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Physiology is defined as ‘the scientific study of the bodily function of living organisms and their parts'. There is a natural symbiosis between function (physiology) and structure (anatomy) from which physiology emerged as a separate discipline in the late 19th century. A good understanding of anatomy and physiology is an essential prerequisite for understanding what happens when things go wrong – the structural abnormalities and pathophysiology of disease – and as such underpins all biomedical studies and medicine itself. Following a century of reductionism, where the focus of research has progressively narrowed down to the function of individual proteins and genes, there is now a resurgence in integrative physiology, as it has been realized that to make sense of developments such as the Human Genome Project we have to understand body function as an integrated whole. This is considerably more complex than just the sum of its parts because of the multiplicity of interactions involved. True understanding of the role of a single gene, for example, can only be gained when placed in the context of the whole animal, as reflected by the often unexpected effects of knock-out of single genes on the phenotype of mice.
This volume is designed as a concise guide and revision aid to core topics in physiology, and should be useful to all students following a first-year physiology course, whether they are studying single honours, biomedical sciences, nursing, medicine or dentistry. It should also be useful to those studying system-based curricula. The layout of Physiology at a Glance follows that of the other volumes in the At a Glance series, with a two-page spread for each topic (loosely corresponding to a lecture), comprising a large diagram on one page and concise explanatory text on the other. For this fourth edition we have extensively revised the text and figures, there are three completely new chapters, on Cell signalling, Thermoregulation, and Altitude and aerospace physiology, and we have added a Glossary.
Physiology is a large subject, and in a book this size we cannot hope to cover anything but the core and basics. Physiology at a Glance should therefore be used primarily to assist basic understanding of key concepts and as an assistance to revision. Deeper knowledge should be gained by reference to full physiology and system textbooks, and in third-year honours programmes to original peer-reviewed papers. Students may find one or two sections of this book difficult, such as that on the physics of flow and diffusion, and detailed elements of cell signalling. Though such material may not be included in some introductory physiology courses, an understanding of these concepts can assist in learning how body systems behave in the way they do, and in understanding primary research papers.
In revising this fourth edition we have been helped immensely by constructive criticism and suggestions from our colleagues and students, and junior and senior reviewers of the last edition. We thank all those who have given us such advice; any errors are ours and not theirs. We would also like to thank the team at Wiley-Blackwell who provided great encouragement and support throughout the project.
Jeremy Ward
Roger Linden
Some figures in this book are taken or modified from:
Ward, J.P.T., Ward, J. and Leach, R.M. (2012) The Respiratory System at a Glance (4th edition). Wiley-Blackwell, Oxford.
Aaronson, P.I., Ward, J.P.T. and Connolly, M.J. (2012) The Cardiovascular System at a Glance (4th edition). Wiley-Blackwell, Oxford.
Mehta, A. and Hoffbrand, V. (2009) Haematology at a Glance (3rd edition). Wiley-Blackwell, Oxford.
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Chapters
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.
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.
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 determined by the sequence of amino acids (Figure 1.3). Genetic mutations that alter this sequence can have profound effects on the functionality of the final molecule. Such gene polymorphisms are the basis of many genetically based disorders. The final shape of the molecule (the tertiary structure), however, results from a process of folding of the amino acid chain (Figure 1.4). Folding is a complex process by which a protein achieves its lowest energy conformation. It is determined by electrochemical interactions between amino acid side-chains (e.g. hydrogen bonds, van der Waals’ forces), and is so vital that it is overseen by molecular chaperones, such as the heat shock proteins, which provide a quiet space within which the protein acquires its final shape. In healthy tissue, cells can detect and destroy misfolded proteins, the accumulation of which damages cells and is responsible for various pathological conditions, including Alzheimer's disease and Creutzfeldt–Jakob disease. Folding ensures that the functional sequences of amino acids (domains) that form, e.g. binding sites for other molecules or hydrophobic segments for insertion into a membrane, are properly orientated to allow the protein to serve its function.
The relatively weak nature of the forces that cause folding renders them sensitive to changes in the environment surrounding the protein. Thus, alterations in acidity, osmotic potential, concentrations of specific molecules/ions, temperature or even hydrostatic pressure can modify the tertiary shape of a protein and change its interactions with other molecules. These modifications are usually reversible and are exploited by some proteins to detect alterations in the internal or external environments. For instance, nerve cells that respond to changes in CO2 (chemoreceptors; Chapter 32) possess ion channel proteins (Chapter 4) that open or close to generate electrical signals (Chapter 5) when the acidity of the medium surrounding the receptor (CO2 forms an acid in solution) alters by more than a certain amount. However, there are limits to the degree of fluctuation in the internal environment that can be tolerated by proteins before their shape alters so much that they become non-functional or irreversibly damaged, a process known as denaturation (this is what happens to egg-white proteins in cooking). Homeostatic systems prevent such conditions from arising within the body, and thus preserve protein functionality.
Osmosis is the passive movement of water across a semi-permeable membrane from regions of low solute concentration to those of higher solute concentration. Biological membranes are semi-permeable in that they usually allow the free movement of water but restrict the movement of solutes. The creation of osmotic gradients is the primary method for the movement of water in biological systems. This is why the osmotic potential (osmolality) of body fluids is closely regulated by a number of homeostatic mechanisms (Chapter 38). A fluid at the same osmotic potential as plasma is said to be isotonic; one at higher potential (i.e. more concentrated solutes) is hypertonic and one at lower potential is hypotonic. The osmotic potential depends on the number of osmotically active particles (molecules) per litre, irrespective of their identity. It is expressed in terms of osmoles, where 1 osmole equals 1 mole of particles, as osmolarity (osmol/L), or osmolality (osmol/kg H2O). The latter is preferred by physiologists as it is independent of temperature, though in physiological fluids the values are very similar. The osmolality of plasma is ∼290 mosmol/kg H2O, mostly due to dissolved ions and small molecules (e.g. glucose and urea). These diffuse easily across capillaries, and the crystalloid osmotic pressure they exert is therefore the same either side of the capillary wall. Proteins do not easily pass through capillary walls, and are responsible for the oncotic (or colloidal osmotic) pressure. This is much smaller than crystalloid osmotic pressure, but is critical for fluid transfer across capillary walls because it differs between plasma and interstitial fluid (Chapter 26). Oncotic pressure is expressed in terms of pressure, and in plasma is normally ∼25 mmHg. Maintenance of plasma osmolality is vital for regulation of blood volume (Chapter 25). Drinking fluids of differing osmotic potentials has distinct effects on the distribution of water between cells and extracellular fluid (Figure 2.2).
Water is the solvent in which almost all biological reactions take place (the other being membrane lipid), and so it is fitting that it accounts for some 50–70% of the body mass (i.e. about 40 L in a 70 kg person). The nature of biological membranes means that water moves freely within the body, but the materials dissolved in it do not. There are two major ‘fluid compartments’: the water within cells (intracellular fluid, ICF), which accounts for about 65% of the body total, and the water outside cells (extracellular fluid, ECF). These compartments are separated by the plasma membranes of the cells, and differ markedly in terms of the concentrations of the ions that are dissolved in them (Figure 2.1; Chapter 4). Approximately 65% of the ECF comprises the tissue fluid found between cells (interstitial fluid, ISF), and the rest is made up of the liquid component of blood (plasma). The barrier between these two fluids consists of the walls of the capillaries (Figure 2.1; Chapter 26).
Many critical biological events, including all bioelectrical signals (Chapter 5), depend on maintaining the composition of physiological fluids within narrow limits. Figure 2.1 shows the concentrations of ions in the three main fluid compartments. It should be noted that, within any one compartment, there must be electrical neutrality, i.e. the total number of positive charges must equal the total number of negative charges. The most important difference between ICF and ECF lies in the relative concentrations of cations. The K+ ion concentration is much higher inside the cell than in ECF, while the opposite is true for the Na+ ion concentration. Ca2+ and Cl− ion concentrations are also higher in ECF. The question arises as to how these differences come about, and how they are maintained. Ion channel proteins allow the cell to determine the flow of ions across its own membrane (Chapter 4). In most circumstances, relatively few channels are open so that the leakage of ions is low. There is, however, always a steady movement of ions across the membrane, with Na+ and K+ following their concentration gradients into and out of the cell, respectively. Uncorrected, the leak would eventually lead to the equalization of the compositions of the two compartments, effectively eliminating all bioelectrical signalling (Chapter 5). This is prevented by the activity of the Na+-K+ ATPase, or Na+ pump (Chapter 3). Of the other ions, most Ca2+ in the cell is transported actively either out of the cell or into the endoplasmic reticulum and mitochondria, leaving very low levels of free Ca2+ in ICF. Cl− ions are differentially distributed across the membrane by virtue of their negative charge. Intracellular proteins are negatively charged at physiological pH. These and other large anions that cannot cross the plasma membrane (e.g. phosphate, PO43−) are trapped within the cell and account for most of the anion content of ICF. Cl− ions, which can diffuse across the membrane through channels, are forced out of the cell by the charge on the fixed anions. The electrical force driving Cl− ions out of the cell is balanced by the chemical gradient driving them back in, a situation known as the Gibbs–Donnan equilibrium. Variations in the large anion content of cells mean that the concentration of Cl− ions in ICF can vary by a factor of 10 between cell types, being as high as 30 mM in cardiac myocytes, although lower values (around 5 mM) are more common.
The main difference between these fluids is that plasma contains more protein than does ISF (Figure 2.1). The plasma proteins (Chapter 9) are the only constituents of plasma that do not cross into ISF, although they are allowed to escape from capillaries in very specific circumstances (Chapter 11). The presence of impermeant proteins in the plasma exerts an osmotic force relative to ISF (plasma oncotic pressure; see previously) that almost balances the hydrostatic pressure imposed on the plasma by the action of the heart, which tends to force water out of the capillaries, so that there is a small net water movement out of the plasma into the interstitial space. The leakage is absorbed by the lymphatic system (Chapter 26). Transcellular fluid is the name given to fluids that do not contribute to any of the main compartments, but which are derived from them. It includes cerebrospinal fluid and exocrine secretions, particularly gastrointestinal secretions (Chapters 40–44), and has a collective volume of approximately 2 L.
The aqueous internal environment of the cell is separated from the aqueous external medium by an envelope of fat molecules (lipids) known as the plasma membrane. About half the cell is filled with cytosol, a viscous, protein-rich fluid between the internal structures. These consist of organelles which are themselves enclosed by lipid membranes, and components of the cytoskeleton such as microtubules and actin filaments which provide structural stability and the ability of the cell to change shape or move. The reticular appearance of the cell interior is due to organelles whose membranes are folded to maximize surface area. These include the rough endoplasmic reticulum and Golgi apparatus, which are involved in protein assembly, and the smooth endoplasmic reticulum which serves as a store for intracellular Ca2+ and is the major site of lipid production (Figure 3.1). The arrangement of structures within the cell is highly organized, but also dynamic; organelles and structures can be rearranged according to need and function (e.g. cell division or migration).
The nucleus (Figure 3.1) contains the chromosomes and nucleolus, a membrane-less structure responsible for production of ribosomes. Ribosomes translocate to the rough endoplasmic reticulum (giving it its appearance), where they are responsible for protein assembly. The endoplasmic reticulum and Golgi apparatus perform post-translational processing of new proteins. This includes trimming amino acid chains to the right length, protein folding, addition of polysaccharide chains (glycosylation) and identification of improperly folded proteins. These and other proteins for recycling are tagged with multiple ubiquitin molecules, allowing them to be recognized and destroyed by proteasomes (proteolytic protein complexes). Proteins are delivered from the Golgi apparatus to specific intracellular destinations. For example, receptor and structural proteins are sent to the membrane and digestive enzymes to lysosomes, and molecules for extracellular action are packaged into secretory vesicles. Lysosomes are small vesicles containing acid hydrolase enzymes which catabolize macromolecules. They work optimally at pH 5.0, and as cytosolic pH is ∼7.2, any leaking into the cytosol cannot attack the cell inappropriately. Lysosomes digest endocytosed, unwanted and defective proteins, thereby recycling raw materials and preventing accumulation of rubbish.
Membrane lipids (mostly phospholipids) comprise a hydrophilic (water-loving) head, with two short hydrophobic (water-repelling) fatty acid tails (Figure 3.2). In an aqueous medium they self-organize into a bilayer with the heads facing outwards and the tails inwards (Figure 3.2). They diffuse freely within each layer (lateral diffusion) so the membrane is fluid. The hydrophobic interior and hydrophilic exterior of the membrane means that lipid-soluble (hydrophobic) substances such as cholesterol incorporate into the membrane, whilst molecules with both hydrophobic and hydrophilic domains such as proteins can be tethered part in and part out of the membrane (the fluid mosaic model; Figure 3.2). Many such molecules provide signalling, transport or structural functions. The latter are provided by proteins such as spectrin, which binds to the inner layer and forms an attachment framework for the cytoskeleton. Lipid-soluble molecules such as O2 and CO2, and small molecules such as water and urea readily pass through the lipid bilayer. However, larger molecules such as glucose and polar (charged) molecules such as ions cannot, and their transport is mediated by transporter and ion channel membrane proteins (Chapter 4). Proteins and large particles can also be engulfed by membrane segments to form intracellular vesicles (endocytosis). Membrane components can diffuse laterally and move around the membrane. However, the cell can control exactly which proteins insert into which portion of the membrane. For example, cells lining the kidney tubules are polarized so that Na+–K+ ATPase transporters (Chapters 4 and 36) are located only on one side of the cell. Most cells are covered by a thin gel-like layer called the glycocalyx, containing glycoproteins and carbohydrate chains extending from the membrane and secreted proteins (Figure 3.2). It protects the membrane and also plays a role in cell function and cell–cell interactions.
Numerous membrane proteins are associated with cell signalling (see Chapter 7). These include enzymes bound to the inner surface (e.g. phospholipase), and transmembrane proteins such as receptors, transporters and ion channels (Figure 3.2) which penetrate the entire thickness of the bilayer. The intramembrane segments of such proteins are composed of hydrophobic amino acid residues whilst the extra- and intra-cellular portions predominantly contain hydrophilic residues. Other transmembrane proteins such as integrins and cadherins provide structural and signalling links with other cells and the extracellular matrix (Figure 3.2). Their cytosolic ends bind to components of the cytoskeleton, including protein kinases which can initiate or modulate processes such as gene transcription, cell growth or changes in cell shape.
Mitochondria use molecular oxygen to, in effect, burn sugar and small fatty acid molecules to produce adenosine triphosphate (ATP), which is used by all energy-requiring cellular reactions. Glucose is first converted to pyruvate in the cytosol by glycolysis, producing in the process a small net amount of ATP and reduced nicotinic adenine dinucleotide (NADH). Glycolysis does not require O2, so when O2 is limited, this anaerobic respiration can supply some ATP, with NADH being reoxidized to NAD+ by metabolism of the pyruvate to lactate (Figure 3.3). However, under normal conditions where there is sufficient O2, oxidative phosphorylation in the mitochondria produces ∼15-fold more ATP for each glucose molecule than does glycolysis. Pyruvate and fatty acids transported into the mitochondrial matrix act as substrates for enzymes that drive the citric acid (Krebs’) cycle, which generates NADH and the waste product CO2. The electron transport chain, a series of enzymes in the inner mitochondrial membrane, then uses molecular O2 to re-oxidize NADH to NAD+. In doing so, it generates a H+ ion gradient across the inner membrane which drives the ATP synthase (Figure 3.3). Note that mitochondria are not solely devoted to ATP production, as they are also involved in other cellular processes, including Ca2+ homeostasis and signalling. The mitochondria are also the major source of body heat production (see Chapter 13).