Physiology: The Science of the Body

by Ernest G. Martin

pin in anywhere for any distance without striking against one or more of them. The capillaries are spread so thickly that there are not many places in the body where living cells are more than a very small fraction of an inch from one of them. The cells do not, however, lie right against the capillaries, but are separated from them by tiny spaces filled with tissue fluid. In order for material to get from the blood to any living cell, then, it must pass through the wall of the capillary into the fluid which fills the tissue space and from that in turn to the cell itself. The wall of the capillaries is so delicate that if the blood flowing through any capillary contains more of any substance than is present in the tissue fluid surrounding that capillary, some of it will pass out through the wall and into the tissue fluid, just about as freely as though there were no wall there at all. The arrangement can be illustrated by a familiar example; if a drop of ink is allowed to fall into a glass of water, it will color only a small part of the water at first, but quickly spreads out until each part of the water is as deeply colored as any other part. If the glass of water were to be divided in half by a very delicate membrane, and the ink dropped in on one side, it would spread out in the same way, passing through the membrane in so doing, until again all the water in the glass was equally colored. Of course, the quickness with which the ink could pass through the membrane would depend on how delicate the membrane was. We could imagine membranes which would not let any ink at all through, and every degree from that up to membranes so delicate as to offer no obstruction at all to the passage of the ink. The walls of the capillaries rank as membranes of such delicacy as apparently to offer almost no obstruction to the passage of materials through them. They hold back the red corpuscles and the platelets fairly well, so that they do not pass out of the blood and into the tissue spaces, unless the capillaries are actually injured. The colorless corpuscles are able to make their way through the capillary walls and so also do nearly all the substances that are dissolved in the blood. It is an interesting fact that the blood proteins do not pass freely through the capillary walls, although the digestion products of food proteins, do. It will be remembered that in the chapter on Body Fluids the importance of the sticky quality of the blood proteins was spoken of. It is now believed that it is because of this gelatinous nature that the blood proteins are not able to pass out through the capillary walls, and this is supposed to be important in the proper working of the circulation. In fact there is a condition of greatly lowered vitality to which the name “shock” is applied, in which the blood proteins escape through the capillary walls to so great an extent as to interfere with the proper working of the body. It has been found possible to prevent this in a very large measure by the simple expedient of injecting some substance like mucilage directly into the blood stream. We are to think of the capillary walls, then, as allowing materials to pass freely through them in either direction, from the blood into the tissue spaces or from the tissue spaces into the blood, with the exception of the red corpuscles, platelets, and the blood proteins, and as thus keeping the tissue fluids supplied with whatever materials the blood contains or taking from the tissue fluids the waste products of cell metabolism, which the cells are pouring out. With this arrangement clearly in mind, all that remains for the understanding of the conveyer system is to see how the blood is kept in motion and distributed among the various organs of the body, and then to consider where the blood in turn gets its supplies of materials which it can pass on to the tissue fluids, or how it gets rid of the substances which the tissue fluids have passed on to it from the cells.

At the beginning of the chapter we said something about the arteries and veins and their branching into smaller and smaller subdivisions with the final connecting link between the smallest arteries and the smallest veins in the form of capillaries. We are now to consider in detail the movement of the blood through these tubes, and to do that it will be necessary to speak again of this arrangement. In describing the circulation we usually begin with the heart. The heart itself will be taken up presently. First let us trace the blood vessels from the heart through the body and back to it again. The large main artery leading from the heart is known as the aorta. This springs from the upper side of the heart, bends over in an arch, and passes down through the chest into the abdomen.

Branches are given off from the aorta all along its length. The very first of these come off before the aorta gets away from the heart, and are the arteries by which the tissues of the heart itself are supplied with blood. A little farther on are large arteries, one for the left arm with a large branch running up the left side of the neck, another for the right arm with a large branch running up the right side of the neck. Each of these in turn gives off branches all along to provide for the tissues of the arms, neck, and head. It is worth while noting that the arteries running up the neck to the head are large in proportion to the size of the head itself; this is because the brain, as the most important organ in the body, requires and receives a disproportionately large blood supply. Besides the brain the head contains numerous muscles and also the salivary and tear glands, all of which carry on active metabolism and therefore require abundant blood supply. The main branches of the aorta to the head and arms are given off from the arch; as the aorta passes down through the chest it gives off small branches to the muscles of the chest wall, and then passes into the abdomen. Here are located two of the three important arrangements for renewing the blood; namely, the digestive organs and the organs of excretion (kidneys). Large branches from the aorta pass to the digestive organs and others to the kidneys; smaller branches lead to the muscles of the abdominal wall, and also to the various secreting glands that are located in the abdomen. At the lower end of the abdomen the aorta divides, giving one large branch for each leg. As we have already seen, if we follow any of these subdivisions through its finer and finer branchings, we shall finally be able, with the aid of a microscope, to trace it to capillaries, where the interchanges between blood and tissue fluid occur, and beyond the capillaries to tiny veins which unite with other tiny veins from other capillaries into larger veins. These again continue to come together into main veins corresponding in every part of the body with the main arteries. All these veins finally unite into two, one for the lower part of the body, called the inferior vena cava, and one for the upper part of the body, called the superior vena cava. These two come together just at the entrance to the heart. One special feature of the blood supply to the digestive organs may as well be mentioned here; it is that the blood which flows through the capillaries of the stomach and intestines is all reassembled into a vein known as the portal vein, which instead of passing directly into the inferior vena cava goes first to the liver, where the vein breaks up into another set of capillaries, the liver capillaries, beyond which is another vein which leads into the inferior vena cava. The result of this arrangement is that all blood passing into the capillaries of the stomach or intestines is obliged to pass again through the capillaries of the liver before going on into the main stream of the circulation. This is an important feature of the renewal of the food supplies of the blood.

We have now traced the blood from the heart through the body back to the heart again, and have seen how in its course some of it will pass through such active tissues as muscles or brain or glands, so that the interchanges can go on by which the fluids in these active tissues can take up needed materials and give off wastes. Also a part of it flows through the digestive organs, where food materials can be taken up, and another part flows into the kidneys, where wastes can be gotten rid of. This leaves us to consider only the passage of the blood through the lungs, where the supply of oxygen is to be taken up and the gaseous waste product, carbon dioxide, is to be disposed of.

The most urgent requirement of the body is the requirement for oxygen. There is under ordinary circumstances at all times some surplus of food materials stored in the cells, so that even though the renewal of their surrounding fluids from the blood should stop, they could keep on going for a time on the material that is stored within them; but there is no such storage of oxygen. The cells in the body lead almost a hand-to-mouth existence so far as their oxygen supply is concerned. They are constantly withdrawing oxygen from the tissue fluids surrounding them, and these fluids are just as constantly withdrawing it from the blood; therefore any failure of the blood to be properly supplied with oxygen results very promptly in a condition of oxygen hunger in the cells. This means prompt cutting off of metabolism, since metabolism is a matter of oxidizing fuel, and oxidation cannot go on unless the oxygen is provided. This urgent need for oxygen is met in the body by having the arrangement for supplying it to the blood much more perfect than the other renewal arrangements. We saw a moment ago that only part of the whole blood stream passes through the digestive organs at any given time and only part of the stream passes through the kidneys. The whole stream, on the other hand, passes through the capillaries of the lungs. This is brought about by having an arrangement whereby the combined venæ cavæ after entering the heart communicate with an outlet in the form of an artery leading to the lungs. This artery, which is called the pulmonary artery, breaks up into capillaries in the lungs, which reunite into the pulmonary vein which comes back to the heart again. It is from the pulmonary vein that connection is made with the aorta, starting the blood on its course through the body again. We see then that the blood passes through the heart twice in each complete round, once as it comes in from the body at large on its way to the lungs, and again as it comes in from the lungs on its way to the body at large.

We have spoken of the heart thus far as a single organ; it is actually two hearts side by side, and these would work just as well if they were at a distance instead of being built into one organ. There has probably been more misunderstanding of the heart by people in general than of any of the other parts of the body. This is because from the earliest times the heart has been looked upon as the seat of the affections, and so powers and properties have been attributed to it to which it is not at all entitled. As we tried to make perfectly clear in a former chapter all intelligence and all feelings are located in the brain; the heart cannot possibly take any more active part in these than can the stomach, liver, kidneys or any of the other parts of the body which are concerned with the maintaining of the tissues in good working order. Probably no one really knows how it came about originally that the heart was endowed with these peculiar gifts. It is true that in time of strong emotion there are changes in the activity of the heart which we can perceive. These occur because the heart is under the same kind of nervous control as are the smooth muscles and glands, and shares with them in the disturbances which accompany emotion; but the real seat of these emotions is, of course, the brain. As a matter of fact the heart is nothing but a muscular pump whose sole function is to keep the blood in motion. From what has already been said it is clear that the heart cannot relax its activity for more than an instant without disastrous results; the pressing need of the tissues for oxygen requires that the blood be kept moving. If there were any other way in which the needs of the cells could be supplied except through the movement of the blood, the heart could be dispensed with perfectly well. We have emphasized this about the heart because it is much easier to understand its working if one thinks of it simply as a pumping organ than if one is attributing to it mystic functions connected with our higher emotions.