It seems odd that the tissues making up the heart must have their own separate blood supply. You might think that the torrent of blood rushing through the heart every minute would more than adequately meet the needs of the organ. The walls of the heart, however, consist of layers of specialized muscle. These walls are quite thick—the wall of the left ventricle is often over 1 inch thick. Since the lining of the heart is watertight, the blood cannot seep through the layers of muscle to provide the nourishment essential to these constantly working masses. Blood is carried through the muscle layers that form the heart wall by means of the two coronary arteries. These two small vessels branch off the aorta just after it leaves the heart and curl back across the surface of the chambers, sending twigs through the walls (Fig. 4-1).

The coronary arteries are so named because of the supposed resemblance to a crown or “corona” of the little arteries as they encircle the heart. These arteries divide into smaller and smaller branches, like all blood vessels in the body, until they become so small that only one blood cell at a time can move through them. At this point the vessels are called capillaries. After the blood has passed through the capillaries, and the tissues have extracted the needed oxygen, it returns by way of veins, which become larger and larger until they, like all other veins in the body, empty into the right atrium. The veins from the wall of the heart, or coronary veins, empty into the right atrium through a structure called the coronary sinus.