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

BOOK 1

marks the end of the cycle; it is followed by silence while the auricles fill again.

In one minute this pump drives about a gallon of used blood to the lungs and sends out an equal amount of pumped blood to the body.

Right Veins

Auricle

Artery to Lungs

Left Auricle

Aorta Valves

Left

Right —~ Ventricle

Ventricle

Fig. 15. The Three Phases of the Heart Pump.

(1) The auricles filling from the veins ; (2) the auricles driving blood into the ventricles ; (3) the ventricles pumping blood into the arteries.

The heart lies in a bag—the pericardium, which has smooth walls and contains a fluid. The purpose of this structure is to lubricate the movements of the heart ; lying in this bag of fluid, the body-pump is 38

THE SCIENCE OF LIFE

CHAPTER 2

able to contract and expand (about seventytwo times per minute) without friction against the adjacent organs.

The arteries which take the blood from the heart have to stand a fairly heavy strain and are strong and elastic to resist it. Every time the heart contracts there comes a tidal rush and the pressure rises to a climax and falls. Accordingly the walls of an artery are strong and elastic; they include a thick layer of muscle-cells and elastic fibres. Blood travels in the main

arteries at a relatively high speed; in the great aorta, which leads directly out of the heart, it travels at about one-and-a-half to two feet per second, and in the limbs its At the

pace is still considerable. wrist a main artery to the hand comes conveniently near the surface and the tidal pulsation of blood can be felt, and here it is that doctors inform themselves of the vigour and excitement of the heart.

As the arteries branch and divide, the force of the pulse becomes less, their walls become thinner, and the blood travels less swiftly and more evenly. Finally, in the capillaries, its velocity has fallen to about a millimetre per second—about one-thousandth of its speed in the aorta. The pulse by that time has disappeared and the pressure is much lower. There is therefore no need for a capillary to have strong, resistant walls. The structure of a capillary is determined by its function, which is to bring the blood into the closest possible relation with the surrounding flesh. For this reason the walls are almost incalculably thin. Moreover, the fact that the blood, instead of flowing along in one broad current, has been divided into a multitude of tiny parallel trickles, ensures that the surface of contact between blood and flesh is as great as possible. The number of the capillaries is enormous ; in a cross-section of muscle,

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L7 Fug. 16. An artery (below) and the corresponding vein, (above) cut across to show the strengths of their walls.