Physiology: The Science of the Body

by Ernest G. Martin

We showed a moment ago that the heart is really a double pump. The relation of the two halves is shown in the diagram. One of the two pumps, that on the right side of the heart, receives the blood from the body at large and pumps it out into the pulmonary artery and through the capillaries of the lungs; the pump on the left side of the heart receives the blood from the lungs through the pulmonary vein and pumps it out into the aorta and so through all the other capillaries of the body. Since the circuit of the body is much more extensive than the circuit of the lungs, the work of pumping is correspondingly greater, and we find the left part of the heart a much more powerful pump than the right. The heart operates as a reciprocating pump, by which we mean that it alternately fills and empties. In this respect it is like ordinary pumps except those of the rotary variety. Any reciprocating pump must have a chamber which will fill and which can then be emptied forcibly. In order that it shall not empty itself back through the pipe from which it filled there must be a valve in the intake pipe which shall close as the pump is being emptied. If, as is the case in the heart, it is emptying itself into a system which permits backflow, there must be another valve in the outlet pipe to prevent the fluid that has been expelled from running back in. Each of the heart pumps consists, then, of a chamber, which alternately fills and empties itself, and two valves, one on the intake and one on the outflow side. In ordinary pumps the forcible emptying is performed by a piston which moves through the pump chamber expelling the liquid ahead of it and then has to draw back, making room for the chamber to fill again. In the heart the forcible emptying is accomplished by muscular action. The wall of the heart consists of a great many muscle fibers so arranged that when they contract they pull the walls of the heart together, making the cavity smaller, or even obliterating it completely. The contraction of these fibers makes up what we are familiar with as the beat of the heart. The frequency with which they contract varies a good deal in different individuals. The average is about seventy-two a minute; but it may be as slow as forty-eight or fifty, or may run up to one hundred and forty or one hundred and fifty a minute. Whatever the rate, in every case there is an alternation of contraction and relaxation; during the relaxation the cavity is filling with blood through the intake valve, the outflow valve being closed, so that no blood that has once been pumped out can rush back in again. By the contraction of the muscles the heart is emptied, the outflow valve being open, and the intake valve being closed to prevent an escape of blood backward into the veins through which it flowed in. The part of the heart that carries on this active pumping work is known as the ventricle; that on the right side, which receives the blood from the body and pumps it to the lungs, is the right ventricle; and that on the left side, which receives the blood from the lungs and pumps it to the body, is the left ventricle.

In addition to the ventricles, which are the active pumps, each side of the heart has an additional chamber known as the auricle, whose purpose is to serve as a reservoir into which blood can flow during the time that the ventricles are emptying themselves. If it were not for the auricles, the movement of blood into the heart would have to stop with every beat, because while the ventricles are contracting the intake valves are closed and there would be no place to which blood could flow, but since each side has its auricle, the flow of blood goes on during the beat of the ventricles, the auricles filling up. The intake valve, in order to operate properly, should be located between the auricle and ventricle, and this is where it is. The vein opens directly into the auricle without any valve between; the auricle opens into the ventricle with the intake valve at the point of junction. The intake valves are given rather formidable names; they are sometimes spoken of as the auriculo-ventricular valves; that on the right side of the heart between the right auricle and the right ventricle is composed of three flaps of membrane, and has therefore been named the tricuspid valve. The intake valve on the left side of the heart, which is composed of but two flaps, is known as the mitral valve. As soon as the beat of the ventricles is over and the ventricular muscle relaxes, the blood which has accumulated in the auricles presses the intake valve open and blood begins to flow through it directly into the ventricle. Both the intake and outflow valves are composed of stout but thin sheets of membrane, so that very little pressure is required to operate them. The weight of the blood that is accumulated in the auricles during the beat of the ventricle is more than sufficient to force the valve open and allow the blood to flow on through into the ventricle. In a heart that is beating seventy-two times a minute, there cannot be much time occupied either in filling or emptying. As a matter of fact both these intervals are measured in tenths of seconds. If we take a heart that is beating at the average rate of seventy-two times a minute the whole of a single beat amounts to eight-tenths of a second. The beat of the ventricle takes about three-tenths of a second or three-eighths of the whole time; the period of relaxation of the ventricle, during which it is filling with blood through the open intake valve, is about five-tenths of a second or five-eighths of the whole time. The movement of blood is rapid enough so that this five-tenths of a second allows the ventricle to fill; in fact much less time than this is required, for in a heart that is beating at twice the average rate, the ventricle still fills with blood between beats.

A word remains to be said about the beat of the auricle. During most of the period when the ventricle is relaxed the auricle is also quiet and blood is pouring directly through it from the veins into the ventricle; but just an instant before the ventricular beat begins, one-tenth of a second to be exact, the auricle contracts, emptying what blood it contains into the nearly filled ventricle; thus, when the ventricle beats, which it does immediately, closing the intake valve at the same time, the auricle is empty and so is able to accommodate the inflow of blood from the vein during the three-tenths of a second that the intake valve is shut. Both sides of the heart work exactly together, the two auricles beating simultaneously, and the two ventricles. Of course it is necessary that this be so, for if they did not keep pace exactly, one with the other, there would be a piling up of the blood either in the lungs or in the veins leading from the body to the heart, and the efficiency of the circulation would be seriously impaired.

We can get a good deal of information about the way our hearts are behaving simply by holding the hand against the chest directly over the heart or by pressing the ear against the chest of some one else and listening to the heart’s action. The physician makes use of a stethoscope, which is merely an apparatus for conducting clearly the sounds which the heart makes, so that it is not necessary to apply the ear to the chest. When one listens thus to the heart he finds that with every beat there are two distinct sounds: the first is a rather dull sound which comes just at the beginning of the beat of the ventricle, the second is a sharp sound occurring just at the end of the ventricular beat. As we saw in Chapter IX, sound is always the result of vibrations, and a great deal of study has been devoted to an attempt to find out where the vibrations come from that cause the heart sounds. It is now generally believed that the first sound is partly the result of vibrations set up in the contracting heart muscle and partly due to vibrations from the sharp closing of the intake valves. The second sound is known to be wholly due to the sudden closing of the outflow valves. The sounds are chiefly of importance in that they enable the physician to determine whether the valves are holding tight or whether there is a leakage of blood through them. In case the intake valve leaks, there will be a backward jet of blood from the ventricle into the auricle with every beat of the heart. This will cause a sort of hissing or murmur which can be heard with the stethoscope in connection with the first sound. If the outflow valve is the one that leaks, blood will squirt back into the ventricle from the aorta, while the ventricle is relaxing. The murmur in this case will come just after the second sound. The skillful physician by comparing the loudness of the murmur when the stethoscope is pressed at different points on the chest and back can determine whether the leaky valves are on the right side or the left side. Thus an accurate diagnosis of imperfect valves can be obtained. Of course the heart will not work well if its valves are not tight any more than will an ordinary pump, so that persons suffering from this trouble cannot have as good a circulation as those whose valves have nothing the matter with them. It is true that in most cases of imperfect valve action there is a compensation in the form of an increase in the size and strength of the heart muscle, so that the circulation is maintained in spite of poor valve action by harder work on the part of the heart. It is evident that in a case of this kind exceptional strains on the heart are more dangerous than if the heart is normal to begin with, so that persons with faulty valve action must avoid physical strains, such as sharp running after street cars or trains, which would be borne with impunity by ordinary individuals. Since faulty valves are a frequent result of acute rheumatism, which in turn comes from pus pockets, and since no way is known to cure a defective valve, once the trouble has developed, it is evident that prevention is of the utmost importance. Physical efficiency is very seriously hampered by poor heart action.

One feature of the heart action with which we are all perfectly familiar is that both the rate and the vigor of the heartbeat vary greatly from time to time. When one is lying quietly, the heartbeat is at its lowest point. It becomes more rapid as one sits up, still more rapid upon standing, increasing still more with the taking of any form of muscular exercise, and in vigorous muscular exercise attains its greatest rapidity and force. The rate in this latter case may be fully double that of the quiet standing position, and, as the vigorous thumping tells us, the force is also very much increased. As we saw in Chapter VII the heart muscle works automatically, contracting and relaxing without being stimulated through the nervous system. The variations in rate and force, however, are the result of nervous action. The heart muscle, as we have already seen, is under the same sort of nervous control as the smooth muscles and glands. It has passing to it two sets of nerves, one to slow it down, the other to speed it up. Both these sets of nerves arise from centers in the brain stem, and both these centers appear to be discharging continuously. So it works out that the heart muscle is under the constant influence of two opposing sets of nerves, and its actual rate and vigor depend upon the balance between them. This has the effect of making the heart extremely responsive to nervous influences. The slightest relaxation on the part of the nerves whose function is to cause slowing will lead to a prompt increase of rate, since the nerves that tend to cause increase are active all the time. Various things may bring about changes in the nervous balance governing the heart; chief of these are muscular activity and emotional disturbance. Practically all the changes in the heart action that we observe from moment to moment can be explained as being due to one or the other of these causes. There are, however, two additional points to be noted briefly; the first is that after muscular exercise the heart slows down very gradually, not returning to its ordinary resting rate for a half hour to an hour after the exercise is over, depending on how long the exercise was kept up. The explanation of this long-continued rapid beat is found in the great outpouring of waste products as the result of the exercise. We have already learned that the functional metabolism of muscular work involves the oxidation of a large amount of energy-yielding material and therefore brings about the production of large amounts of oxidation products. Their presence in the blood serves as a stimulus to