Why? The Science of Athletics

I' I I I 80 WHY?-THE SCIENCE OF ATHLETICS on the one hand for their sustenance, and on the i>trreF to the lungs for purification, for reasons which will be explained more fully later. The heart is divided into two halves, each having an auricle for receiving the blood and a larger chamber, called a ventricle, for pumping it. Both are regulated by valves permitting the blood to flow only in one direction when the heart is pumping. We saw in the last chapter how the heart, beating 72 to the minute (_approximate average) when a man is resting and much faster when he is at work, sends all the blood in the body twice round that body in the space of a single minute, passing from the heart into the aorta, on through the arteries and right to the tiny capillaries, · some of them not more than r /goooths of an inch in thick– ness. The capillaries have to be so thin in order that the nourishment in the blood can flow through into the tissues and the waste matter in the tissues can be taken back into the blood ; in fact the blood vessels become finer and finer until the walls are so thin that the nutritive material can pass through them, when the cells in each tissue of the body select their own food and oxygen. After the oxygen has passed through the lungs and into the blood it is carried round the arteries fairly rapidly, until it reaches the capillaries, where the corpuscles establish intimate contact, one by one, with the muscles. The union of oxygen and carbon in the working muscle is different frotn what we find in ordinary combustion, and during the period of muscular work the number of capil~ laries which are opened to allow the blood to pass is greatly increased. This increases the volume of blood circulation per minute, but causes it to flow, if anything, a trifle slower than when a man is resting. Blood assisting a working muscle has a dual duty to perform, since it has both to supply oxygen and also to remove carhon dioxide, produced by combustion. This it carries away to the lungs, which in their turn breathe it out. The question of circulation brings us to an interesting