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

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

to the lungs, where the oxygen-content is restored. In the lungs the blood travels through another network of capillaries across the wall of which it comes into contact, not with active and hungry flesh, but with oxygen-containing air. Instead

Fig. 13. The course followed by the blood in its unending flow round the body.

Vessels carrying fresh blood are shaded ; used blood are black.

those carrying of being impoverished it is refreshed. From these lung capillaries the oxygenated blood is collected as before into veins and returns again to the heart. It is now fit to be sent once more to feed, stimulate, and oxygenate the active tissues, and it is pumped by the heart, along a branching system of arteries, into the vast network of capillaries that permeates every organ in the body; there It repeats the exchanges, the marketing of oxygen and nutritive matter for waste products, with which our description began. The circulation of a man or a mouse is, therefore, a completely double one—it is not so much a circle as a figure of eight. There is the journey through the lungs, during which the blood is charged with oxygen and parts with surplus carbon dioxide, and there is the longer journey, from the heart to a capillary in some other part of the body and back, during which the oxygen is surrendered. The former round is called the pulmonary (lung) and the latter the Systemic (body) circulation. Manifestly this could be managed by two distinct pumps—

a body-pump to disperse the blood everywhere, and a smaller less powerful pump to send the blood on its special trip to the lungs. But the mammal’s heart combines both these pumps into one organ. It is really two pumps side by side. Each has a thinwalled upper part, the awricle, to receive the inflow of blood, and a powerful muscular part, the ventricle, which, as there are valves to prevent any back flow, impels the blood forward into the great arteries of the body or into the lungs, as the case may be. In Figure 14 we have a cross-section of the ventricles, and the reader can see how much feebler and flabbier the right (pulmonary) ventricle is than the left, which has to force the blood to scalp and toes and everywhere.

Tt is quite easy to hear anyone’s heart at work and distinguish the phases of its action. By means of a stethoscope, which is simply a listening tube, one end of which is put to the ear and the other pressed against the left side of the chest, it is possible to hear the sounds made by the beating heart. There is a silence, then a long sound, then a short, sharp sound, then silence again—Lubb . . . dup, Lubb . . . dup, Lubb ... dup. The fillmg of the heart from the veins and the contraction of the auricles to fill the ventricles are noiseless ; the sudden, violent rush of blood when, in turn, the ventricles contract gives the long sound, Lubb. . . . The short sound depends on a property of the arteries. The arteries have elastic walls. When blood is forced into them under considerable pressure by the ventricles they give, and when the ventricular contraction ceases they recoil. If it were not for the valves at their openings, the result of this recoil would be to drive blood back into the heart, but actually it

Left

Right Ventricle

Ventricle

Fig. 14. The ventricles of the heart cut across to show the relative strengths of their walls.

closes the valves. The aorta, the main arterial trunk which supplies all the body except the lungs, is abruptly cut off from the ventricle and the slam of its valves causes that second heart sound... dup. ‘This sound