Physiology: The Science of the Body

by Ernest G. Martin

We have described the control of smooth muscles and glands through reflexes and in connection with the emotions. We have still to mention briefly a third way in which nervous disturbances leading to these organs may be originated. In the last chapter the brain was described as consisting of three chief parts: the cerebrum, which is the seat of memory and association, and so of all thought processes; the cerebellum, which governs the reflex of locomotion; and the brain stem about which nothing was said except that it is a region through which all nervous disturbances have to pass on their way into or out from the brain. The brain stem has a special function of its own in connection with the control of what are often called the “vital processes.” Since most of these vital processes are concerned with smooth muscle or gland action they are of interest to us here. In the brain stem are located groups of nerve cells to which are given the name of “centers.” From these extend nervous pathways to various bodily structures. For example, there are two centers having to do with the activity of the heart, and both of these have nerve pathways leading to that organ. Over one of them pass the discharges which cause the heart to beat more rapidly, over the other the discharges which slow it down. There are also centers for controlling the caliber of the various small blood vessels and so the amount of blood that flows through the various organs of the body. Still another center has control of the secretion of sweat. These are a few of the centers which are present in the brain stem. It is evident that these centers must be played upon directly from the sense organs in the reflex control of the smooth muscles and glands and also from the cerebrum in their emotional control. The third way in which these centers can be affected is by a direct action of the blood which circulates through them. Of course, every center is made up of nerve cells which are alive and which share with all other living cells the necessity of being sufficiently nourished. This means that there must be small blood vessels here and there among the nerve cells, so that interchanges of material can take place between the blood and the fluid immediately surrounding the cells. Among the materials which may come from the blood are some special chemical substances which have the ability to arouse these cells to activity. For example, the very rapid beating of the heart following vigorous muscular exercise is due, in part at least, to the presence of chemical substances that are poured out into the blood from the laboring muscles and which have the effect of arousing the center whose action speeds up the heart. In at least one case, that of the center controlling the sweat glands, the nerve cells are aroused by an increase in temperature; on a very hot day or when one exercises briskly the temperature of the blood begins to go up; this arouses the center which controls the secretion of sweat; nervous disturbances are poured out to the sweat glands and they become active. The effect of their activity is to remove heat from the body, and so help to lower its temperature. Here we have an example of an automatic regulating device in which nerve cells are controlled through the blood. Except for this one instance of the effect of blood temperature the actions of this class are thought to be wholly by means of chemical substances, and for that reason we speak commonly of the third method of control of the smooth muscles and glands as chemical control. We have then altogether three methods of control; through reflexes, through the emotions, and through chemical substances in the blood.

Since we have spoken of chemical control it will be appropriate to close this chapter with just a further word about it. In addition to the regulation of smooth muscle and gland activity there are a number of other bodily processes which are affected more or less through chemical agents brought to them by the blood. Many of these processes are of a kind that cannot well be controlled through the nervous system. For example, growth is something which goes on very slowly and yet which distinctly requires some kind of control. It has not perhaps occurred to us that it is more than chance that our ears are substantially the same size, our arms about the same length, our legs the same length, our feet about the same size, and so on. Yet if we pause a moment we realize that as the ears grow there would seem to be no particular reason why they should both stop when they have reached the same size, unless there is some definite regulation. This regulation is present in the form of chemical substances. Of late years a very large amount of study has been given to the materials which have the power of acting as chemical regulators, and a good deal has been learned about them, although there is every reason to believe that a great deal more remains to be found out. It has been necessary to invent a name for substances which act as chemical regulators, because, without such a name, talking about them becomes too cumbersome. The name that has been adopted is hormone, from a Greek word meaning to arouse or to stimulate, referring to the ability these substances have of acting upon living cells. Thus far we have spoken of hormones only in connection with their ability to excite those nerve cells by which smooth muscles and glands are controlled. As a matter of fact, their action is much wider than that. They have the power of affecting the metabolism of very many body cells. Some of them appear to have the special function of regulating the rate of metabolism. These we shall talk about in detail, when we return to the study of metabolism in a later chapter. One of the hormones is interesting in connection with the relation of emotions to smooth muscle and gland activity. This is a secretion manufactured by a pair of small glands located in the abdomen near the kidneys. They are known as the adrenal bodies. These glands secrete a substance to which has been given the name of adrenalin, which has been shown to be a very efficient chemical excitant to a number of bodily processes. We are particularly interested here in a property it has of arousing smooth muscles and glands in precisely the same way as they are aroused in connection with the disagreeable emotions, such as fright or anger. For example the injection of a little of this adrenalin into the veins of a person or animal will cause his heart beat to be quickened, the blood flow into the brain and the muscles to be increased at the expense of the flow through the skin and abdominal organs, and the other effects to occur that were described above as accompanying these emotions. These facts about the effects of adrenalin had been known for some time before it was realized that they fit in with the reaction by which the body prepares itself to operate efficiently in time of stress. Now we have learned to look upon adrenalin as the “emergency hormone,” meaning that it is a chemical substance which does the same things to the body that are done through the emotions in time of emergency. We look upon this as a reenforcement of the nervous action, indicating the efficiency with which the fitting of the body for time of stress works out. The adrenal bodies are themselves glands and are acted upon like other glands through the part of the nervous system that we have just been describing. In time of emergency a stream of nervous discharges pours into them; they secrete an abundant supply of adrenalin which passes out into the blood and is carried by the blood to all parts of the body, exerting its special functions wherever it comes. There is no doubt that the ability of the body to care for itself in time of need is helped by the outpouring of adrenalin. In one respect, however, this arrangement does not work out perfectly. This is because chemical substances that are poured out into the blood cannot be gotten rid of instantly, and are likely to linger for a shorter or longer time after the actual emergency subsides, so the bodily effects persist for a time after the occasion for them has disappeared. We are all familiar with the fact, for example, that the heart goes on thumping for a long time after a sudden shock. It may not have occurred to us to wonder what kept it going at that rate after the immediate disturbance was passed, but we now see that it must do so until the adrenalin which was poured out in connection with the shock has been gotten rid of, which is a more or less gradual process.

CHAPTER XIII
THE BODY FLUIDS

WE have learned to think of the cells which make up the body as dependent on the fluid which surrounds them for the various materials they require, and as a place into which they discharge the products of their metabolism. We have seen furthermore that the fluids which bathe the cells directly must be constantly renewed. The renewal is accomplished by interchanges between this fluid and the blood, which constantly flows through the tiny blood vessels that are everywhere present in the body. In its course, in turn, it passes through the blood vessels of the organs in which it is to be itself renewed; the digestive organs for food supplies, the lungs for oxygen, the kidneys for the discharge of waste material. We must now look further into the nature and action of the various body fluids. Of course the foundation of all of them is water. In this water must be dissolved everything that is used by any of the cells for food or anything that any of these cells produces. Under this latter head we have the waste products of ordinary metabolism, or in the case of some cells special products of functional metabolism. The presence of all these various materials would be bound to make the body fluid an extremely complex mixture. In addition to these various materials there are certain substances present besides water to make up what we may call the structure of the body fluids as distinguished from the materials which they are carrying from one place to another. These structural materials include a number of salts of which ordinary table salt (sodium chloride) is the most abundant as well as the most familiar. In addition there are salts of lime and potash and magnesium; all these latter in very small proportions. Just why the body fluids should contain these salts is not very clear. We know that if they were not present the cells of our bodies could not live, yet it is true that there are a great many kinds of living cells that get along perfectly well in fresh water, which may have no salts dissolved in it at all. On the other hand there are more kinds living in the ocean and exposed to the rather strong concentration of salts which make up ocean water, and there are even some kinds of animals that live in strong brine, so that evidently living protoplasm can adjust itself to surroundings in which the strength of the salt in solution is widely different. It is true that very few kinds of animals can endure being changed suddenly from ocean water to fresh water or the reverse. One of the best ways to clean the bottom of a ship that has become foul through long sailing about the sea is to transfer it into a fresh water lake or stream, where the accumulation of living animals and plants will be killed and will drop off. Most kinds of ocean animals die rather promptly if changed to fresh water, or fresh water animals if put into the ocean. There are a few kinds of fish, like the salmon and shad, which live in the ocean but lay their eggs in rivers, and these are able to endure the change from the one kind of water to the other without being destroyed. They, to be sure, make the transition gradually, swimming up from the ocean into water that is less and less salty, until they finally reach the fresh water stream itself. There are other kinds of living things which can endure a much more abrupt change from salt to fresh water and back again. In various parts of the world are large rivers emptying into the ocean and so situated relative to towns and cities that steamers make regular round trips from the ocean up to the fresh water of the river and back again, and it will be found that on the bottoms of these steamers are various plants and water animals which endure the frequent shift from salt water to fresh and back again without harm.

The percentage of salt in the body fluids of all the higher animals including ourselves is only about one-fifth that in the ocean. Furthermore the fluids of most kinds of land animals have about the same percentage of salts dissolved in them. Naturally there has been much speculation as to why there should be this percentage of salt in preference to any other. One ingenious theory supposes that back in the beginning of things, when the earth cooled down below the boiling point of water, so that it was possible for water to collect on the earth, the water in all the oceans was fresh. This would have to be true, since water must have fallen in the form of rain; but in course of time some of the salts in the earth’s crust would be dissolved, making the water salty, and as time went on the ocean would become saltier and saltier. This is still happening, for every river that discharges into the ocean carries with it materials that it has dissolved from the underlying soil during its passage from its source, and such material, when it once enters the ocean, must stay there until the ocean water becomes saturated with that particular substance. According to this theory there was a time, then, when the ocean water was just about as salty as our body fluids at the present time, and it supposes also that that was the time when the ancestors of present land animals crawled out of the ocean and took up their abode on land. Of course there is no way to prove that this is so, but it does account for the particular percentage of salts in our bodies as well as any other explanation we know anything about.

In addition to these various salts our body fluids contain in solution moderate amounts of very complex chemical substances belonging to the class of proteins. A fact about proteins which has not yet been emphasized is that they make liquids in which they are dissolved sticky or gelatinous. An excellent example of this is ordinary raw white of egg, which is a solution of protein in water, and which shows the gelatinous character very strikingly. Because of the protein that is in solution in the body fluids, they have also this gelatinous character, although to a much less extent than in the white of egg, because the amount of protein in solution is so much less. As we shall show later, this sticky quality of the fluid is of a good deal of importance in its actual use in the body. It may be that the protein in the body fluids serves other purposes as well. One interesting fact that needs to be emphasized is that it is not used for fuel or building material for the cells. The protein that comes to them as part of their food supply is entirely distinct from that about which we are now talking, which is part of the permanent structure of the body fluids. We shall speak of the protein that serves to nourish the cells in a moment, when we are talking about the relations of the fluids to the transportation of food material.

In addition to the salts and proteins we have also dissolved a great many very complex materials which may be looked upon as permanent or relatively permanent constituents of the fluid, but about which we know practically nothing chemically. We are sure that they are present, because of certain effects which they produce, but the substances themselves have never been made out by chemical analysis. These are materials which are concerned with the resistance of the body to infectious disease, and it will be necessary to say just a word about infection to make clear the part played by them.

What we call an infection is the invasion of the body from the outside by minute living organisms, either plant or animal, and the establishing of them within the body, so that they grow and multiply. They carry on their metabolism just as do all other living cells and produce various chemical products as a result. There are many organisms living within our bodies whose metabolic products apparently do us no harm, and so we serve as hosts for these unbidden guests year in and year out without even knowing of their existence. The products from other kinds of organisms are poisonous to us, and when some of these organisms multiply within us we discover it, because we are poisoned and become ill. Only organisms whose products of metabolism are poisonous are counted ordinarily as causing infection. In the strict sense we might be said to be infected by the harmless organisms of which so many thrive within us, but in the usual use of the word we speak of one as having an infection only when his body has been invaded by organisms that produce poison. In the case of most kinds of injurious organisms their multiplication within the body unchecked would lead finally to its destruction. It is necessary, therefore, that the body have some means either of checking the development of the organisms or of neutralizing the poisons which they produce. The body has this power, and the machinery for it consists very largely of substances or structures in the body fluids. The detailed story of these is too complex to be told here. We shall content ourselves by saying that when an infection becomes established, as of scarlet fever, for example, the poisons that are produced are poured out by the organisms into the body fluids of the part where they happen to be located, and are taken up from there by the blood and distributed all over the body. The fever, headache, and other disagreeable symptoms are due to these poisons. The interesting thing about it is that the poisons themselves act toward the body as chemical regulators or hormones, exciting some or perhaps all of the cells of the body to a special kind of functional metabolism, which results in the manufacture of materials which neutralize the poisons. Thus, one of the very important properties of living protoplasm is to respond to the poisons from the metabolism of other cells by producing neutralizing material. Whether one dies from an infection or recovers from it depends on whether the cells are able to produce enough neutralizing material to prevent themselves from being killed or whether the poison is so abundant or so malignant that the cells are destroyed in spite of their activity in pouring out the neutralizing material.

Still another interesting thing about this whole matter is that every kind of infecting organism has its own kind of poison, which differs from that of the other kinds, and so the chemical effect of the poison upon the cells is not the same for one infection as for another. The functional metabolism of the cells in turn is adjusted to the kind of poison, so that the material they pour out is suitable to neutralize the particular poison which aroused them to activity in the first place, and in most cases no other. If one gets well from any infection, there is a surplus of the neutralizing material left in his body fluids, and, as long as it remains, he is secure from another infection of the same kind. This condition is defined as immunity. Since the neutralizing materials are different for different infections, immunity against one is in most cases of no avail against another. One may be immune against scarlet fever, but be just as likely to catch pneumonia as a person who has never suffered from any infection at all. It follows that an individual who has had and recovered from a great many infections has a correspondingly large assortment of neutralizing materials in his body fluids. Some of these appear to persist throughout life, others disappear fairly soon.

The next group of permanent constituents to be described consists of some materials which seem to have nothing at all to do as long as everything is going well: the body fluids bathing the cells, or, in the case of the blood, circulating about from part to part through the blood vessels. These come into play only when, as the result of injury, the fluids begin to escape; namely, in the case of bleeding, or, as it is technically called, hemorrhage. It is clear that if any injury is started sufficient to allow the fluid to escape, there would be no more reason why it should stop running out, unless prevented, than there is why water should stop running out of an open faucet of itself. Since, as we know, we do not bleed to death every time we get a slight cut, but after a long or shorter time the bleeding stops of itself, there must be some automatic arrangement by which the opening is plugged. All of us are familiar with the way in which this is done. We know that in the course of a few minutes after the bleeding begins, the blood tends to set into a firm jelly, which is called the clot. This clotting is the result of a chemical transformation which goes on in the blood as the result of its escape from the blood vessels and its exposure to the outside. The details of the chemical processes are too complicated to be described here; all we need to remember is that the blood within the body contains certain special materials which are soluble and therefore float in solution along with the other soluble materials. When any blood vessel is injured, so that the blood begins to escape, a series of chemical changes is started automatically by which this soluble material is changed and becomes insoluble, so that instead of remaining dissolved in the blood it is precipitated out. In this form it is called fibrin. Fibrin is a very sticky, stringy mass which forms a spongy network, spreads itself over the injury, and clings firmly to the edges, thus plugging the opening, unless the rush of blood is so strong that it keeps washing the fibrin away as fast as it is formed. Bleeding from small wounds will presently stop of itself, but if the hemorrhage is too great for this, it is necessary that the blood flow be slowed down or stopped artificially. This is done by locating and pressing upon the large blood vessel through which the blood is escaping. In order to do this successfully, one has to know something about the course of the blood vessels, and this will be described in a later chapter.