HEART, ILLUSTRATION. ´HEART, ILLUSTRATION The heart is the organ that makes blood circulate. Its muscle, the myocardium, receives stimulation from nerves, then contracts and propels the blood around the body. It is composed of 4 cavities, 2 atria and 2 ventricles. Each ventricle corresponds to an atrium, and the two are connected by valves.The heart is divided into 2 parts, then, with the blood leaving the right heart, passing through the lungs, and returning to the left heart.´