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

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

seen without any elaborate apparatus. ‘The redness of skin depends on the amount of blood it contains, and the flush produced by a hot fire, or the blushing that accompanies various emotional states, is due to this factor.

The essential response of the circulation to muscular exercise is this increase in blood-flow through the tissues. But there are other changes too. It is clear that a mere dilatation of the capillaries, unsupported by correlated changes in other parts of the circulation, would be of little avail. The speed of the circulation as a whole depends on the output of the heart, the pump which keeps it all in motion, and any widespread dilatation of the capillaries would result in a slowing rather than a quickening of the circulation, unless the output of the heart were augmented at the same time. The two factors work together. The disastrous effects of capillary dilatation unsupported by the heart are seen in the condition called surgical shock, which follows any extensive tearing of the flesh. It appears that torn flesh pours a substance known as histamine into the blood, and that histamine causes the capillaries to relax completely without having any effect on the heart output. We have already had a glimpse of histamine at work in the early stages of a small wound (Chapter 2 § 8). The result, however, of an extensive production of histamine, since the total volume of the capillaries is enormously increased (about seven hundred times more than normal) and the heart is only pumping just enough blood for a normal resting circulation, is that blood stagnates in the bloated capillaries, and the whole blood-stream becomes sluggish. Clearly this is advantageous in one respect, because it will diminish bleeding from the wound, but this advantage is gained by sacrificing the efficiency of the transport system. The oxygen supply of the body becomes inadequate, the delicate brain-cells are soon affected, and the subject suffers from collapse.

When, during exertion, the capillaries enlarge, a similar condition would be produced, were it not for corresponding changes in other parts. Of these the most important is an increase in the output of the heart. Unlike the rhythm of the muscles concerned in respiration, the beating of the heart is spontaneous and automatic, and will continue for days after the organ is isolated from the body. It can, however, be controlled by nervous and chemical influences; there are, for example, nerves

F

which make it beat more slowly, and others which make it beat faster. During exercise the latter come into play; they increase the number of beats per minute and the volume of blood pumped at each beat, so that when exercise is taken and the capillaries dilate, the output of the heart also increases, and the blood-supply to the muscles ds duly adjusted.

Besides changes in capillary volume and heart output there are yet other changes in the circulation during exertion. Among them it is interesting to note a compensatory mechanism, only very recently brought to light, which involves the most mysterious organ in the human abdomen—the spleen.

The spleen is a purple organ, roughly oblong and about five inches long, lying behind the stomach and receiving a copious blood-supply direct from the dorsal aorta. Its minute structure is curious, for in its substance the blood is not confined within definite vessels as it is in any other organ ; the arterioles and vessels have open ends, and instead of passing through a capillary network the blood flows freely in the spongy tissue, and directly bathes the spleen cells. The spleen has long been a puzzle to physiologists, partly because it is an organ with many functions. Probably, for example, it is concerned with the breaking down of old and worn blood-cells, and with manufacturing new ones to replace them. But at present we are concerned with another aspect of its activity, which depends on the presence of a well-defined system of musclefibres, forming a capsule round the organ and sending interlacing strands through its substance. ;

During rest, the heart is pumping just enough blood to keep it circulating at an effective speed through the partly-closed capillaries. If, as a result of exertion, the capillaries suddenly open up, the bloodstream will evidently be slowed unless the heart labours strenuously to keep up its speed. But if at the same time some other part of the circulation, which normally holds a considerable volume of blood, reduces its own capacity and ejects its contents into the general circulation, then the expansion of the capillaries will be compensated for. Now there exist in the body certain bloodreservoirs which have precisely this function, and of these the spleen is chief. During rest the muscle-fibres of the spleen are for the most part relaxed; they are rhythmically contractile, beating slowly and indolently, like a very lazy heart, and so keeping a sluggish flow of blood through the organ.

65