their activity is usually activity, the cells would quickly deplete the regulated. The cell membrane is relatively well oxygen and nutrient stores within the fluids permeable to hydrophobic molecules such as and flood their surroundings with waste prod- gases. This is useful for the exchange of O2 and ucts if organs capable of maintaining a stable CO2 and for the uptake of lipophilic signal sub- internal environment had not developed. This stances, yet exposes the cell to poisonous gases is achieved through homeostasis, a process by such as carbon monoxide (CO) and lipophilic which physiologic self-regulatory mecha- noxae such as organic solvents. The cell mem- nisms (see below) maintain steady states in brane also contains other proteins—namely, the body through coordinated physiological receptors and enzymes. Receptors receive sig- activity. Specialized organs ensure the con- nals from the external environment and con- tinuous absorption of nutrients, electrolytes vey the information to the interior of the cell and water and the excretion of waste products (signal transduction), and enzymes enable the via the urine and feces. The circulating blood cell to metabolize extracellular substrates. connects the organs to every inch of the body, Let us imagine the primordial sea as the ex- and the exchange of materials between the ternal environment of the unicellular or- blood and the intercellular spaces (interstices) ganism ( A). This milieu remains more or less creates a stable environment for the cells. Or- constant, although the organism absorbs gans such as the digestive tract and liver ab- nutrients from it and excretes waste into it. In sorb nutrients and make them available by2 spite of its simple structure, the unicellular or- processing, metabolizing and distributing Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  15. 15. A. Unicellular organism in the constant external environment of the primordial sea Substance absorption Signal receptionPrimordial and excretionsea Internal and External Environment Heat Ion exchange Genome Digestion Water O2 Exchange Motility of gases CO2 Excretion Plate 1.1 B. Maintenance of a stable internal environment in humans Integration through External signals nervous system and hormones O2 CO2 Exchange Emission of heat Internal of gases (water, salt) signals Behavior Regulation Lungs Blood Skin Extra- cellular Interstice space Intracellular space Uptake of nutrients, water, salts, etc. Kidney Distribution Excretion of excess – water Waste and – salts toxins Liver Excretion of – acids Digestive 3 tract waste and toxinsDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  16. 16. them throughout the body. The lung is re- Although behavioral science, sociology, and sponsible for the exchange of gases (O2 intake, psychology are disciplines that border on CO2 elimination), the liver and kidney for the physiology, true bridges between them and excretion of waste and foreign substances, and physiology have been established only in ex- the skin for the release of heat. The kidney and ceptional cases. lungs also play an important role in regulating the internal environment, e.g., water content, Control and Regulation osmolality, ion concentrations, pH (kidney, lungs) and O2 and CO2 pressure (lungs) ( B). In order to have useful cooperation between1 Fundamentals and Cell Physiology The specialization of cells and organs for the specialized organs of the body, their func- specific tasks naturally requires integration, tions must be adjusted to meet specific needs. which is achieved by convective transport over In other words, the organs must be subject to long distances (circulation, respiratory tract), control and regulation. Control implies that a humoral transfer of information (hormones), controlled variable such as the blood pressure and transmission of electrical signals in the is subject to selective external modification, nervous system, to name a few examples. for example, through alteration of the heart These mechanisms are responsible for supply rate ( p. 218). Because many other factors and disposal and thereby maintain a stable in- also affect the blood pressure and heart rate, ternal environment, even under conditions of the controlled variable can only be kept con- extremely high demand and stress. Moreover, stant by continuously measuring the current they control and regulate functions that en- blood pressure, comparing it with the refer- sure survival in the sense of preservation of the ence signal (set point), and continuously cor- species. Important factors in this process in- recting any deviations. If the blood pressure clude not only the timely development of re- drops—due, for example, to rapidly standing productive organs and the availability of fertil- up from a recumbent position—the heart rate izable gametes at sexual maturity, but also the will increase until the blood pressure has been control of erection, ejaculation, fertilization, reasonably adjusted. Once the blood pressure and nidation. Others include the coordination has risen above a certain limit, the heart rate of functions in the mother and fetus during will decrease again and the blood pressure will pregnancy and regulation of the birth process normalize. This type of closed-loop control is and the lactation period. called a negative feedback control system or a The central nervous system (CNS) processes control circuit ( C1). It consists of a controller signals from peripheral sensors (single with a programmed set-point value (target sensory cells or sensory organs), activates out- value) and control elements (effectors) that can wardly directed effectors (e.g., skeletal adjust the controlled variable to the set point. muscles), and influences the endocrine glands. The system also includes sensors that continu- The CNS is the focus of attention when study- ously measure the actual value of the con- ing human or animal behavior. It helps us to lo- trolled variable of interest and report it (feed- cate food and water and protects us from heat back) to the controller, which compares the ac- or cold. The central nervous system also plays a tual value of the controlled variable with the role in partner selection, concern for offspring set-point value and makes the necessary ad- even long after their birth, and integration into justments if disturbance-related discrepancies social systems. The CNS is also involved in the have occurred. The control system operates development, expression, and processing of either from within the organ itself (autoregula- emotions such as desire, listlessness, curiosity, tion) or via a superordinate organ such as the wishfulness, happiness, anger, wrath, and central nervous system or hormone glands. envy and of traits such as creativeness, inquisi- Unlike simple control, the elements of a con- tiveness, self-awareness, and responsibility. trol circuit can work rather imprecisely This goes far beyond the scope of physiology— without causing a deviation from the set point which in the narrower sense is the study of the (at least on average). Moreover, control circuits 4 functions of the body—and, hence, of this book. are capable of responding to unexpected dis- Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  17. 17. C. Control circuit Set point value Prescribed set point Controller Actual value = set point ? Negative feedback Control and Regulation I Control signal Actual value Control element 1 Control Sensor element 2 Control Plate 1.2 element n Controlled 1 system Control circuit: principle Disturbance Set point Actual pressure = set point ? Autonomic nervous system Circulatory centers Nerve IX Nerve X Presso- Arterioles sensors Heart rate Venous return Peripheral resistance Blood pressure 2 Control circuit: blood pressure Orthostasis etc. 5Despopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  18. 18. turbances. In the case of blood pressure regu- tends the settling time ( E, subject no. 3) and lation ( C2), for example, the system can re- can lead to regulatory instability, i.e., a situa- spond to events such as orthostasis ( p. 204) tion where the actual value oscillates back and or sudden blood loss. forth between extremes (unstable oscillation, The type of control circuits described above E, subject no. 4). keep the controlled variables constant when Oscillation of a controlled variable in re- disturbance variables cause the controlled sponse to a disturbance variable can be at- variable to deviate from the set point ( D2). tenuated by either of two mechanisms. First, Within the body, the set point is rarely invaria- sensors with differential characteristics (D1 Fundamentals and Cell Physiology ble, but can be “shifted” when requirements of sensors) ensure that the intensity of the sensor higher priority make such a change necessary. signal increases in proportion with the rate of In this case, it is the variation of the set point deviation of the controlled variable from the that creates the discrepancy between the set point ( p. 312 ff.). Second, feedforward nominal and actual values, thus leading to the control ensures that information regarding the activation of regulatory elements ( D3). expected intensity of disturbance is reported Since the regulatory process is then triggered to the controller before the value of the con- by variation of the set point (and not by distur- trolled variable has changed at all. Feedfor- bance variables), this is called servocontrol or ward control can be explained by example of servomechanism. Fever ( p. 224) and the ad- physiologic thermoregulation, a process in justment of muscle length by muscle spindles which cold receptors on the skin trigger coun- and γ-motor neurons ( p. 316) are examples terregulation before a change in the controlled of servocontrol. value (core temperature of the body) has actu- In addition to relatively simple variables ally occurred ( p. 224). The disadvantage of such as blood pressure, cellular pH, muscle having only D sensors in the control circuit can length, body weight and the plasma glucose be demonstrated by example of arterial pres- concentration, the body also regulates com- sosensors (= pressoreceptors) in acute blood plex sequences of events such as fertilization, pressure regulation. Very slow but steady pregnancy, growth and organ differentiation, changes, as observed in the development of as well as sensory stimulus processing and the arterial hypertension, then escape regulation. motor activity of skeletal muscles, e.g., to In fact, a rapid drop in the blood pressure of a maintain equilibrium while running. The regu- hypertensive patient will even cause a coun- latory process may take parts of a second (e.g., terregulatory increase in blood pressure. purposeful movement) to several years (e.g., Therefore, other control systems are needed to the growth process). ensure proper long-term blood pressure regu- In the control circuits described above, the lation. controlled variables are kept constant on aver- age, with variably large, wave-like deviations. The sudden emergence of a disturbance varia- ble causes larger deviations that quickly nor- malize in a stable control circuit ( E, test sub- ject no. 1). The degree of deviation may be slight in some cases but substantial in others. The latter is true, for example, for the blood glucose concentration, which nearly doubles after meals. This type of regulation obviously functions only to prevent extreme rises and falls (e.g., hyper- or hypoglycemia) or chronic deviation of the controlled variable. More pre- cise maintenance of the controlled variable re- quires a higher level of regulatory sensitivity 6 (high amplification factor). However, this ex- Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  19. 19. D. Control circuit response to disturbance or set point (SP) deviation SP Controller SP Controller SP Controller Sensor Sensor Sensor Controlled Controlled Controlled system Disturbance system Disturbance system Disturb- ance Control and Regulation II Set point Actual value Time Time Time 1 Stable control 2 Strong disturbance 3 Large set point shift E. Blood pressure control after suddenly standing erect 80 Subject 1 Plate 1.3 75 Quick and complete 70 return to baseline 65 100 Subject 2 90 Slow and incomplete adjustment (deviation from set point) Mean arterial pressure (mmHg) 80 100 Subject 3 90 80 Fluctuating adjustment 70 110 Subject 4 100 90 Unstable control 80 10 20 30 40 50 60 70 80 s Reclining Standing 7 (After A. Dittmar & K. Mechelke)Despopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  20. 20. inal. In cell division, this process is the means The Cell by which duplication of genetic information The cell is the smallest functional unit of a (replication) is achieved. living organism. In other words, a cell (and no Messenger RNA (mRNA) is responsible for smaller unit) is able to perform essential vital code transmission, that is, passage of coding functions such as metabolism, growth, move- sequences from DNA in the nucleus (base ment, reproduction, and hereditary transmis- sequence) for protein synthesis in the cytosol sion (W. Roux) ( p. 4). Growth, reproduction, (amino acid sequence) ( C1). mRNA is and hereditary transmission can be achieved formed in the nucleus and differs from DNA in1 Fundamentals and Cell Physiology by cell division. that it consists of only a single strand and that Cell components: All cells consist of a cell it contains ribose instead of deoxyribose, and membrane, cytosol or cytoplasm (ca. 50 vol.%), uracil (U) instead of thymine. In DNA, each and membrane-bound subcellular structures amino acid (e.g., glutamate, E) needed for known as organelles ( A, B). The organelles of synthesis of a given protein is coded by a set of eukaryotic cells are highly specialized. For in- three adjacent bases called a codon or triplet stance, the genetic material of the cell is con- (C–T–C in the case of glutamate). In order to centrated in the cell nucleus, whereas “diges- transcribe the DNA triplet, mRNA must form a tive” enzymes are located in the lysosomes. complementary codon (e.g., G–A–G for gluta- Oxidative ATP production takes place in the mate). The relatively small transfer RNA mitochondria. (tRNA) molecule is responsible for reading the The cell nucleus contains a liquid known codon in the ribosomes ( C2). tRNA contains as karyolymph, a nucleolus, and chromatin. a complementary codon called the anticodon Chromatin contains deoxyribonucleic acids for this purpose. The anticodon for glutamate (DNA), the carriers of genetic information. Two is C–U–C ( E). strands of DNA forming a double helix (up to RNA synthesis in the nucleus is controlled 7 cm in length) are twisted and folded to form by RNA polymerases (types I–III). Their effect chromosomes 10 µm in length. Humans nor- on DNA is normally blocked by a repressor pro- mally have 46 chromosomes, consisting of 22 tein. Phosphorylation of the polymerase oc- autosomal pairs and the chromosomes that curs if the repressor is eliminated (de-repres- determine the sex (XX in females, XY in males). sion) and the general transcription factors at- DNA is made up of a strand of three-part tach to the so-called promoter sequence of the molecules called nucleotides, each of which DNA molecule (T–A–T–A in the case of poly- consists of a pentose (deoxyribose) molecule, a merase II). Once activated, it separates the two phosphate group, and a base. Each sugar strands of DNA at a particular site so that the molecule of the monotonic sugar–phosphate code on one of the strands can be read and backbone of the strands (. . .deoxyribose – transcribed to form mRNA (transcription, phosphate–deoxyribose. . .) is attached to one C1a, D). The heterogeneous nuclear RNA of four different bases. The sequence of bases (hnRNA) molecules synthesized by the poly- represents the genetic code for each of the merase have a characteristic “cap” at their 5! roughly 100 000 different proteins that a cell end and a polyadenine “tail” (A–A–A–. . .) at the produces during its lifetime (gene expression). 3! end ( D). Once synthesized, they are im- In a DNA double helix, each base in one strand mediately “enveloped” in a protein coat, yield- of DNA is bonded to its complementary base in ing heterogeneous nuclear ribonucleoprotein the other strand according to the rule: adenine (hnRNP) particles. The primary RNA or pre- (A) with thymine (T) and guanine (G) with cy- mRNA of hnRNA contains both coding tosine (C). The base sequence of one strand of sequences (exons) and non-coding sequences the double helix ( E) is always a “mirror (introns). The exons code for amino acid image” of the opposite strand. Therefore, one sequences of the proteins to be synthesized, strand can be used as a template for making a whereas the introns are not involved in the new complementary strand, the information coding process. Introns may contain 100 to 8 content of which is identical to that of the orig- 10 000 nucleotides; they are removed from the Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  21. 21. A. Cell organelles (epithelial cell) Tight junction Cell membrane Cytosol Cytoskeleton Lysosome Smooth ER Golgi vesicle Rough ER Mitochondrion Golgi complex Nucleus Chromatin Vacuole The Cell I Nucleolus B. Cell structure (epithelial cell) in electron micrograph Plate 1.4 Cell membrane Brush border 1 µm Vacuole Tight junction Free ribosomes Cell border Mitochondria Lysosomes Rough endoplasmic reticulum Autophagosome Golgi complex Basal labyrinth (with cell membranes) Basal membrane 9 Photo: W. PfallerDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  22. 22. primary mRNA strand by splicing ( C1b, D) brane of the endoplasmic reticulum (ER), and then degraded. The introns, themselves, which is described below ( F). contain the information on the exact splicing The mRNA exported from the nucleus site. Splicing is ATP-dependent and requires travels to the ribosomes ( C1), which either the interaction of a number of proteins within float freely in the cytosol or are bound to the a ribonucleoprotein complex called the cytosolic side of the endoplasmic reticulum, as spliceosome. Introns usually make up the lion’s described below. Each ribosome is made up of share of pre-mRNA molecules. For example, dozens of proteins associated with a number they make up 95% of the nucleotide chain of of structural RNA molecules called ribosomal1 Fundamentals and Cell Physiology coagulation factor VIII, which contains 25 in- RNA (rRNA). The two subunits of the ribosome trons. mRNA can also be modified (e.g., are first transcribed from numerous rRNA through methylation) during the course of genes in the nucleolus, then separately exit the posttranscriptional modification. cell nucleus through the nuclear pores. As- RNA now exits the nucleus through nuc- sembled together to form a ribosome, they lear pores (around 4000 per nucleus) and en- now comprise the biochemical “machinery” ters the cytosol ( C1c). Nuclear pores are for protein synthesis (translation) ( C2). Syn- high-molecular-weight protein complexes thesis of a peptide chain also requires the pres- (125 MDa) located within the nuclear en- ence of specific tRNA molecules (at least one velope. They allow large molecules such as for each of the 21 proteinogenous amino transcription factors, RNA polymerases or cy- acids). In this case, the target amino acid is toplasmic steroid hormone receptors to pass bound to the C–C–A end of the tRNA molecule into the nucleus, nuclear molecules such as (same in all tRNAs), and the corresponding an- mRNA and tRNA to pass out of the nucleus, and ticodon that recognizes the mRNA codon is lo- other molecules such as ribosomal proteins to cated at the other end ( E). Each ribosome travel both ways. The (ATP-dependent) pas- has two tRNA binding sites: one for the last in- sage of a molecule in either direction cannot corporated amino acid and another for the one occur without the help of a specific signal that beside it (not shown in E). Protein synthesis guides the molecule into the pore. The above- begins when the start codon is read and ends mentioned 5! cap is responsible for the exit of once the stop codon has been reached. The ri- mRNA from the nucleus, and one or two bosome then breaks down into its two sub- specific sequences of a few (mostly cationic) units and releases the mRNA ( C2). Ribo- amino acids are required as the signal for the somes can add approximately 10–20 amino entry of proteins into the nucleus. These acids per second. However, since an mRNA sequences form part of the peptide chain of strand is usually translated simultaneously by such nuclear proteins and probably create a many ribosomes (polyribosomes or polysomes) peptide loop on the protein’s surface. In the at different sites, a protein is synthesized much case of the cytoplasmic receptor for glucocor- faster than its mRNA. In the bone marrow, for ticoids ( p. 278), the nuclear localization sig- example, a total of around 5 1014 hemoglobin nal is masked by a chaperone protein (heat copies containing 574 amino acids each are shock protein 90, hsp90) in the absence of the produced per second. glucocorticoid, and is released only after the The endoplasmic reticulum (ER, C, F) hormone binds, thereby freeing hsp90 from plays a central role in the synthesis of proteins the receptor. The “activated” receptor then and lipids; it also serves as an intracellular Ca2+ reaches the cell nucleus, where it binds to store ( p. 17 A). The ER consists of a net-like specific DNA sequences and controls specific system of interconnected branched channels genes. and flat cavities bounded by a membrane. The The nuclear envelope consists of two mem- enclosed spaces (cisterns) make up around 10% branes (= two phospholipid bilayers) that of the cell volume, and the membrane com- merge at the nuclear pores. The two mem- prises up to 70% of the membrane mass of a branes consist of different materials. The ex- cell. Ribosomes can attach to the cytosolic sur-10 ternal membrane is continuous with the mem- face of parts of the ER, forming a rough endo- Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  23. 23. C. Transcription and translation Genomic DNA Nucleus Cytoplasm Nuclear pore RNA polymerase Transcription factors and signal RNA 5’ a 2 Translation in ribosomes Transcription Primary RNA mRNA Ribosome subunits 3’ b end Splicing The Cell II Stop tRNA mRNA amino acids c mRNA export Plate 1.5 d mRNA Growing Ribosomes Ribosome peptide chain breakdown tRNA amino acids1 Cytosolic Start Finished protein peptide chain e tRNA 5’ end Translation amino acids Ribosomes Membrane-bound and export proteins Control (cf. Plate F.) D. Transcription and splicing E. Protein coding in DNA and RNA Coding for amino acid no. ... 3’ 5’ 1–15 16 – 44 45 – 67 DNA T A A A A T G C T C T CGenomicDNA Codogen Transcription Transcription and Splicing 5’ 3’ Export from nucleusPrimary end Exon Intron endRNA(hnRNA) 5’ 3’ mRNA A U U U U A C G A G A G Codon Anti- 3’-poly-A tail Reading direction codon C U C A 5’ A A A A Ribosome cap tRNAGlu C 5’ C Splicing Introns Protein A A NH2 Ile Leu Arg Glu A A A AmRNA 11 1 15 44 67 Growth of peptide chainDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  24. 24. plasmic reticulum (RER). These ribosomes syn- Hence, the Golgi apparatus represents a thesize export proteins as well as transmem- central modification, sorting and distribution brane proteins ( G) for the plasma mem- center for proteins and lipids received from the brane, endoplasmic reticulum, Golgi appara- endoplasmic reticulum. tus, lysosomes, etc. The start of protein synthe- Regulation of gene expression takes place sis (at the amino end) by such ribosomes (still on the level of transcription ( C1a), RNA unattached) induces a signal sequence to modification ( C1b), mRNA export ( C1c), which a signal recognition particle (SRP) in the RNA degradation ( C1d), translation ( C1e), cytosol attaches. As a result, (a) synthesis is modification and sorting ( F,f), and protein1 Fundamentals and Cell Physiology temporarily halted and (b) the ribosome (me- degradation ( F,g). diated by the SRP and a SRP receptor) attaches The mitochondria ( A, B; p. 17 B) are the to a ribosome receptor on the ER membrane. site of oxidation of carbohydrates and lipids to After that, synthesis continues. In export pro- CO2 and H2O and associated O2 expenditure. tein synthesis, a translocator protein conveys The Krebs cycle (citric acid cycle), respiratory the peptide chain to the cisternal space once chain and related ATP synthesis also occur in synthesis is completed. Synthesis of membrane mitochondria. Cells intensely active in meta- proteins is interrupted several times (depend- bolic and transport activities are rich in mito- ing on the number of membrane-spanning chondria—e.g., hepatocytes, intestinal cells, domains ( G2) by translocator protein clo- and renal epithelial cells. Mitochondria are en- sure, and the corresponding (hydrophobic) closed in a double membrane consisting of a peptide sequence is pushed into the phos- smooth outer membrane and an inner mem- pholipid membrane. The smooth endoplasmic brane. The latter is deeply infolded, forming a reticulum (SER) contains no ribosomes and is series of projections (cristae); it also has im- the production site of lipids (e.g., for lipo- portant transport functions ( p. 17 B). Mito- proteins, p. 254 ff.) and other substances. chondria probably evolved as a result of sym- The ER membrane containing the synthesized biosis between aerobic bacteria and anaerobic membrane proteins or export proteins forms cells (symbiosis hypothesis). The mitochondrial vesicles which are transported to the Golgi ap- DNA (mtDNA) of bacterial origin and the paratus. double membrane of mitochondria are relicts The Golgi complex or Golgi apparatus ( F) of their ancient history. Mitochondria also has sequentially linked functional compart- contain ribosomes which synthesize all pro- ments for further processing of products from teins encoded by mtDNA. the endoplasmic reticulum. It consists of a cis- Lysosomes are vesicles ( F) that arise from Golgi network (entry side facing the ER), the ER (via the Golgi apparatus) and are in- stacked flattened cisternae (Golgi stacks) and a volved in the intracellular digestion of macro- trans-Golgi network (sorting and distribution). molecules. These are taken up into the cell Functions of the Golgi complex: either by endocytosis (e.g., uptake of albumin ! polysaccharide synthesis; into the renal tubules; p. 158) or by phagocy- ! protein processing (posttranslational modi- tosis (e.g., uptake of bacteria by macrophages; fication), e.g., glycosylation of membrane pro- p. 94 ff.). They may also originate from the teins on certain amino acids (in part in the ER) degradation of a cell’s own organelles (auto- that are later borne as glycocalyces on the ex- phagia, e.g., of mitochondria) delivered inside ternal cell surface (see below) and γ-carboxy- autophagosomes ( B, F). A portion of the en- lation of glutamate residues ( p. 102 ); docytosed membrane material recycles (e.g., ! phosphorylation of sugars of glycoproteins receptor recycling in receptor-mediated en- (e.g., to mannose-6-phosphate, as described docytosis; p. 28). Early and late endosomes below); are intermediate stages in this vesicular trans- ! “packaging” of proteins meant for export port. Late endosomes and lysosomes contain into secretory vesicles (secretory granules), the acidic hydrolases (proteases, nucleases, li- contents of which are exocytosed into the ex- pases, glycosidases, phosphatases, etc., that12 tracellular space; see p. 246, for example. are active only under acidic conditions). The Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.
  25. 25. F. Protein synthesis, sorting, recycling, and breakdown Nucleus Cytosol Transcription mRNA Free ribosomes Cytosolic ER-bound proteins ribosomes Protein and lipid synthesis The Cell III Mitochondrion Endoplasmatic reticulum (ER) Plate 1.6 cis-Golgi network Auto- Protein and lipid modification Golgi stacks phagosome Micro- tubule f g Sorting trans-Golgi Breakdown network of macro- Protein breakdown molecules M6P receptor Recycling Lysosome Secretory Late vesicle endosome Signal Early endosome Cytosol Phagocytosis Protein inclusion in cell membrane Extra- Recycling cellular of receptors space Clathrin Exocytose Endocytosis ControlledBacterium protein Constitutive secretion secretion 13 ControlDespopoulos, Color Atlas of Physiology © 2003 ThiemeAll rights reserved. Usage subject to terms and conditions of license.
  26. 26. membrane contains an H+-ATPase that creates below. Cholesterol (present in both layers) re- an acidic (pH 5) interior environment within duces both the fluidity of the membrane and the lysosomes and assorted transport proteins its permeability to polar substances. Within that (a) release the products of digestion (e.g., the two-dimensionally fluid phospholipid amino acids) into the cytoplasm and (b) ensure membrane are proteins that make up 25% (my- charge compensation during H+ uptake (Cl– elin membrane) to 75% (inner mitochondrial channels). These enzymes and transport pro- membrane) of the membrane mass, depend- teins are delivered in primary lysosomes from ing on the membrane type. Many of them span the Golgi apparatus. Mannose-6-phosphate the entire lipid bilayer once ( G1) or several1 Fundamentals and Cell Physiology (M6 P) serves as the “label” for this process; it times ( G2) (transmembrane proteins), binds to M6 P receptors in the Golgi membrane thereby serving as ion channels, carrier pro- which, as in the case of receptor-mediated en- teins, hormone receptors, etc. The proteins are docytosis ( p. 28 ), cluster in the membrane anchored by their lipophilic amino acid resi- with the help of a clathrin framework. In the dues, or attached to already anchored proteins. acidic environment of the lysosomes, the Some proteins can move about freely within enzymes and transport proteins are separated the membrane, whereas others, like the anion from the receptor, and M6 P is dephosphory- exchanger of red cells, are anchored to the cy- lated. The M6 P receptor returns to the Golgi toskeleton. The cell surface is largely covered apparatus (recycling, F). The M6 P receptor by the glycocalyx, which consists of sugar no longer recognizes the dephosphorylated moieties of glycoproteins and glycolipids in proteins, which prevents them from returning the cell membrane ( G1,4) and of the extra- to the Golgi apparatus. cellular matrix. The glycocalyx mediates cell– Peroxisomes are microbodies containing cell interactions (surface recognition, cell enzymes (imported via a signal sequence) that docking, etc.). For example, components of the permit the oxidation of certain organic glycocalyx of neutrophils dock onto en- molecules (R-H2), such as amino acids and dothelial membrane proteins, called selectins fatty acids: R-H2 + O2 R + H2O2. The peroxi- ( p. 94). somes also contain catalase, which transforms The cytoskeleton allows the cell to maintain 2 H2O2 into O2 + H2O and oxidizes toxins, such and change its shape (during cell division, etc.), as alcohol and other substances. make selective movements (migration, cilia), Whereas the membrane of organelles is re- and conduct intracellular transport activities sponsible for intracellular compartmentaliza- (vesicle, mitosis). It contains actin filaments as tion, the main job of the cell membrane ( G) well as microtubules and intermediate fila- is to separate the cell interior from the extra- ments (e.g., vimentin and desmin filaments, cellular space ( p. 2). The cell membrane is a neurofilaments, keratin filaments) that extend phospholipid bilayer ( G1) that may be either from the centrosome. smooth or deeply infolded, like the brush border or the basal labyrinth ( B). Depending on the cell type, the cell membrane contains variable amounts of phospholipids, cholesterol, and glycolipids (e.g., cerebrosides). The phos- pholipids mainly consist of phosphatidylcho- line ( G3), phosphatidylserine, phosphati- dylethanolamine, and sphingomyelin. The hy- drophobic components of the membrane face each other, whereas the hydrophilic com- ponents face the watery surroundings, that is, the extracellular fluid or cytosol ( G4). The lipid composition of the two layers of the membrane differs greatly. Glycolipids are14 present only in the external layer, as described Despopoulos, Color Atlas of Physiology © 2003 Thieme All rights reserved. Usage subject to terms and conditions of license.