Heart and blood circualtory system

 THE HEART'S OWN SUPPLY The heart, being a muscle, and a hard-working one, needs nutritious blood. And nature has seen to it that it gets it by special means. The heart's special system begins at the aorta, from which branch off two arteries, the coronaries, each about the size of a thick knitting needle. One enters the heart muscle on the right side; the other, on the left. And the two together form a kind of wreath about the heart. The coronaries divide and divide to feed every part of the heart muscle. And after the blood has deposited its oxygen supply and picked up waste, it is carried by a system of veins to the right atrium to be passed, along with blood returning from other parts of the body, to the lungs. As we have noted, the heart has remarkable ability to adapt to demands of the body-to beat faster, contract more completely, and thus pump much more blood when necessary.

 In turn, the coronary circulation has remarkable ability to adapt to the heart's needs when they increase. When the heart must work harder, it needs more nourishment -and the coronary circulation accommodates. Ordinarily, the coronary arteries take only a small fraction of the blood moving through the aorta. But when the heart is working harder, the arteries will take more, even up to half of the total flow through the aorta. An unusual feature of the coronary circulation is the presence of extra capillaries, many of which form connections between the two coronary arteries. These extra tiny vessels lie unused and empty except when you are exerting yourself to the point of putting bigger demands on the heart. Then they go to work to bring more blood and oxygen to the heart muscle. These same capillaries help, too, if some of the regular blood channels no longer function effectively because of disease. Then the spare capillaries go into regular use as substitute pathways. There is another safeguard in the fact that each coronary artery doesn't supply only its own side of the heart.

Branches extend over to the other side so that many heart areas have blood supplied from both coronary arteries. Thus, if one of these vital vessels should become dis- eased and narrowed, all is not necessarily lost.