The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

Normally, adjustment is brought about in the following way. If after a period of rest there is a sudden spell of exertion, the muscles will consume more oxygen and produce more carbon dioxide; therefore the blood becomes poor in the former gas and rich in the latter. But it is the excess of carbon dioxide and not the shortage of oxygen which stimulates the respiratory centre. It is the unneeded and not the needed gas that makes the breathing increase and restores the proportion of these gases in the blood to the normal.

This arrangement will work all right if the oxygen-lack is accompanied by simultaneous carbon dioxide excess, and this always occurs during exertion. But sometimes the oxygen shortage may be due to causes acting not from within but from without ; in that case there is not necessarily any excess of carbon dioxide and the adjusting mechanism may fail. Suppose, for example, that a man goes into a room where the air is deficient in oxygen—containing, say, half the normal -amount—but where the carbon dioxide concentration is normal. There is, as has already been pointed out, a slight sensitiveness to oxygen-lack, and he therefore pants a little. But by panting he blows out most of the carbon dioxide in his blood, and since this is the more potent controlling factor, his breathing slows down again. He suffers no further discomfort, but a certain lassitude comes upon him, and presently he falls unconscious from oxygenlack. Or suppose that a man climbs a high mountain or soars in an aeroplane to a region of low atmospheric pressure. Both oxygen and carbon dioxide will be deficient ; because of the lack of the latter he does not respond by increased breathing to the shortage of the former, and oxygen-want may take him by surprise. The paradox presents itself therefore that although carbon dioxide is a poison there must be a certain amount of it present if the respiratory system is to work. ‘This does not injure an animal or a savage living a normal life ; any oxygen shortage in the blood of either will be due to bodily effort and therefore accompanied by carbon dioxide production. But it is beginning to endanger man. Exploring and venturing into places with strange atmospheres, into deep mines or above the snow line, is a thing man does against nature and tradition, and it is profoundly interesting to note that he has no certain adjustments to such adventures.

The oxygen demands of an active body are met not merely by increased breathing, they

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THE SCIENCE OF LIFE

CHAPTER 3

are also supplied by more rapid circulation. The most important adjustment of the circulation is made by the capillaries. The capillaries are not rigid tubes; they are contractile, and can narrow or widen their bore from its normal condition. During exercise the capillaries open more widely, permitting a greater volume of blood to flow through the tissue. The importance of this fact is enormous. It has been calculated that as a result of the simultaneous relaxation of the millions of capillaries when the body is violently exerted, the volume of blood in the voluntary muscles is increased about three hundred times. Moreover, since a large cylinder has a greater surface than a small one, the surface of the capillaries at which exchange of dissolved gases can take place is also greatly increased. The blood-flow through the muscles of a resting man is gentle and restrained ; as his exertions increase it becomes a swollen torrent.

Contractility is not confined to the capillaries ; the smaller arteries and veins are also capable of marked changes in diameter. The factors which can bring about these changes in bore are very various. Like the breathing centre, the capillaries can be controlled by nervous or chemical agencies. There are nerves which make the blood-vessels contract and others which make them relax, and there are a variety of chemical substances which, if they are present in blood, can influence the diameter of the vessels. With some of these mechanisms we shall deal in a moment; for the present we may note that either an excess of carbon dioxide or a shortage of oxygen can cause the capillaries to widen, and so increase the blood-supply of any part.

This action differs from the responses of the respiratory centre or heart in that its effects are purely local. If carbon dioxide accelerates breathing through the respiratory centre, the result is to put up the oxygen supply in the whole circulation. But if exercise is confined to one small regionto a hand, for example—and the rest of the body is relatively quiet, the carbon dioxide produced by that single group of muscles may not be enough to affect the respiratory centre. It can, however, exert a _ local action by causing the local blood-vessels to dilate, and in this way the blood-supply to the active part is increased. During exertion which involves the whole body the capillary dilatation is general, and in this case other nervous and chemical agents come into play.

Variation in size of the capillaries may be