If we should be moved to inquire why so small a fraction of its whole store of oxygen is given up by the blood to the tissues ordinarily, we shall find the answer in remembering that the demand of the body for oxygen is extremely variable; every increase in functional metabolism means an increase in the amount of fuel that is oxidized and therefore an increase in the amount of oxygen that is required. Actual measurements have shown that in very vigorous muscular exercise the oxygen consumption may be approximately ten times as great as in complete rest. In order that this very greatly increased metabolism may be carried on it must be possible for the blood to deliver approximately ten times as much oxygen to the active tissues as it delivers to them when quiet. There are just two ways in which this can be done; one is by a more complete decomposition of the oxyhemoglobin by which all its oxygen is set free; the other is by a more rapid movement of the blood. It is by a combination of these two that the oxygen requirements of the body in times of vigorous metabolism are taken care of. As has already been said, the heart rate is just about doubled in vigorous exercise. There has also been shown to be some increase in the amount of blood that it pumps with every beat. The result is that more than twice as much blood leaves the heart in a minute under these circumstances as in time of rest. The oxyhemoglobin is also completely decomposed when the tissues are active, and these two facts together are sufficient to account for the great increase in the oxygen supply.
Hand in hand with the increased consumption of oxygen, there is of course an increased production of carbon dioxide and of water, since the oxidation of fuel substances produces these waste products. The cells are always pouring both out into the tissue fluids, but to a very much greater extent when they are actively functioning. We need make no effort to keep track of the water, since it merely adds itself to the water already present, and we shall consider later how the water supplies of the body are handled. The carbon dioxide, however, must be gotten rid of, and the mechanism for getting rid of it must work efficiently, otherwise metabolism itself will be hampered, since it is a familiar law of chemical action that if the products of an action are allowed to accumulate they interfere with its further progress. The method of getting rid of carbon dioxide is by simple diffusion from the cells into the tissue fluids and from the tissue fluids into the blood. Carbon dioxide is many times as soluble as oxygen, so that a great deal more of it can be handled by merely dissolving. This is not sufficient, however, to take care of all the carbon dioxide; the remainder must go into chemical combination with some substances that are in the blood. There is no single conspicuous material for carrying carbon dioxide like the hemoglobin which transports the oxygen. There are, however, a number of compounds in the blood which are able to combine with carbon dioxide, among them the blood proteins of which much was made in Chapter XIII. The carbon dioxide distributes itself among these various substances and so is transported. It should be noted that the blood does not become saturated with carbon dioxide as it does with oxygen. Arterial blood ordinarily carries practically all the oxygen it is able to take up; venous blood on the other hand probably never comes anywhere near being as fully charged with carbon dioxide as it is able to be.
During the passage of the blood through the capillaries of the lungs an outward diffusion of carbon dioxide into the air in the lung sacs is going on simultaneously with the inward movement of oxygen from this air into the blood. The diffusion is never so complete as to deprive the blood of all its carbon dioxide; there is in fact only a little less of it in arterial blood than in venous, although the diffusion is sufficiently rapid so that as much carbon dioxide as is produced in the whole body in a minute is passed out into the air of the lung sacs in the same time. The effect of this outward diffusion is naturally to increase the amount of carbon dioxide in the air of the lung sacs, and if this increase is allowed to go on unhampered, there will presently be so much carbon dioxide there as to stop further outward diffusion, and so to put an end to the escape of carbon dioxide from the blood. This is avoided by lung ventilation. Every time a breath is drawn some air that is almost free from carbon dioxide enters the lung spaces to replace the carbon dioxide-laden air that was expelled at the previous exhalation.
The description of gas transportation that we have just given opens the way for an account of the control of breathing. From what has just been said it should be clear that the amount of carbon dioxide in the blood corresponds closely with the amount that is in the air of the lung sacs. As the percentage of carbon dioxide in this air goes up, outward diffusion becomes less free, and so the amount of carbon dioxide in the blood will have to increase. The tissues are all the time producing and pouring out carbon dioxide, and so there will be a steady increase in the amount of carbon dioxide in the blood. This applies to the arterial blood as well as to the venous, since, as we saw a moment ago, there is nearly as much carbon dioxide in the former as in the latter. This is the fact which is utilized in the body for operating the breathing machinery. The respiratory center in the brain stem is susceptible to carbon dioxide; the more of this gas there is in the blood, the more tendency there will be for the center to discharge. There is a certain level of carbon dioxide below which it is entirely inactive, but when this level is passed nervous discharges begin and become more and more powerful as the amount of carbon dioxide in the blood goes up. Now we can see what makes us breathe. Let us imagine that there is not very much carbon dioxide in our blood, but that the tissues are constantly producing it and giving it off. Since we are supposing the amount is not enough to excite the respiratory center, there will be no movements of breathing. There will be a steady increase of the amount of carbon dioxide in the blood and at the same time a corresponding increase of the amount of carbon dioxide in the air sacs of the lungs; presently there will be enough in the blood to arouse the respiratory center to discharge. This will cause a breath to be drawn; the effect of this will be to sweep out much of the accumulated carbon dioxide from the lung sacs; this in turn enables more rapid diffusion of carbon dioxide from the blood to occur, and so the amount of it in the blood may fall below the level at which the respiratory center is made active. In a moment, of course, the continued outpouring of carbon dioxide from the tissues will raise the level again to the point of exciting the respiratory center, and so we will have a rhythmically recurring discharge of that center causing a rhythmic drawing of breath.
According to the account just given the activity of the respiratory center is determined exclusively by the carbon dioxide in the blood; it could be so regulated, but, as a matter of fact, in all higher animals, including man, the carbon dioxide control of the respiratory center is interwoven with a complicated nervous control whose effect is to make us breathe more often in a minute, but to make the individual breaths shallower than they would be if the control of breathing were exclusively by means of carbon dioxide. The net result in lung ventilation is exactly the same, but the rapid shallow breaths are advantageous in that they avoid large fluctuations in the amount of carbon dioxide in the blood, while they do serve fully to provide sufficient oxygen.
The rate and vigor of breathing are ordinarily adjusted automatically to the amount of carbon dioxide in the blood stream, but, as we know, we can, of our own will, breathe quite differently. Let us see what will happen if while we are sitting quietly we begin to breathe deeply and rapidly, overventilating the lungs. So far as oxygen is concerned, this will make no difference at all, since, as we have already seen, the ordinary automatic breathing keeps the blood charged with all the oxygen it can hold. What overventilation does is to sweep out the carbon dioxide from the lung sacs more rapidly than usual and this permits of a correspondingly more rapid outward diffusion of carbon dioxide from the blood. The result will be that carbon dioxide will leave the blood faster than it is being poured into it from the tissues, and so the total amount of the gas in the body will be cut down. The first effect of this we would expect to be the removal of the automatic stimulation of the respiratory center, so that, after a period of excessive breathing, one would not at once resume breathing spontaneously. This, as a matter of fact, is the case; anyone can easily prove on himself, by breathing deeply and rapidly for a minute or two, that the automatic control of breathing is temporarily suspended immediately after. It follows naturally that one can hold the breath a good deal longer if the lungs are overventilated for a short time just before the attempt is made. This also can be easily proved. Prolonged overventilation of the lungs has, likewise, a number of other effects, all of which are due to cutting down the total amount of carbon dioxide in the body. The most conspicuous is a feeling of dizziness or light-headedness that comes on. If pushed to excess, there is a very definite feeling as though one were about to soar away into space, and this is followed by unconsciousness. Certain religious cults in India have interpreted this sensation resulting from deep breathing as an actual severance of soul from body, and maintain that during the time of unconsciousness the spirit really floats freely in space. Without venturing any statement as to the relation between the soul and the body during either consciousness or unconsciousness, we would point out that these bodily sensations are definitely due to the very simple fact that there is less carbon dioxide in the blood than is normal on account of the overventilation of the lungs, and just as soon as the metabolism that goes on all of the time in the tissues pours out enough carbon dioxide to bring the amount up to normal, consciousness will return and the ordinary condition of affairs will be resumed.
Although this finishes what we have to say about the movements of gases into and out of the body, the general subject cannot be completed without a word concerning the conditions that should be maintained in the air immediately surrounding us. This makes up the topic of ventilation. We all know that some air is much more fit to breathe than other; until very recently, however, our ideas as to the conditions which make air fit or unfit to breathe have been hazy or entirely erroneous. Fortunately, of late years, the subject of ventilation has been actively investigated and we now have a satisfactory knowledge of its laws.
There are, of course, two things that must be true of any air that is to be breathed; these are that it must contain enough oxygen and must not contain too much carbon dioxide. So far as the oxygen supply is concerned we may state that only with the greatest difficulty are conditions reached in which there is not enough oxygen in the air. As we all know, the air becomes thinner the higher we go above the surface of the earth; both mountain climbers and aviators have attained heights at which the amount of oxygen in the air is only about one-third that of ordinary air and have been able to obtain enough oxygen for their bodily needs even under those extreme conditions. It is quite evident that a room could scarcely be so poorly ventilated as to bring the oxygen supply down below this figure, so that no attention need be paid to the oxygen supply in working out practical methods of ventilation. Air which contains carbon dioxide to the extent of four per cent could not be breathed because the carbon dioxide being produced in the body would not diffuse out fast enough into an atmosphere containing that amount of carbon dioxide to keep the body alive. This again is a percentage of carbon dioxide that is practically never reached. Probably the most famous case in history of death from poor ventilation is the “Black Hole of Calcutta,” a dungeon room about twenty feet square with only two small windows, in which one hundred and fifty British soldiers were imprisoned over one night; all but twenty-three of these died, but it is doubtful whether their death was actually due either to deficiency of oxygen or to excess of carbon dioxide. This is because there were enough other factors which would make the air unbreathable to bring on death before either of these could come into play. The modern science of ventilation concerns itself with these other factors; chief among them is the factor of moisture. As we shall see in the next chapter our bodies are constantly giving off from the lungs and by evaporation from the sweat glands water vapor into the air. This causes the humidity to go up rapidly in rooms where people are congregated, and particularly so where there are many people present. Also everyone gives off a great deal of heat. We now know that the feeling of closeness which we ascribe to a poorly ventilated room is due to the combination of warmth and moisture. We also know that the discomfort which comes from being in such rooms is due to the same causes. Actual vitiation of the air is much less disagreeable than is the accumulation of heat and moisture. In theory, of course, the best ventilation is secured by keeping rooms flooded with outdoor air. In practice, however, this does not always work out; for example, in many cities the air is so laden with dust and smoke as to be bad for everybody and even dangerous for sick people. Before such air is breathed the smoke and dirt should be gotten out of it. This is done sometimes by forcing it through fine mesh cloth bags, or the most modern scheme is by passing it through a thin screen of water and so washing the dirt and smoke out of it. The second practical difficulty with flooding rooms with outside air is the expense in cold weather of warming the large volumes that would be required. For this reason it has been found feasible in churches and public halls that are occupied only occasionally to use the same air over and over by keeping down the temperature and moisture. Of course, this cuts down very greatly the expense of heating.
There is one other source of harmful effect from bad air besides the high humidity and undue warmth; this is the presence in it of ammonia and other poisonous compounds that are given off from the bodies of people. It used to be believed that organic poisons were exhaled from the lungs with every breath, but we now know that the amount of these, if any are present, is too small to be important in comparison with the very much larger amounts that come off from the evaporating sweat, from decaying teeth, and from the digestive tract; there is no doubt that in any assemblage of people the air will be vitiated by organic poisons from these sources. The more cleanly the individuals are, the less will be the contamination. It is generally believed, although perhaps not absolutely proven, that the bad health found in sweatshops and crowded slums generally is due largely to chronic poisoning from the constant breathing of effluvia from the unwashed bodies and clothing of the inhabitants. The obvious remedy is insistence upon personal cleanliness, although this does not lessen the desirability of breathing as pure air as can be gotten. The point to be emphasized is that where personal cleanliness prevails, the closeness of rooms is chiefly due to excessive moisture ordinarily accompanied by too high a temperature. Ventilation measures should be carried out with this in mind.