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What is physiology!

This chapter aims to explain:

  • The subject matter of physiology

  • The hierarchical organisation of the body

  • The concept of homeostasis

What a piece of work is a man! How noble in reason! How infinite in faculty! In form, in moving, how express and admirable! ... The paragon of animals!

William Shakespeare Hamlet Act 2

What is physiology $

1.1 Introduction

Physiology is the study of the functions of living matter. It is concerned with how an organism performs its varied activities: how it feeds, how it moves, how it adapts to changing cir­cumstances, how it spawns new generations. The subject is vast and embraces the whole of creation. The success of physiology in explaining how organisms perform their daily tasks is based on the notion that they are intricate and exquisite machines whose operation is governed by the laws of physics and chemistry. Although some processes are similar across the whole spectrum of biology—the replication of the genetic code for example— many are specific to particular groups of organisms. For this reason it is necessary to divide the subject into various parts such as bacterial physiology, plant physiology, and animal physio­logy. The focus of this book is the physiology of mammals, particularly that of man.

To study how an animal works it is first necessary to know how it is built. A full appreciation of the physiology of an organ­ism must therefore be based on a sound knowledge of its anatomy. Experiments can then be carried out to establish how particular parts perform their functions. Although there have been many important physiological investigations on human volunteers, the need for precise control over the experimental conditions has meant that much of our present physiological knowledge has been derived from studies on other animals, such as frogs, rabbits, cats, and dogs. When it is clear that a specific physiological process has a common basis in a wide variety of animal species, it is reasonable to assume that the same prin­ciples will apply to man. The knowledge gained from this approach has given us a great insight into human physiology and endowed us with a solid foundation for the effective treatment of many diseases.

^ 1.2 The organization of the body

The building blocks of the body are the cells which are grouped together to form tissues. The principal types of tissue are: epithe­lial, connective, blood, lymphoid, nervous, and muscular, each with its own characteristics. Connective tissues have relatively

few cells embedded within an extensive extracellular matrix, while smooth muscle consists of densely packed layers of muscle cells linked together via specific cell junctions. Organs such as the brain, the heart, the lungs, the intestines, and the liver are formed by the aggregation of different kinds of tissue. The organs are themselves parts of distinct physiological systems so that the heart and blood vessels form the cardiovascular system; the lungs, trachea, and bronchi, together with the chest wall and diaphragm, form the respiratory system; the skeleton and skele­tal muscles form the musculo-skeletal system; the brain, spinal cord, autonomic nerves and ganglia, and peripheral somatic nerves form the nervous system, and so on.

Cells differ widely in form and function but they have certain common characteristics. First, they are bounded by a limiting membrane, the plasma membrane. Secondly, they have the ability to break down large molecules to smaller ones to liberate energy for their activities. Thirdly, at some point in their life history, they possess a nucleus which contains genetic informa­tion in the form of deoxyribonucleic acid (DNA). Further details of the fine structure of cells will be considered in Chapter 3.

Living cells continually transform materials. They break down glucose and fats to provide energy for other activities such as motility and the synthesis of proteins for growth and repair. These chemical changes are collectively called metabolism. The breakdown of large molecules to smaller ones is called catabolism and the synthesis of large molecules from smaller ones anabolism.

In the course of evolution, cells began to differentiate to serve different functions. Some developed the ability to contract (muscle cells), others to conduct electrical signals (nerve cells). A further group developed the ability to secrete different sub­stances, such as hormones (endocrine cells) or enzymes (e.g. the acinar cells of the salivary glands). During embryological development this process of differentiation is re-enacted as many different types of cell are formed from the fertilized egg. Most tissues contain a mixture of cell types. For example, blood con­sists of red cells which transport oxygen around the body, white cells which play an important role in the defense against infec­tion, and platelets which are vital components in the process of blood clotting. Connective tissue contains fibroblasts, macrophages, and mast cells. Nerve tissue contains nerve cells (of which there are many different kinds) and glial cells.

^ 1 What is physiology?

The principal organ systems

The cardiovascular system

The cells of large multicellular animals cannot derive the oxygen and nutrients they need directly from the external environment. These must be transported to the cells. This is one of the prin­cipal functions of the blood, which circulates within blood vessels by virtue of the pumping action of the heart. The heart, blood vessels, and associated tissues form the cardiovascular system. The heart consists of two pumps arranged side by side. The right side of the heart pumps the blood around the lungs where it absorbs oxygen from the air, while the left side pumps oxygenated blood around the rest of body to supply the tissues. Fluid exchanged between the blood plasma and the tissues passes into the lymphatic system which eventually drains into the blood.

^ The respiratory system

The energy required for performing the various activities of the body is ultimately derived from respiration. This process involves the oxidation of foodstuffs (principally sugars and fats) to release the energy they contain. The oxygen needed for this process is absorbed from the air in the lungs and carried to the tissues by the blood. The carbon dioxide produced as a result of the respiratory activity of the tissues is carried to the lungs by the venous blood to be lost in the expired air.

^ The digestive system or gastrointestinal tract

The nutrients needed by the body are derived from the diet. Food is taken in by the mouth and broken down into its com­ponent parts by enzymes in the gastrointestinal tract (or gut). The digestive products are then absorbed into the blood across the wall of the intestine and pass to the liver via the portal vein. The liver makes nutrients available to the tissues both for their growth and repair and for the production of energy.

^ The kidneys and urinary tract

In the course of metabolism non-volatile waste products are pro­duced which need to be eliminated from the body. This is one of the key functions of the kidneys. Another is the control of the composition of the extracellular fluid (the fluid which bathes the cells). To perform these functions, the kidneys produce urine which is temporarily stored in the bladder before voiding.

^ The reproductive system

Reproduction is one of the fundamental characteristics of living organisms. The gonads (the testes in the male and ovaries in the female) produce specialized sex cells known as gametes. At the core of sexual reproduction is the creation and fusion of rhe male and female gametes, the sperm and ova (eggs), with the result that the genetic characterisrics of rwo separate individuals are mixed to produce offspring which differ genetically from their parents.

^ The musculoskeletal system

This consists of the bones of the skeleton, skeletal muscles, and their associated tissues. Its primary function is to provide a means of movement which is required for locomotion, for the main­tenance of posture and for breathing. It also provides physical support for the internal organs.

^ The endocrine and nervous systems

The activities of the differenr organ systems need to be co­ordinated and regulated so that they act together to meet the needs of the body. Two coordinating systems have evolved: the nervous system and the endocrine system. The nervous system uses electrical signals to transmit information very rapidly to specific cells. Thus the nerves pass electrical signals to the skele­tal muscles to control their contraction. The endocrine system secretes chemical agents, hormones, which travel in the blood­stream to the cells upon which they exert a regulatory effect. Hormones play a major role in the regulation of many different organs and are particularly important in the regulation of the menstrual cycle and other aspects of reproduction.

^ The immune system

The immune system provides the body's defenses against infec­tion, both by killing invading organisms and by eliminating dis­eased or damaged cells.

Although it is helpful to study how each organ performs its functions, it is essential to recognize rhat the activity of the body as a whole is dependent on the intricate interactions between the various organ systems. If one part fails, the consequences are found in other organ systems throughout the whole body. For example, if the kidneys begin to fail, the regulation of the inter­nal environment is impaired, which in turn leads to disorders of function elsewhere.

1.3 Homeostasis

Complex mechanisms are at work to regulate the composition of the extracellular fluid, and individual cells have their own mechan­isms for regulating their internal composition. The regulatory mechanisms stabilize the internal environment despite variations borh in the external world and in the activity of the animal. The process of stabilization of the internal environment is called homeostasis and is essential if the cells of the body are to function normally.

To take one example, rhe beating of the heart depends on the rhythmical conrractions of cardiac muscle cells. This activity depends on electrical signals which, in turn, depend on the con­centration of sodium and potassium ions in the extracellular and intracellular fluids. If there is an excess of potassium in the extra­cellular fluid, the cardiac muscle cells become too excitable and may contract at inappropriate times rather than in a coordinated manner. Thus rhe concentration of potassium in the extracellular

1.3 Homeostasis

fluid must be kept within a narrow range if the heart is to beat normally.

How does the body regulate its own composition?

For each chemical constituent of the body there is a desirable concentration range which the control mechanisms are adapted to achieve. For example, the concentration of glucose in the plasma (the fluid part of the blood) is about 4-5 mmoles I-, between meals. If plasma glucose rises above this level (e.g. shortly after a meal) there is an increased secretion of the hormone insulin by the pancreas which acts to bring the con­centration down. If the concentration of glucose falls too low, the secretion of insulin falls. In each case, the changes in the cir­culating level of insulin act (together with other mechanisms) to restore the plasma glucose to the appropriate level.

This type of regulation is known as negative feedback. A nega­tive feedback loop is a control system that acts to maintain the level of some variable within a given range following a disturb­ance. Although the example given above refers to plasma glucose, the basic principle can be applied to other physiological variables such as body temperature, blood pressure, and the osmolality of the plasma.

A negative feedback loop requires a sensor of some kind that responds to the variable in question but not to other physio­logical variables. Thus an osmoreceptor should respond to changes in osmolality of the body fluids but not to changes in body temperature or blood pressure. The information from the sensor must be compared in some way to the desired level

(known as the 'set point' of the system) by some form of com­parator. If the two do not match, an error signal is transmitted to an effector, a system that can act to restore the variable to its desired level. The basic features of a negative feedback loop are summarized in Fig. 1.1. These features of negative feedback can be appreciated by examining a simple heating system. The controlled variable is room temperature which is sensed by a thermostat. The effector is a heater of some kind. When the room temperature falls below the set point, the temperature difference is detected by the thermostat which switches on the heater. This heats the room until the temperature reaches the pre-set level whereupon the heater is switched off.

Although negative feedback is the principal mechanism for maintaining a constant internal environment, it does have certain disadvantages. First, negative feedback control can only be exerted after the controlled variable has been disturbed. Secondly, the correction to be applied can only be assessed by the magnitude of the error signal (the difference between the desired value and the displaced value of the variable in question). In practice this means that negative feedback systems provide incomplete correction. Thirdly, overcorrection has the potential for causing oscillarions in the controlled variable. These dis­advantages are largely overcome in physiological systems by means of multiple regulation. In the example above, blood glucose is maintained within a narrow range by two mechanisms that act in opposition (push—pull). Insulin acts to lower plasma glucose while another pancreatic hormone, glucagon, acts to mobilize glucose from the body's stores.

While it is difficult to overemphasize the importance of negative feedback control loops in homeostatic mechanisms, they are frequently reset or overridden in stresses of various kinds. For example, arterial blood pressure is monitored by receptors known as baroreceptors which are found in the walls of the aortic arch and carotid sinus. These receptors are the sensors for a negative feedback loop that maintains the arterial blood pressure within close limits. If the blood pressure rises, compensatory changes occur that tend to restore it to normal. In exercise, however, this mechanism is reset. Indeed, if it were not, the amount of exercise we could undertake would be very limited.

Negative feedback loops operate to maintain a particular variable within a specific range. They are a stabilizing force in the economy of the body. In some circumstances, however, positive feedback occurs. In this case the feedback loop is inher­ently unstable as the error signal acts to increase the initial deviation. An example from everyday life is the howling that occurs when a microphone is placed near one of the loudspeakers of a public address system. The microphone picks up the initial sound and this is amplified by the electronic circuitry. This drives the loudspeaker to emit a louder sound which is again picked up by the microphone and amplified so that the loud­speaker makes an even louder sound and so on until the amplify­ing circuitry reaches the limit of its power—and the hearers run for cover!

1 What is physiology?

An example of the interaction between negative and positive feedback mechanisms is the hormonal regulation of the men­strual cycle. Cyclical alterations in the plasma levels of two hor­mones from the pituitary gland, known as follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are involved in the regulation of fertility. Steroid hormones from the ovaries can exert both negative and positive feedback control on the output of FSH and LH, depending upon the concentration of hor­mone present. Low or moderate levels of a hormone called estradiol-17/3, tend to inhibit secretion of FSH and LH (negative feedback). If, however, estradiol-17/3 is present in high con­centrations for several days, it stimulates the secretion of FSH and LH (positive feedback). As a result, there is a sharp increase

in the output of both FSH and LH just before mid-cycle. This rise is responsible for ovulation. Once ovulation has taken place, estrogen levels fall sharply and output of FSH and LH drops as negative feedback reasserts control.

Recommended reading

Houk, J.C. (1980). Homeostasis and control principles. In Medical physiology, (14th edn), (ed. V.B. Mountcastle), Chapter 8, pp. 246—267. Mosby, St Louis.

Paton, W.D.M. (1993). Man and mouse. Animals in medical research, (2nd edn). Oxford University Press, Oxford.


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