The heart is nourished not by the blood passing through its chambers but by a specialized network of blood vessels. Known as the coronary arteries, these blood vessels encircle the heart like a crown.

 About 5 percent of the blood pumped to the body enters the coronary arteries, which branch from the aorta just above where it emerges from the left ventricle. Three main coronary arteries—the right, the left circumflex, and the left anterior descending—nourish different regions of the heart muscle. From these three arteries arise smaller branches that enter the muscular walls of the heart to provide a constant supply of oxygen and nutrients. Veins running through the heart muscle converge to form a large channel called the coronary sinus, which returns blood to the right atrium.