Physiology: The Science of the Body

by Ernest G. Martin

In concluding this part of our discussion we should note that artificial stimulation to intestinal activity should be regarded as a measure of last resort and under no circumstances as a habitual means of inducing evacuation. Persons who allow themselves to become dependent upon laxatives are laying up for themselves trouble, since these invariably become less and less effective, making it necessary to increase the dose and finally establishing a condition in which only the vigorous efforts of a physician will restore the body to normal. A safe general rule is that anyone who finds himself becoming dependent upon laxatives should immediately put himself under competent medical care for the purpose of restoring his system to normal functioning in which he will not be dependent upon drugs.

CHAPTER XVI
THE SERVICE OF SUPPLY OF OXYGEN

EVERY cell must have oxygen for its metabolism. This it must get from the tissue fluid upon which it fronts, and tissue fluid in turn must get it from the circulating blood. The blood in turn has to get it from somewhere and the place from which the blood gets it is called a lung or a gill, according as the animal breathes air directly or gets its oxygen out of the water. The purpose of the present chapter is to trace oxygen from the air through the blood to the tissue fluids and so to the living cells. We saw in the chapter on “Blood” that we have a special substance, the hemoglobin, which helps in the transportation of oxygen. We shall have occasion here to see how it does this. It is well to bear in mind that for practical purposes we include the methods by which carbon dioxide is gotten rid of along with the study of the supply of oxygen, so that although this chapter carries the heading “The Service of Supply of Oxygen,” we shall also study in it how the carbon dioxide is carried away. This is a convenient way of dealing with the subject, because the two gases are handled in very much the same manner; it is also made necessary by the fact that the control of the apparatus by which these gases are handled is so interwoven that the transportation of one could not well be studied without giving attention also to the transportation of the other.

The problem of the oxygen supply and of the removal of the carbon dioxide is in theory very simple; the air contains a large percentage of oxygen and a very small percentage (three parts in ten thousand) of carbon dioxide. If the blood is exposed to air with nothing between but a very delicate membrane, oxygen will diffuse from the air into the blood until the blood has taken up all that it is capable of holding. As the blood circulates around the body and comes to the tissue spaces where there is little or no oxygen, because the living cells are constantly taking it up and using it, the oxygen which previously diffused into the blood will diffuse out into the tissue spaces. The only special arrangements that have to be provided are a sufficiently great surface of exposure to the air, so that no matter how rapidly the blood may be flowing it shall be able to take up all the oxygen it can hold, and an arrangement for making sure that the blood can hold and carry as much oxygen as the tissues need. The first of these requirements is met by the special construction of the lung or gill; the second by having present in the blood stream a chemical substance (hemoglobin) which automatically takes up large quantities of oxygen and so insures that sufficient shall be transported.

In principle the structure of a gill corresponds with that of a lung; since we are particularly interested in the working of our own bodies we shall content ourselves with describing only a lung. We saw in the chapter on the Circulation that the pulmonary artery which leads away from the right side of the heart breaks up into a system of capillaries. These capillaries are thousands in number and they are spread out over the whole lung surface. The lung itself consists of a hollow bag with very thin and very elastic walls connecting with the throat by means of the windpipe. In reality the bag is double, for the windpipe splits at its lower end into two tubes, known as the chief bronchial tubes, and these subdivide repeatedly until their fine terminals end in the elastic lung sacs themselves. We spoke of the lung as a bag; in reality it is a system of thousands upon thousands of separate tiny bags. The structure is comparable to that of a bunch of grapes, the stem representing the chief bronchial tube, the smaller stems the subdivisions and the individual grapes the lung sacs proper. The advantage of this arrangement is, of course, in the very large surface which it gives; every one of the individual lung sacs has its wall filled with capillaries and there are so many of the tiny individual sacs that the total surface over which the blood is spread is measured in hundreds of square feet. We cannot imagine any other arrangement by which so large a surface of exposure could be packed away into a cavity the size of the human chest.

Of course, we see immediately one serious defect of this arrangement of the lung surface; every one of the individual sacs is full of air and so the blood vessels which line its wall have exposure to air, but between the individual lung sac and the outside atmosphere is, first, the very tiny bronchial tube with which the sac connects, then the somewhat larger one into which that opens, then a still larger one, and so on until we come by way of the chief bronchial tube and windpipe up to the throat, and so through the mouth or nose to the outside. It is quite evident that this system of passages does not permit of a very free movement of air. We must realize also that the blood which flows through the walls of the lung sacs must constantly take up oxygen from the air within the sacs, if it is to meet the needs of the body tissues. Simple diffusion through the narrow bronchial tubes could not possibly bring oxygen into the lung sacs fast enough to supply the requirements of the blood flowing through their walls. The situation is met by active lung ventilation; that is, by forcibly changing the air in the lungs at frequent intervals. The way in which this is done is, as we all know, by breathing. Breathing is nothing but a bellows movement of the chest by which air is alternately expelled and allowed to enter. It does not require a very active lung ventilation to keep the air in the lung sacs sufficiently supplied with oxygen under conditions of bodily quiet. Our ordinary breathing movements are gentle, less than a quart of air is breathed in and out again in every breath, and we breathe only fifteen or sixteen times a minute. Of course, when there is vigorous functional metabolism, as in brisk muscular exercise, the oxidation processes in the tissues go on at a very much more rapid rate, and correspondingly larger amounts of oxygen must be carried by the blood to meet the demand. Under these circumstances there is an improvement in lung ventilation, the movements of the chest are greater and also happen more times in a minute.

The act of breathing is carried on by ordinary skeletal muscles. This is the only act connected with bodily maintenance of which this is true. Our other “vital” organs are operated by means of smooth muscles. On account of this difference we have a certain degree of control over our movements of breathing. As we all know, we can hold the breath for a short time without difficulty, or can breathe more quickly or more deeply any time we choose. In this respect breathing differs strikingly from either the heart action or the movements of the digestive organs, over which we have no direct control at all. Our control of the muscles of breathing is, however, rather limited; we cannot hold the breath indefinitely. This means that the nervous mechanism which causes the muscles to contract will work in spite of the efforts of our will to prevent it. The actual machinery is very much like that which has already been talked about in connection with other “vital processes.” We have a “center” in the brain stem from which the nervous discharges come. This center is located immediately adjoining the vasoconstrictor center about which we learned in Chapter XIV. Because of the location of these two important centers in a single very small space, the spot where they are was named by a French physiologist more than a hundred years ago “the vital knot”; the point of this is that death can be induced more quickly and with less actual tissue destruction here than anywhere else in the body.

The center which controls breathing has been named the “respiratory center.” It discharges automatically about fifteen or sixteen times a minute, causing the muscles of breathing to contract and so the bellows action of the chest to be carried on. Like the other centers in the brain stem this one can be acted upon by nervous disturbances passing into the brain stem from the sense organs. Perhaps the best example of this is the modification of breathing that comes as the result of a dash of cold water on the skin. Most of us have noticed that we give a sort of gasp upon stepping suddenly under a cold shower or plunging into cold water. It may not have occurred to us that this gasp is entirely involuntary, but we can easily prove that it is by trying to breathe with perfect regularity at the moment of stepping under a cold shower. We shall easily convince ourselves that this modification of the breathing is something over which we have no control. It is, as a matter of fact, an excellent example of a simple reflex. Coughing and sneezing are other reflexes in which sensory irritation of some sort acts upon the respiratory center modifying its discharges. In addition to these reflex changes in breathing we have also the familiar effects of muscular exercise. We know that after even moderate exercise the breathing is quickened somewhat, and after vigorous exercise it becomes very rapid and deep, and that after very severe exertion, particularly in an untrained person, the puffing and blowing is not only pronounced but even distressful. We shall see presently how muscular exercise brings these changes about, but before doing so it will be necessary for us to take up the movement of the gases between the lungs and the tissues, by way of the blood; oxygen from lungs to tissues, carbon dioxide from tissues to lungs.

By lung ventilation the air in the tiny individual lung sacs is kept supplied with oxygen and also measurably free from carbon dioxide. From this air there is a continuous diffusion of oxygen into the blood. The first oxygen that diffuses in may dissolve in the blood liquid just as oxygen will dissolve in any water to which it is exposed, but as this goes on the hemoglobin of the red corpuscles begins to take up oxygen, forming a chemical compound to which is given the name of oxyhemoglobin. If there is enough of the gas present, every molecule of hemoglobin will take up oxygen to its full capacity. The amount that will dissolve directly in blood is so slight that to all intents and purposes the ability of the blood to carry oxygen depends on the extent to which hemoglobin can combine with it. It is important to emphasize this, because it means that there is a definite limit to the amount of oxygen that the blood can carry, a limit which is reached as soon as the hemoglobin is saturated. Hemoglobin has so great a power of combining with oxygen that the moderate lung ventilation which ordinary quiet breathing gives suffices usually to saturate it. Now and then we encounter statements which give the impression that there is a virtue in deep breathing in improving the amount of oxygen which becomes available for our tissues. As a matter of fact, this is not the case; ordinary quiet breathing when the body is at rest saturates the blood with oxygen, which means that it is carrying its full cargo, and evidently no amount of deep breathing can make it do more than that. We should not be understood as intimating that deep breathing is not a valuable exercise; the point which we wish to emphasize is that its value does not lie in affording an increased supply of oxygen.

Oxyhemoglobin is of a bright scarlet color; hemoglobin itself, not combined with oxygen, is a very dark purplish color; partial combinations are brighter and brighter as they contain more oxygen, so that an expert can judge of the degree to which any specimen of hemoglobin is combined with oxygen by noting its color in comparison with fully saturated hemoglobin. The combination of hemoglobin with oxygen takes place as the blood is passing through the capillaries of the lungs; therefore the blood which leaves the lungs has the bright scarlet color characteristic of oxyhemoglobin. This blood is called arterial blood, the reason is that it is the kind that is found in the arteries of the body in general. It happens that it makes its first appearance in the pulmonary vein, by which it is conveyed from the lungs to the left side of the heart, so that the expression arterial blood does not mean anything in particular except to describe blood in which the hemoglobin is saturated with oxygen. This blood is pumped out by the left side of the heart to all the parts of the body; in its passage through the capillaries it is in a region where there is active utilization of oxygen by the living cells. These are steadily taking up oxygen from the tissue fluids about them, so the blood in the capillaries, which is carrying an abundant supply of oxygen, is brought in contact with tissue fluids containing little or none, with only the delicate wall of the capillaries between. Under these circumstances rapid diffusion of oxygen from the blood into the tissue fluids takes place and accompanying this there is a breakdown of oxyhemoglobin, so that not only most of the oxygen which was dissolved in the blood passes out, but also a considerable part of that which was formerly in combination with hemoglobin. Under ordinary circumstances only from one-fourth to one-third of the oxyhemoglobin decomposes during the rapid passage of the blood through the capillaries; thus the blood that goes on into the veins will still be carrying two-thirds or more of the total oxygen cargo. The color of venous blood will be darker than that of arterial blood, because it contains a good deal less oxyhemoglobin, but it is nowhere near so dark as is blood in which all the oxyhemoglobin has been decomposed. This is well recognized in melodramatic fiction where the wounds of persons who have met death by strangulation are described as oozing black blood. It is an actual fact that blood from which all the oxygen has been withdrawn is so much darker than ordinary venous blood that it gives the impression of being black, although it is really a dark purple.