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 waterto 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 theocean, 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 thatserves 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 theword 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 clearthat 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 theclot. 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 calledfibrin. 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 beslowed 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.
There are a few people whose blood lacks some of the necessary chemical substances to enable it to clot; such persons are known technically as “bleeders.” Even a slight injury in one of them will cause serious, or even fatal, hemorrhage, unless the escape of blood is stopped artificially, since it will not stop of itself. An interesting fact about this condition is that it runs in families; in other words it is hereditary.
In addition to all these constituents of the body fluids which aredissolvedin them, there are in that part of the fluids confined to the blood vessels, which we call the blood, three kinds of structures floating; these we have next to describe briefly. The first of these are thered corpuscles. They give the blood its red color, although if looked at singly they appear yellowish rather than red. Red corpuscles are almost inconceivably tiny. They are red flexible disks, a little bit thinner in the middle than at the edges, about one three-thousandth of an inch in diameter. Some idea of the enormous numbers in the body can be gathered, when we say that a drop of blood the size of the head of a pin would contain four or five million of them. The red corpuscles are made up of a sort of framework of protein within which is inclosed a red coloring matter or pigment, known ashemoglobin. It is this pigment that gives the blood its color, and in some respects it is one of the most important of the nonliving substances in
STRUCTURE OF A DROP OF BLOOD AS SEEN UNDER THE MICROSCOPE Two white (colorless) corpuscles a appear. The remainder are red corpuscles sticking together, forming rouleaux. (From Martin’s “Human Body.”)STRUCTURE OF A DROP OF BLOOD AS SEENUNDER THE MICROSCOPETwo white (colorless) corpuscles a appear. The remainder are red corpuscles sticking together, forming rouleaux. (From Martin’s “Human Body.”)
Two white (colorless) corpuscles a appear. The remainder are red corpuscles sticking together, forming rouleaux. (From Martin’s “Human Body.”)
the body. This is because it is the means by which the cells obtain sufficient supplies of oxygen. As we have already seen, every cell is constantly drawing from the body fluids about it the oxygen which is required for carrying on its metabolism. The fluids in turn get oxygen from the blood. It is necessary, therefore, for the blood to convey abundant supplies. Oxygen will dissolve in water, as is proven by the fact that fish and other aquatic animals are able to get enough oxygen from the water in which they live to serve their needs; but it is not sufficiently soluble to supply the needs of an active body like that of man. It is necessary, therefore, to have a special additional means of conveying oxygen besides its simply dissolving in the blood. This additional means is furnished by the hemoglobin, which is an “oxygen-carrying” pigment. What this means is that the hemoglobin has the property of taking up oxygen chemically, whenever it is exposed to a region where there is oxygen in abundance, and of giving it up again whenever it passesthrough a region where oxygen is scarce. It is thus that oxygen is conveyed from the lungs to the active tissues of the body. We shall have more to say about this in the chapter devoted especially to the matter of the oxygen supply.
We said a moment ago that the red corpuscles consist of a protein framework inclosing hemoglobin. They are not living. They must, therefore, have been made by living cells and poured out into the blood stream. We might suppose that this was done once for all and that the same red corpuscles are floating in our blood now that started floating there when the blood was first formed; but, as a matter of fact, this is not the case. There is a continuous breaking down of red corpuscles which must be made good by a continuous manufacture of new ones. Most of the larger bones in our bodies have a sort of spongy framework by which the ends, where the joints are, are made stronger. Within the space of these frameworks is a kind of marrow, known asred marrow, because it has such a very abundant blood supply. It is in this red marrow that the manufacture of red corpuscles goes on. There are throughout the red marrow living cells which are constantly dividing and subdividing, forming more and more so-calleddaughter cells. Within these daughter cells hemoglobin is presently deposited; a little later they lose the nucleus and probably the remainder of the living protoplasm as well, leaving just the framework of nonliving protein with its contained store of hemoglobin. This is the finished red corpuscle, and it breaks loose from the red marrow and floats out into the blood stream. The rate of manufacture of red corpuscles is very rapid; undoubtedly millions of them areformed daily in the various red marrow regions of the body. The total number of red corpuscles does not increase correspondingly, because they are broken down at the same rate as they are formed. It is now believed that in thespleen, which is a large organ of the abdomen whose function has always been obscure, those red corpuscles which are destined for destruction are picked out of the blood and broken down. We commonly suppose that this fate overtakes corpuscles that are worn out and are no longer efficient oxygen carriers, but we do not know, as a matter of fact, that the corpuscles do lose their effectiveness in course of time, nor have we any idea how the spleen could select out of the millions in the blood those particular ones which are no longer useful.
What the spleen does to the corpuscle is to break it up so that the protein and the hemoglobin in it are set free in the blood stream. We do not know what becomes of the protein; probably it is taken up and utilized. We do know that the hemoglobin is decomposed in the liver. One constituent of hemoglobin, in fact the constituent which gives it its ability to carry oxygen, is the element iron. Iron is not particularly abundant in living things, and we find that the body is thrifty with regard to it. When the liver decomposes the hemoglobin, the iron is saved in some way which enables the blood to carry it back to the red marrow, where it can be used over again. There are also some portions of the hemoglobin which are valuable as food material; the remainder, which is of no further use, is discharged from the body as a part of the bile.
Besides the red corpuscles the blood contains what are known as the white or, better, the colorless corpuscles. These, instead of being dead structures,as are the red corpuscles, are actual living cells, which float along in the blood stream or have the power of clinging to the walls of the blood vessels and crawling along them; in places where the blood vessels are very thin, they work their way right through, and so get out into the spaces among the tissues. There are not nearly so many colorless as red corpuscles, in fact, the latter are eight or nine hundred times as numerous as the former. A great deal of study has been devoted to colorless corpuscles to find out just what they do. We are by no means certain that we understand fully all their activities, but we do know that one very important thing that they do is to absorb into their own bodies tiny foreign particles that may be present in the blood stream or in the spaces among the tissues. There are not many kinds of foreign particles that can get into these places. In fact, about the only sort that can are the tiny plant or animal cells which are responsible for disease. When these foreign organisms invade the body, the colorless corpuscles may engulf them into their own bodies and destroy them. In this way a great deal of infection is prevented. Of course it may happen that the invading organisms are so numerous that the colorless corpuscles can not get rid of all of them, or the corpuscles themselves may show a diminished activity. It is an interesting fact that both the number of colorless corpuscles in the body and the vigor with which each colorless corpuscle attacks foreign organisms vary from time to time, so that we are much more secure against infection at some times than at others. In general we may say that when the body is in a condition of vigorous health the colorless corpuscles will be efficient. When we arerun down, on the other hand, these cells share with all other body cells this state of low vitality. This explains why people who allow themselves to become run down are more likely to fall victims to infection than those who are in vigorous health, and emphasizes, of course, the importance of a habit of life that shall keep the body vigorously healthy at all times. It should be understood that the colorless corpuscles do not show the same effectiveness against all kinds of foreign organisms; some kinds of disease germs are able to bring about infection in the body quite regardless of the activity of the colorless corpuscles. From other kinds, on the other hand, they give the body very complete protection.
The most familiar example of the action of the colorless corpuscles is in the formation of what we all know as pus. There are a few kinds of organisms that, instead of getting into the body and becoming scattered through its fluids, establish themselves at certain points and by growth and multiplication accumulate at those places in large numbers. Examples are pimples and boils. In these cases the pus-forming organisms have located just under the skin and are multiplying there at a great rate. They produce poison which is absorbed from the place where they are and distributed through the body. This poison appears to have some sort of chemical attraction for the colorless corpuscles; at any rate the corpuscles gather from all around to the place where these organisms are located and engulf as many of them as they are able, but in so doing they themselves are destroyed, and pus, as we know it, is simply a mass made up of the dead bodies of the colorless corpuscles along with the organisms which they have destroyed and which in turn havedestroyed them. In the ordinary pimple or boil the colorless corpuscles ultimately win the combat and the infection is completely overcome. It happens occasionally, however, that a pus-forming infection becomes established in some place where it manages to maintain itself in spite of the attacks of the colorless corpuscles. This happens very frequently in the tonsils, so that persons who have infected tonsils may have pus formation going on month in and month out. This may also take place at the roots of the teeth. In fact it is now commonly believed that whenever the nerve to the tooth dies, infection is certain sooner or later to become established at the tip of the root. These places where pus formation is going on continually are known as “pus pockets.” For a long time very little attention was paid to them. Persons occasionally suffered acute distress from gumboils or had attacks of sore throat owing to the infected condition of the tonsils, but beyond these no particular attention was paid to the pus-forming organisms, unless, as occasionally happened, an especially virulent type became established which brought on the condition commonly known as blood poisoning. Within the last few years the discovery has been made that there is a steady discharge of poison from every pus pocket into the body fluids and so to the blood stream. The amount is ordinarily so small that the body as a whole is not seriously affected, but now and then, either because of a larger production of poison or because of a lower resistance on the part of the body, serious ill effects are produced. Among them may be mentioned acute (inflammatory) rheumatism. This is not only an extremely painful condition, but it is very likely to leave serious after-effects, as, for example, injuredheart valves, to give trouble for the rest of life. The discovery of these facts has given us great respect for pus pockets, so we no longer treat them carelessly. Infected tonsils are looked after, not simply because they bring about sore throat now and then, but even more because of the poison which they are likely to send through the body. It is probably not too much to say that the practice of dentistry has been revolutionized since the significance of pus pockets has been discovered. Formerly the killing of nerves to relieve aching teeth or to insure them against future aching was a common practice. The modern dentist, on the other hand, kills nerves to teeth with the very greatest reluctance and only as a last resort, because he knows full well that in so doing he is opening the way for the establishment of pus pockets with the train of ills that is likely to follow. At the present time methods of curing pus pockets at the base of the teeth are not very adequate, so that the extraction of the tooth has to be resorted to, but there is every reason to believe that the dental profession will shortly find methods by which pus pockets can be controlled without having to sacrifice the teeth.
The third kind of structures which are present in the blood stream are much smaller than the red corpuscles, but are nowhere near so numerous. They are called theplateletsand are disk-shaped bodies composed chiefly of protein material, and probably, although not certainly, living cells. Their presence in the blood remained unsuspected up to about the end of the last century, not so much because of their very small size, as because they go to pieces very quickly after the blood is shed. By the time a drop of blood could be gotten under the microscope theywould be all gone. They were discovered only as the result of the adoption of a method of treating the blood which preserved them long enough so that they would still be present when the blood was looked at under the microscope. They seem to have something to do with the changes that take place in the blood by which it is caused to clot. Whether this is their only function, we do not know.
We have now described the substances which are present in the blood and in the other tissue fluids as a fairly permanent part of their make-up. In addition to these there are present all the materials that are in transit to the cells or from them. These include all kinds of foodstuffs on their way from the digestive organs to the cells. As we shall see in detail later, the digestive organs work over the food that we eat before passing it on to the blood, so that the actual food materials that are being transported by the blood are the digested products of the food rather than the food itself. For example, there are to be found in the blood, in addition to the regular blood proteins which were described a moment ago, the digestion products of the food proteins on their way to serve the needs of the various cells of the body. There will be found also the digestion products of the other classes of foodstuffs. The body fluids contain also the waste products of cell metabolism on the way to be discharged from the body and the special products, such as the hormones, which are manufactured by certain cells and carried through the blood stream to act upon other cells. Of course we realize that not all of the materials that are manufactured by cells are poured out into the blood stream. Such materials as saliva, gastric juice and the like are passed directly from the cells in whichthey are manufactured into tubes by which they are conveyed to the digestive canal, where they carry on their special work of digesting the food.
Among the things which are present in the body fluids should be mentioned the two gases oxygen and carbon dioxide. We should expect oxygen to be present in the body fluids, because it is necessary for the metabolism of cells and can get to them only by being carried in the blood stream. We have seen in the red corpuscles the special method by which an abundance of oxygen is transported. Carbon dioxide is the gaseous product of the oxidation of carbon and is found in large amounts wherever there is burning, since carbon is the chief constituent of all fuel and whenever carbon is burned, carbon dioxide is formed. Since the fuel materials that are burned in the living cells consist largely of carbon, carbon dioxide is produced in them as well. They have to get rid of it, and the only way they can do so is by passing it out into the fluids that surround them, which in turn pass it on to the blood. The way in which the body handles these two gases makes up a special subdivision of the subject of physiology and will be so treated in the chapter on respiration.
IN the last chapter we talked about the body fluids and saw that they can be subdivided into the tissue fluids, which surround the cells, and the blood, which is inclosed in a system of pipes and which carries the materials to and from the tissue fluids. We now have to take up this matter of transporting the material in more detail. The first step will be to see how materials that are in the blood get from it to the tissue fluids, and how materials that are in the tissue fluids get from them into the blood. Unless these interchanges can take place freely, there is no way in which the blood can serve as a conveyer system. In order to see how the interchanges are carried on, we shall have to look first at some features of the construction of the system of pipes through which the blood flows. As we all know, the large blood vessels are either arteries or veins, the arteries being blood vessels which are carrying blood away from the heart, and the veins vessels that are carrying blood toward the heart. If we start with an artery and trace it through the body, we find that it is continually giving off branches which in turn give off smaller branches, until finally the subdivisions are so small that we cannot trace them any further with the naked eye. In the days before the microscope was discovered, there was a great deal of question as to how these finest branches ended. At firstno one suspected that there was connection between the fine subdivisions of the arteries and the fine subdivisions of the veins through which the blood could pass.
About the beginning of the seventeenth century William Harvey became convinced that there must be fine vessels leading across from the smallest arteries to the smallest veins and that the blood must pass through these. He came to this conclusion without ever having seen these small vessels, since at that time there was no microscope by which such tiny structures could be seen. Before Harvey’s time it was supposed that the blood ebbed and flowed in the arteries and in the veins. He showed that the blood flows in one direction constantly, leaving the heart by way of the arteries and coming back into it by the veins. Harvey was, therefore, the discoverer of the circulation, one of the most important discoveries in physiology. After the microscope was perfected the tiny tubes connecting the finest arteries with the finest veins were made out. They were found to be very small in diameter and to have very thin and delicate walls. They were also found to be extremely numerous. The finest subdivisions of the arteries that can be seen with the naked eye are scattered very thickly through all the tissues of the body which have a blood supply, and they go on subdividing microscopically, so that the finest vessels are scattered more and more thickly through the mass of living substance. The very finest of all, which are the tubes connecting the smallest arteries with the smallest veins, are calledcapillaries, from a Latin word meaning a hair, to indicate their very small size. They are so close together in most parts of the body that it would be difficult to thrust a
Photo, Paul Thompson THE TEST FOR BLOOD PRESSUREPhoto, Paul ThompsonTHE TEST FOR BLOOD PRESSURE
Photo, Paul Thompson PART OF PROCESS OF URINALYSIS INDICAN TESTPhoto, Paul ThompsonPART OF PROCESS OF URINALYSIS INDICAN TEST
A NETWORK OF CAPILLARIES The artery a and vein v (highly magnified). (From “The Human Mechanism,” Hough and Sedgwick)A NETWORK OF CAPILLARIESThe artery a and vein v (highly magnified). (From “The Human Mechanism,” Hough and Sedgwick)
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 itwill 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 arenot 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 describingthe 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 theaorta. This springs from the upper side of the heart, bends over in an arch, and passes down through the chest into the abdomen.
THE HEART AND BLOOD VESSELS DIAGRAMMATICALLY REPRESENTED L, lung; M, intestine; P, liver; dotted lines lymphatics. (Martin’s “Human Body”)THE HEART AND BLOOD VESSELS DIAGRAMMATICALLY REPRESENTEDL, lung; M, intestine; P, liver; dotted lines lymphatics. (Martin’s “Human Body”)
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 armwith 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, onefor the lower part of the body, called theinferior vena cava, and one for the upper part of the body, called thesuperior 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 theportal 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 ordinarycircumstances 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 thepulmonary artery, breaks up into capillaries in the lungs, which reunite into thepulmonary veinwhich 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 theheart 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 thetissues 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.
DIAGRAM SHOWING THE RELATION OF THE TWO HALVES OF THE HEART ra and rv, right auricle and ventricle; la and lv, left auricle and ventricle; ao, aorta; vc, venæ cavæ; pa, pulmonary artery; pc, pulmonary capillaries; pv, pulmonary vein. (Martin’s “Human Body.”)DIAGRAM SHOWING THE RELATION OF THETWO HALVES OF THE HEARTra and rv, right auricle and ventricle; la and lv, left auricle and ventricle; ao, aorta; vc, venæ cavæ; pa, pulmonary artery; pc, pulmonary capillaries; pv, pulmonary vein. (Martin’s “Human Body.”)
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 pulmonaryartery 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 areciprocating 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 asthe 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 theventricle; 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 theauricle, 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 whereit 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 theauriculo-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 thetricuspid valve. The intake valve on the left side of the heart, which is composed of but two flaps, is known as themitral 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 rapidenough 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 distinctsounds: 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 thematter 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. Thevariationsin 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