Getting Introduced to PhysiologyCHAPTER 1

The human body is made up of trillions of cells. In other words, human beings are multicellular organisms. They are very complex as compared to unicellular organisms, whose body has just one cell. But human cells have a lot of similarity with unicellular organisms. Therefore we can understand human physiology much better if we understand how a unicellular organism functions.

A living cell also burns fuel (we call it food) with the help of oxygen to get energy, and in the process produces carbon dioxide and water. Food also has nitrogenous organic compounds (proteins), the burning of which may produce ammonia in addition to carbon dioxide and water. Carbon dioxide, 2 water and ammonia are thrown out of the cell. One wonderful thing about the process of combustion in a cell is that it is carried out with the help of enzymes, which carry out the process in a step-wise fashion, so that the energy obtained from the fuel is not released all at once. That is why the energy can be used for life-processes, and does not burn the cell—it just makes the cell a little warm.

A unicellular organism is totally self-reliant. It lives in sea water. When it finds in the sea a particle which may be used as food, it throws pseudopodia around it, engulfs it into a vacuole, digests it, and burns it to get energy. The oxygen required for the process is obtained from the air dissolved in the sea water. It moves into the cell by diffusion. Waste products such as carbon dioxide and ammonia formed in the cell also move out of the cell by diffusion. Hence the waste products do not accumulate in the cell. The unicellular organism uses the food not only for producing energy but also for growth and repair. In addition, it also reproduces by dividing into two identical cells. Thus the single cell of a unicellular organism is fully equipped to carry out all the functions of life (Fig. 1.1).

A living cell is a wonderful machine. It uses such simple substances from the environment to do so much. It continues to function on its own for so long, it repairs its defects, and it survives through its progeny. A comparable machine is yet to be made by man.

For a unicellular organism, these conditions are easy to meet. Food and oxygen are available in the surrounding sea water. A little cell cannot consume much food and oxygen. Therefore the removal of some food and water by these tiny organisms cannot finish the food and oxygen in the sea. Similarly, dumping a little carbon dioxide or ammonia by these tiny organisms in the sea cannot pollute the sea enough to make it too poisonous for the organisms to live in it.³ The sea water is rather constant in its warmth.⁴ The sea water surrounding the unicellular organisms is so huge in quantity that its pH and osmolarity are not materially affected by the life processes of the tiny organisms. You might have observed that the key to the survival of the unicellular organism resides in the sea water, which forms its immediate environment. The environment provides the optimum conditions for its survival.

Our cells are not surrounded by sea water, but they do have a thin layer of fluid around them (Fig. 1.2). Thus our cells may not have the luxury of the sea, but each cell at least has a private pond of its own. The fluid surrounding the cells is called the interstitial fluid. The interstitial fluid forms the immediate environment of our cells, just as the sea forms the immediate environment of organisms like the ameba. It is interesting that the composition of the interstitial fluid is very similar to that of sea water. But here we have a problem. The quantity of interstitial fluid is so small that the cell would use up all the nutrients and oxygen in it in a very short time. The waste products would also quickly build up to toxic levels. Therefore it is essential that nutrients in the interstitial fluid be replenished, and waste products removed, continually and promptly. This has been made possible by having a set of tubes, which we call capillaries (Fig. 1.3). The fluid in the capillaries exchanges these substances with the interstitial fluid, and the interstitial fluid exchanges them with the cells. Thus the fluid in the capillaries (blood) acts as an extension of the immediate environment of the cells.

It may be noted that the heart sends blood to all parts of the body. Two important functions that the blood performs everywhere it goes are to supply nutrients and pick up waste products. But while passing through the lungs, the kidneys or the gut, it does something in addition: it picks up nutrients from, or gives up waste products to the external environment. In some of these organs (lungs and kidneys), blood discharges to the external environment waste products picked up from the internal environment of the cells (interstitial fluid) all over the body. By acting as a link between the external and the internal environment, the digestive, respiratory and excretory systems replenish the 5 nutrients used up by the body, and dispose of the waste products formed. The ultimate aim of these systems is to help achieve constancy in the characteristics of the thin layer of fluid surrounding every cell of the body. This fluid, the interstitial fluid, constitutes the immediate environment of the cells, or the internal environment of the body. The importance of the constancy of the internal environment (in French, milieu interieur) was first stressed by Claude Bernard in 1857. Claude Bernard was not just an eminent physiologist, but is also considered the father of experimental medicine. The concepts propounded by him were further supported by the extensive experimental work of Walter Cannon in the early twentieth century. Cannon coined the term homeostasis to describe the constancy of the internal environment (homoios, similar; stasis, position).

You might have observed that each system of the body makes a contribution to homeostasis. In turn, it benefits from the contributions which other systems make. For example, the gut replenishes nutrients in the interstitial fluid. In turn, it benefits from the oxygen that has been added to the interstitial fluid by the lungs and also from the removal of waste products by the kidneys. Thus the body is like a society in which there is specialization and division of labor. A health professional specializes in giving health care, and in turn depends for food on the farmer, for clothes on the tailor, and for shelter on the mason. Similarly, in the body each system has specialized in doing one thing for all cells of the body. In turn, cells belonging to other systems of the body fulfill all the remaining needs of this system. Our own body is an excellent example of cooperation and interdependence.

We have seen that the human body is essentially a society of cells. In a society, we also need an organizer or coordinator to see that different members of the society work in such a way that the interests of the society as a whole are well looked after. In the body, the function of co-ordination is performed by the nervous and endocrine systems. To give an example, when we take exercise, our requirement of oxygen goes up. To meet this additional demand, the heart beats faster, and the lungs work harder. How do the lungs know when to step up their activity? This is the job of the coordinating systems. These systems detect the changes resulting from exercise. These changes are contraction of muscles, movement of joints, slight fall in oxygen tension, slight increase in carbon dioxide tension, slight increase in temperature, etc. The coordinating systems process all this information, and then come up with a response. The response is to send messages to the heart and lungs to step-up their activity so that homeostasis is maintained in spite of additional oxygen requirement. Thus coordination has two basic components: communication and information processing (Fig. 1.6). First, there has to be a channel of communication with areas where changes are taking place. In the above example of exercise, the information from muscles, joints, etc. has to reach the coordinator through such a channel of communication. This information-gathering channel is called the sensory limb of the coordinator. Since information is coming from multiple sources, the coordinator has to process this information. Processing means putting together all this information, or synthesizing it, so as to understand what it means in terms of the disturbance in homoestasis which it can produce.

We have seen above that although the coordinating systems, i.e. the nervous and endocrine systems do not contribute directly to homeostasis, they make an indirect contribution to it. Their contribution is to make several other systems work towards homeostasis at the right time at the required level so that homeostasis is maintained in spite of changing circumstances. But there are still a few other systems to be considered. The musculoskeletal system also makes no direct contribution to homeostasis. But apart from giving a shape to the body, it enables the animal to move. Animals need to move to find food, to hunt their prey, and to escape from predators. The gut can replenish nutrients only if the animal finds food in the first place. Hence the musculoskeletal system also makes an essential, although indirect, contribution to homeostasis. Human beings may not find their food the way animals do, but move they also must to earn a living. Even highly civilized societies can support only a few paralyzed individuals and that too rather poorly. How about the skin? Apart from playing a major role in regulation of body temperature, skin is the first line of defense against a wide variety of dangerous microorganisms which our environment is full of. These microorganisms are unable to enter the body because the skin offers an effective barrier. Finally, let us have a look at the reproductive system. The reproductive system seems to contribute to homeostasis neither directly nor indirectly. But it has an important function without which homeostasis becomes meaningless. Reproduction is responsible for perpetuation of the species. No matter how effective the homeostasis, it is not perfect, and therefore every organism dies one day. Therefore, a species which cannot reproduce would soon vanish from the surface of the earth.

We have seen that every part of the body makes some contribution to the survival of the whole organism by doing something to maintain the composition, pH and temperature of the internal environment at an optimal level. The optimal level is that at which the enzyme systems function best, nutrients are available in quantity and quality commensurate with the activity of the tissue in question, and waste products remain well below toxic levels. As you go on to a detailed study of individual systems, keep in mind that the central purpose of all systems is to make a contribution to homeostasis. It is easy to get lost in details and forget this fundamental fact. But keeping this basic fact at the back of your mind will make the study of the details much easier, more interesting and more meaningful.

A similar strategy is adopted by the body to maintain homeostasis. For example, if a person loses blood by bleeding, the blood pressure falls. When this happens, the heart starts beating faster and more forcefully, and the blood vessels constrict. This raises the blood pressure towards normal. To take another example, if a person has just taken a meal, her blood glucose level starts rising. In response to the rise in blood glucose, the pancreas releases insulin. Insulin 7 lowers the blood glucose level. As the blood glucose level falls slightly below the desirable level, the secretion of insulin is switched off. If the person now does not eat for the next eight hours, not only will insulin secretion remain off, glucagon secretion will be switched on. Glucagon raises blood glucose. Thus blood glucose will be maintained at a reasonable level in spite of fasting or feasting. The mechanisms which prevent the blood glucose level from rising too high or falling too low are called the control system for regulating blood glucose. The control system for a variable maintains the value of the variable within a narrow range.

A control system (Fig. 1.7) is basically designed to maintain a controlled variable close to a set point. The value of the controlled variable is continuously monitored by a sensor. The current value of the controlled variable is conveyed by the sensor to the controller in the form of a feedback signal. The feedback signal and the set point constitute the inputs for the controller. The feedback signal is naturally affected by any disturbance which alters the value of the controlled variable.

Once a person is sick, recovery often depends on the same type of responses which operate in good health. For example, fever may be looked upon as the resetting of the thermostat of the body.⁷ Therefore when the temperature of a person starts rising towards fever, she may have shivering. This happens because the thermostat is set to a higher level (say, 40°C), and the body temperature is 37°C. Therefore the control system behaves as if the body is cold. It initiates responses which will take the body temperature towards the set point, which is now 40°C. This can be done by shivering, and therefore the person shivers. In the same way, when the person is recovering from fever, she may have sweating. Since the person has recovered, the hypothalamic thermostat is once again set at the normal level of 37°C. But the body temperature is still 40°C. The mismatch between the set point and the body temperature initiates responses for heat loss, such as sweating.

In light of the above discussion, we can keep ourselves healthy by adopting a two-pronged strategy. First, we should improve the capacity of our body to meet challenges. Secondly, we should try to minimize the extent to which the body is challenged. Both these measures are not mutually exclusive. However, of the two, the first is more important and practical. Adequate and appropriate nutrition, regular physical exercise, and mental peace are the three most important ingredients of the lifestyle that improves the fighting capacity of the body. Ancient disciplines, such as yoga, promote a healthy lifestyle. The second part of the strategy for staying healthy is to avoid needless challenges to the body. This may be done by avoiding inhalation and ingestion of germs, harmful chemicals and pollutants. Further, one should avoid overuse, misuse or abuse of any part of the body. For example, we should not overuse our back by trying to lift a 100 kg weight, misuse our eyes by spending long hours at the computer, or abuse our lungs by smoking.

Doing all this can keep us fairly healthy but is no guarantee against illness. In case of illness, a doctor or nurse usually takes one of the following four measures. First, she reassures the patient that soon everything will be fine. The reassurance is not hollow: it is based on her knowledge of homeostatic mechanisms. Secondly, she tells the patient not to be impatient. This is because she knows that homeostatic mechanisms take time. Thirdly, she suggests some measures which assist the body in its struggle against the disease. For example, she may suggest hot fomentation of an inflamed area so that the accelerated blood flow through the area may increase further and drain away undesirable substances. Or, she may prescribe antibiotics which may kill some disease-causing germs in addition to those killed by the defence mechanisms of the body. And finally, it may be discovered that the disease was due to a deficiency. It could be a dietary deficiency, or deficiency of some physiological substance produced by the body. In that case the doctor or nurse tries to supply the deficient substance in an effort to restore physiological function. For example, in iron deficiency, she gives iron; in water and salt deficiency (as in diarrhea), she gives water and salt; and in insulin deficiency (diabetes), she gives insulin. It is worth observing that in each of the four approaches outlined above, the doctor or nurse works with nature, not against it. And, each of the four approaches needs knowledge of physiology. Truly, physiology forms the basis of medicine.

However, there are a few situations in the body where positive feedback operates, and serves a useful purpose. For example, uterine contractions during labor push the fetus down towards the cervix. The downward movement of the fetus stretches the cervix. Stretching of the cervix stimulates uterine contraction. Uterine contraction pushes the fetus down. The fetus going down stretches the cervix. Stretching of the cervix stimulates uterine contraction, and so on. The sequence continues till the baby has been delivered. Here the downward movement of the fetus is a change from a ‘set position’ which has been brought about by uterine contraction (the disturbance). A negative feedback system would inhibit further uterine contraction, and would try to restore the fetus to the set position. But instead, here the response of the system is to stimulate contraction of the uterus, which in turn moves the fetus further away from its set position in the uterus. Therefore, this is a positive feedback system. But here it serves a purpose. The purpose of labor is to move the fetus towards the vaginal opening so that it can be delivered.