Physiology at a Glance

Jeremy P. T. Ward, Roger W. A. Linden

• Publisher: Wiley-Blackwell
• Year: 2017
• ISBN:
  9781119247319
• Edition: 4
• Topic: Medicina
• Subtopic: Teoría, práctica y referencia médicas

Extensively revised and updated, this fourth edition of Physiology at a Glance continues to provide a thorough introduction to human physiology, covering a wealth of topics in a comprehensive yet succinct manner.

This concise guide breaks this often complex subject down into its core components, dealing with structures of the body from the cellular level to composite systems. New to this edition are three chapters on cell signalling, thermoregulation, and altitude and aerospace physiology, as well as a glossary of terms to aid medical, dental, health science and biomedical students at all levels of their training.

Featuring clear, full-colour illustrations, memorable data tables, and easy-to-read text, Physiology at a Glance is ideal as both a revision guide and as a resource to assist basic understanding of key concepts.

Part 1 Introduction

Chapters

1. 1: Homeostasis and the physiology of proteins
2. 2: Body water compartments and physiological fluids
3. 3: Cells, membranes and organelles
4. 4: Membrane transport and ion channels
5. 5: Biological electricity
6. 6: Conduction of action potentials
7. 7: Cell signalling
8. 8: The autonomic nervous system
9. 9: Blood
10. 10: Platelets and haemostasis
11. 11: Defence: inflammation and immunity
12. 12: Principles of diffusion and flow
13. 13: Thermoregulation
14. 14: Altitude and aerospace physiology

1 Homeostasis and the physiology of proteins

Claude Bernard (1813–1878) first described ‘le mileau intérieur’ and observed that the internal environment of the body remained remarkably constant (or in equilibrium) despite the ever changing external environment. The term homeostasis was not used until 1929 when Walter Cannon first used it to describe this ability of physiological systems to maintain conditions within the body in a relatively constant state of equilibrium. It is arguably the most important concept in physiology.

Homeostasis is Greek for ‘staying the same’. However, this so-called equilibrium is not an unchanging state but is a dynamic state of equilibrium causing a dynamic constancy of the internal environment. This dynamic constancy arises from the variable responses caused by changes in the external environment. Homeostasis maintains most physiological systems and examples are seen throughout this book. The way in which the body maintains the H⁺ ion concentration of body fluids within narrow limits, the control of blood glucose by the release of insulin, and the control of body temperature, heart rate and blood pressure are all examples of homeostasis. The human body has literally thousands of control systems. The most intricate are genetic control systems that operate in all cells to control intracellular function as well as all extracellular functions. Many others operate within organs to control their function; others operate throughout the body to control interaction between organs. As long as conditions are maintained within the normal physiological range within the internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis and in turn, each cell contributes its share towards the maintenance of homeostasis. This reciprocal interplay provides continuity of life until one or more functional systems lose their ability to contribute their share. Moderate dysfunction of homeostasis leads to sickness and disease, and extreme dysfunction of homeostasis leads to death.

Negative feedback control

Most physiological control mechanisms have a common basic structure. The factor that is being controlled is called the variable. Homeostatic mechanisms provide the tight regulation of all physiological variables and the most common type of regulation is by negative feedback. A negative feedback system (Figure 1.1) comprises: detectors (often neural receptor cells) to measure the variable in question; a comparator (usually a neural assembly in the central nervous system) to receive input from the detectors and compare the size of the signal against the desired level of the variable (the set point); and effectors (muscular and/or glandular tissue) that are activated by the comparator to restore the variable to its set point. The term ‘negative feedback’ comes from the fact that the effectors always act to move the variable in the opposite direction to the change that was originally detected. Thus, when the partial pressure of CO₂ in blood increases above 5.3 kPa (40 mmHg), brain stem mechanisms increase the rate of ventilation to clear the excess gas, and vice versa when CO₂ levels fall below 5.3 kPa (Chapter 32). The term ‘set point’ implies that there is a single optimum value for each physiological variable; however, there is some tolerance in all physiological systems and the set point is actually a narrow range of values within which physiological processes will work normally (Figure 1.2). Not only is the set point not a point, but it can be reset in some systems according to physiological requirements. For instance, at high altitude, the low partial pressure of O₂ in inspired air causes the ventilation rate to increase. Initially, this effect is limited due to the loss of CO₂, but, after 2–3 days, the brain stem lowers the set point for CO₂ and allows ventilation to increase further, a process known as acclimatization (Chapter 14).

A common operational feature of all negative feedback systems is that they induce oscillations in the variable that they control (Figure 1.2). The reason for this is that it takes time for a system to detect and respond to a change in a variable. This delay means that feedback control always causes the variable to overshoot the set point slightly, activating the opposite restorative mechanism to induce a smaller overshoot in that direction, until the oscillations fall within the range of values that are optimal for physiological function. Normally, such oscillations have little visible effect. However, if unusually long delays are introduced into a system, the oscillations can become extreme. Patients with congestive heart failure sometimes show a condition known as Cheyne–Stokes’ breathing, in which the patient undergoes periods of deep breathing interspersed with periods of no breathing at all (apnoea). This is partly due to the slow flow of blood from the lungs to the brain, which causes a large delay in the detection of blood levels of CO₂.

Some physiological responses use positive feedback, causing rapid amplification. Examples include initiation of an action potential, where sodium entry causes depolarization which further increases sodium entry and thus more depolarization (Chapter 5), and certain hormonal changes, particularly in reproduction (Chapter 53). Positive feedback is inherently unstable, and requires some mechanism to break the feedback loop and stop the process (an off switch), such as time-dependent inactivation of sodium channels in the first example and the birth of the child in the second.

Protein form and function are protected by homeostatic mechanisms

The homeostatic mechanisms that are described in detail throughout this book have evolved to protect the integrity of the protein products of gene translation. Normal functioning of proteins is essential for life, and usually requires binding to other molecules, including other proteins. The specificity of this binding is determined by the three-dimensional shape of the protein. The primary structure of a protein is determi...

Table of contents

• Cover
• QR Code
• Title Page
• Copyright
• Preface
• Acknowledgements
• Abbreviations
• About the companion website
• Part 1: Introduction
• Part 2: Muscles
• Part 3: The cardiovascular system
• Part 4: The respiratory system
• Part 5: The renal system
• Part 6: The gut and metabolism
• Part 7: Endocrinology and reproduction
• Part 8: The sensory and motor systems
• Glossary
• Appendix I: Comparison of the properties of skeletal, cardiac and smooth muscle
• Appendix II: Normal physiological values
• Index
• EULA