Physiology: The Science of the Body

by Ernest G. Martin

We must remember that once every second, or oftener, the heart is shooting a jet of blood into the large artery which is already stretched with blood and which can empty itself only through the tiny capillaries at the tips of its finest subdivisions. On account of the inertia of the blood stream, room is made for this additional jet of blood by stretching the arteries near the heart more than they were stretched before. The result is that there is an inequality in the amount to which the arteries are stretched, those near the heart being stretched more than those farther along. As quickly as possible this inequality of stretch will be equalized by a spreading of the additional tension out over all the arteries in the form of a wave. This wave makes up what we know as the pulse. It can be felt in any artery that is near enough to the surface so that the finger tips can press upon it. The radial artery at the wrist is the one commonly used by physicians for feeling it. There is a large artery in the neck in which the pulse can also be felt readily, and if one takes the pulse of another person in the neck with one hand and in the wrist with the other he can easily satisfy himself that the pulse in the neck always comes an instant earlier than that in the wrist. This is simply because the pulse spreads from the heart as a wave, and the distance to the neck is not so great as that to the wrist. By the time the finest subdivisions have been reached, the tension is equalized throughout the arterial system, and there is no more pulse. The advantage of this is that the blood flows through the capillaries in a steady stream and not in a series of jerks. This, in fact, is the chief, but not the only, benefit we derive from having elastic arteries. Since the heart operates as an intermittent pump, it is evident that unless the arteries were elastic and so could take up the shock, the blood would have to pass through the capillaries in a series of jerks, exactly corresponding with the beats of the heart. There is abundant proof, which we shall return to in a moment, that to have the blood move through the capillaries in this jerky fashion would be disastrous. Before taking that up, however, we wish to show that by having elastic arteries the actual work of the heart is less than it would be if the arteries were stiff. The reason is really very simple. As was stated a few pages back, the heart is actually emptying itself only during three-eighths of every beat. If the arteries were stiff tubes, and therefore not able to take up any of the blood within themselves, exactly as much would have to pass out through the capillaries during this three-eighths of the beat as was pumped in by the heart. In other words, if the heart were pumping five quarts a minute, five quarts would have to pass through the capillaries in three-eighths of the minute instead of having the whole minute in which to do it. Since the arteries are actually elastic, they are able, by stretching a little more, to make room for part of the blood and so spread the time of its passage through the capillaries out over the whole time instead of confining it just to the period when the ventricle is contracting. Evidently it would take more work to pump five quarts of blood through the capillaries in three-eighths of a minute than in a whole minute.

We measure the work of the heart by what we call blood pressure, about which we are hearing so much these days, so that it will be well to explain as clearly as possible just what is meant by it. The blood pressure really means the pressure of the blood within the large arteries. It could be measured with any ordinary pressure gauge, if it were not for the fact that we cannot very well cut into our bodies to apply gauges to the arteries. For this reason it has been necessary to invent means of finding out what the blood pressure is from the outside. The way it is done is to put a band around the arm, press this band down upon the arm until it squeezes the large arm artery shut, and then, by means of a suitable gauge, find out how much pressure was required. Of course, it is necessary to be able to tell when the artery has actually been squeezed shut, so that the determination of blood pressure in human beings is the work of an expert. Furthermore, blood pressure, as should be clear from what has already been said, varies with every heartbeat. It is at its maximum the instant the heart finishes emptying itself into the artery, and falls off steadily, reaching a minimum just before the next beat comes. The more elastic the arteries, the less difference there will be between the maximum and the minimum blood pressure. The reason for this will be clear when we think that if the arteries were entirely rigid there would have to be a very high pressure during the time the heart was actually beating to force the blood out through the capillaries, but that between beats the pressure would fall off to zero. The more elastic the arteries are, the more nearly do they exert a steady pressure on the blood within them, and so the less will be the difference between the maximum and the minimum pressure. When the physician determines blood pressure, he really determines both the maximum and the minimum pressure, in order that he may be able to judge whether or not the arteries are as elastic as they should be. High blood pressure just by itself might not mean much more than that the heart was beating more rapidly than it should, but a high maximum pressure and a low minimum pressure means nonelastic arteries. This in turn means that the blood is forced through the capillaries in jets rather than in a steady stream, and we may judge of the importance of having a steady flow through the capillaries when we recall the well-known medical proverb that “a man is as old as his arteries.” It is an actual fact that the chronological age of an individual need not have much to do with his physical age. If his arteries continue elastic over a long period of years, he will be physiologically youthful, while if his arteries become rigid he will be physiologically aged, no matter how few his actual years upon earth may have been. Unfortunately we do not know as much as we would like to about the causes of loss of elasticity in the arteries. It does appear, however, that self-indulgence of various kinds is apt to lead to loss of elasticity. For example, even the moderate use of alcohol is now generally recognized by the medical profession as a cause of impairment of elasticity in the arteries. It is probable that intemperance in the use of various foods and drugs leads also to this same condition.

We have just seen that the heart is obliged to maintain high blood pressure in order to force the blood through the tiny capillaries. It will be clear that the actual amount of pressure will depend in part upon how much blood is forced through in a minute and in part upon the extent to which the capillaries offer resistance. It is a familiar law of friction that the smaller the tube the greater will be the resistance it will offer to the passage of liquid through it, so that if the capillaries change in size their resistance to the flow of blood through them is bound to vary. The walls of the capillaries are very sparsely provided with muscle fibers, but the very finest subdivisions of the arteries, which are really no larger in diameter than the capillaries, have much more smooth muscle in their walls. These muscles, as we have already seen, can by their contraction or relaxation make the tiny vessels smaller or larger. We have examples of this in the flushing and pallor of the skin. What we wish to do now is to show how the flow of blood through different parts of the body is controlled by changes in the caliber of these tiny tubes. The muscles in the vessel walls have the double nervous control commonly found in smooth muscles, and both sets of nerves trace back to centers in the brain stem. One of these centers causes the muscles to contract and the vessels to become smaller; the effect of this is, of course, to increase the resistance to the passage of blood through them. The nervous center which brings this about is called vasomotor, or, more properly, the vasoconstrictor center. Vasoconstriction means literally causing contraction of the vessels, which is exactly what this center does. Not all the blood vessels in the body are acted upon through the vasoconstrictor center. Those of the skin and of the abdominal organs are under its control, but those of the skeletal muscles are not. The result of activity of this center, then, is to make it more difficult for blood to flow through the skin and through the abdominal organs, but the ease of flow of blood through the muscles is not affected. Of course, it will follow automatically that the blood stream will be diverted in a large part from the former regions into the latter. The skin and abdomen together make up so large a part of the whole body that marked constriction of the blood vessels in these two regions is bound to cause a considerable increase in blood pressure.

A fairly high blood pressure is necessary for bodily well-being, because only thereby is the brain assured of sufficient nourishment. To see why this is so, we have only to remember that the brain is unfavorably situated for receiving ample supplies of blood. It is at the top of the body, so that the influence of gravity has to be overcome in forcing blood up to it. Also it is completely inclosed in the bony skull, which it in turn fills so completely that there is almost no room for the accommodation of extra blood in it. In all other parts of the body a rise in blood pressure stretches the arteries and so leads to there being actually more blood within them, but there is no room for the arteries in the brain to stretch, so that the total quantity of blood in the brain cannot vary greatly from time to time. The only way in which an increased blood supply can be obtained is by causing the blood to flow more rapidly. This is precisely what happens every time the blood pressure in the body rises. Whenever the vasoconstrictor center sends nervous discharges into the blood vessels of the skin and abdominal organs, causing them to contract, there is a diversion of blood from them directly into the skeletal muscles and also a rise in blood pressure due to the increase in resistance to the circulation, which causes blood to flow more rapidly through the brain. The net result, then, is improvement in the blood supply to the skeletal muscles and to the brain. So important is the blood supply to the brain that the vasoconstrictor center discharges actively throughout the waking part of the day. If for any cause the activity of this center diminishes, there will be an increased circulation in the skin and in the abdominal organs, the blood pressure will fall, the circulation through the brain will decrease, and along with it there will be a decrease in brain function. After this passes a certain point unconsciousness will result. This is what happens when one faints. For some reason or other the vasoconstrictor center becomes inactive, and the series of events just described is set in motion. Fainting ordinarily cures itself automatically, because when consciousness is lost, the individual falls over; this brings his head down on the level with the rest of his body, makes it easier for the blood to flow through it, and so in a moment or two consciousness will be regained. It is a mistake to try to scramble to one’s feet immediately, because until the vasoconstrictor center recovers its ordinary activity, raising of the head above the level of the rest of the body is bound to result in its failure to receive sufficient blood, and so faintness will come on again. It has long been a practice to dash cold water in the face of a fainting person. The physiological value of this is in the sudden stimulation of the sensory nerves in the face by which is set up a stream of nervous discharges which will play upon the vasomotor center, and arouse it again to activity. Almost any vigorous sensory stimulation may have the same effect.

During sleep the vasoconstrictor center is usually not very active; the cause of sleep is not completely understood, but one of the most satisfactory theories regarding it is that during the period of waking the vasoconstrictor center becomes gradually more and more fatigued, and so requires more and more stimulation to keep it active. This stimulation may come in part through the ordinary channels of the sense organs and in part from the higher brain centers, as when one keeps awake by an effort of the will. Upon going to bed sensory stimulation is cut off to a very large extent, also the will to remain awake is no longer present. Under these circumstances the fatigued vasoconstrictor center is under a minimum of stimulation and tends, therefore, to lessen its activity. The result is that the blood pressure falls and presently the circulation through the brain drops below the level of consciousness and the individual is asleep. During the period of sleep the fatigued center recuperates, so that it becomes more susceptible to sensory stimulations and, in course of time, is aroused by such stimuli as accompany the returning day to sufficient activity to restore the brain to consciousness. Of course it will be perceived that there are many things about sleep which are not satisfactorily explained on this theory; in fact no one at the present time pretends that we can account for it completely on this basis of changes of circulation through the brain. It is believed, however, that they have a good deal to do with it and it is certainly true that in healthy individuals the course of sleep follows very closely the activity of the vasoconstrictor center. Undue wakefulness in persons not suffering from disease can nearly always be explained on the basis of excessive activity on the part of this center. The activity may be the result of chemical stimulation, as when persons are kept awake by drinking coffee or strong tea in the evening, or by the persistence of adrenalin in the blood following a period of great excitement. Mental activity, itself, tends to keep the vasoconstrictor center whipped up, so that one who allows his mind to work actively during the time when he should be asleep is very apt to find sleep refusing to come when it is desired. The center is often stimulated from the digestive tract; gastric irritation, even though not acute enough to be recognized as indigestion, may cause wakefulness by arousing nervous disturbances which play upon the vasoconstrictor center. Flatulence, namely the presence of large volumes of gas in the digestive tract, frequently acts as a mechanical source of irritation by which wakefulness is induced. Evidently the factors favoring healthy sleep are the inducing of fatigue, preferably by physical exercise, the avoidance of chemical irritation or of excitement in the hours just before going to bed, and finally the adoption of dietary habits which shall insure good digestion. If an individual in whom all these precautions are combined still continues chronically wakeful, the trouble is sufficiently deep-seated to call for competent medical attention.

Besides the vasoconstrictor center about which we have just been talking there is in the brain stem a center which relaxes the tension of the muscles in the walls of the blood vessels. This is known as the vasodilator center, and it acts in opposition to the vasoconstrictor. In most parts of the body it does not appear to have any very great importance for the simple reason that the blood is under such high pressure that any relaxation of effort on the part of the vasoconstrictor center leads at once to a forcing open of the blood vessels. There are a few regions, however, in which the action of the vasodilator center is of real importance; one of these is in the skeletal muscles. These, as should be perfectly clear by this time, are the seats of our most active functional metabolism. When the muscles are active, great amounts of oxygen and food are being withdrawn from the blood and large amounts of waste material, including carbon dioxide, poured out into it. Only a very rapid circulation will take care of this situation. At the same time the skeletal muscles are the most compact of our living tissues. The cells are crowded together in making up the very strong muscular machine by which our movements are performed. As the muscles contract, they squeeze hard on the blood vessels passing through them. In view of this situation it is very important that the blood vessels be opened as widely as possible during muscular activity, and so we find that the vasodilator center acts to improve the circulation through the muscles, while they are active. In time of special emergency, as we saw a moment ago, the vasoconstrictor center is at the same time engaged in cutting down the blood supply to the skin and to the abdominal organs, thus insuring to the muscles the maximum possible nourishment through the blood stream.

Besides the skeletal muscles there is active functional metabolism in the various secreting glands; a feature of their activity is that it must be very great at certain times, but falls to little or nothing at others; thus during the actual taking and digesting of food the various glands which secrete digestive juices are exceedingly active, but in the intervals they may be doing little or no work. They require a very copious blood supply when they are functioning, but need very little between times. The blood vessels flowing through all these glands are under the influence of the vasodilator center, so that they are opened as widely as possible while the glands are active. They, therefore, receive much more blood in proportion to their size than they would if it were not for the action of this center. Between times their blood supply falls off to that which suffices for inactive tissues generally. In time of emergency the action of the vasoconstrictor center upon the vessels through these glands is such as to cut off the blood supply to them almost completely. This is well illustrated in the dry mouth of the frightened man, showing that the salivary glands have suspended activity, a suspension due largely, if not wholly, to the cutting down of the blood supply through them.

In the section just completed we have tried to give some idea of the way in which the circulating blood provides the various tissues with the materials they require and is adjusted automatically to meet variations in demand from different tissues. Just one more point needs to be noted in completing the account of the conveyer system. The tissue fluids which serve as the connecting links between the circulating blood and the individual cells are necessarily full of waste products, because these come directly from the cells to the tissue fluids and afterward pass on from them to the blood. The result is that the cells are constantly bathed in a solution of their own waste products. There is only one way in which relief from this condition can be obtained and that is by moving the used tissue fluid away bodily and letting it be replaced by fresh fluid. As a matter of fact, this happens; there is an oozing of fluid through the walls of the capillaries from the blood into the tissue spaces; this of course pushes the fluid already in those spaces on ahead of it. If there were no place to which the fluid could go, the result would be a swelling, as the tissue became more and more filled with fluid. This is avoided ordinarily by a drainage system whereby the tissue fluids are carried off as fast as more fluid comes in from the blood, but when liquid is poured out faster than it will drain off, as from a bruise or wrench, we do get a swelling.

The drainage system consists of very delicate vessels known as the lymphatics, which come together into larger and larger vessels, just as the veins do, and finally empty into the vena cava just at the point where it enters the heart. There is no back pressure of blood here, so that the movement of fluid through the lymphatics is not hindered. There is no pump for forcing the lymph to move along; the very gradual motion, which is all that is necessary to keep the fluid from accumulating in the tissue spaces, is brought about by pressure upon the lymphatics resulting from the bodily movements. The lymphatic vessels are like the veins in having valves here and there along them. Whenever by any bodily movement either a lymphatic vessel or a vein is squeezed, the valves insure that the liquid shall be passed along in the direction toward the heart and never in the reverse direction. In the case of the veins this action is not absolutely necessary, since the heart itself is able to maintain the circulation, although it does help, particularly in bringing back the blood from the extremities. In the lymphatics this is the only way by which movement of fluid is brought about. The result is that when the body is perfectly quiet, there is very little movement of fluid through the lymphatics; muscular activity, on the other hand, leads to rather active movement through these vessels. The fact is well illustrated in ourselves. One who sits for a long time humped over a desk finds himself feeling very dull; to obtain relief he stirs about, stretches, and yawns. The dullness was merely the result of the stagnation of fluid in his tissues, causing the cells to be more or less poisoned by their own waste products. By making active movements, these stagnating fluids were forced along to be replaced by fresh liquid direct from the blood and the beneficial effect is felt immediately. Massage properly applied has very much the same effect, although it is doubtful whether as good results can ever be obtained thus as by actual vigorous exercise.

At various places along the lymphatics are little spongelike lumps of tissue known as lymph nodes; the particular spongy substance of which they are composed is called adenoid tissue. This adenoid tissue acts as a filter for the fluid passing through it. Any foreign particles, living or nonliving, that get into the stream are caught in the lymph nodes and held there more or less permanently. Most of the nonliving particles that get into our tissue fluids are from the dust that we inhale, which works its way through the mucous membrane of the respiratory passages and so into the body fluids. The lymphatics that drain the lungs carry along these dust particles and they lodge in lymph nodes at the base of the neck. Persons who have lived in dusty regions or have pursued a dusty livelihood, such as coal heaving, will have by the end of their lives lymph nodes which are literally black with dirt.