Stock Photo: HEART, ILLUSTRATION. HEART, ILLUSTRATION The heart is the motor organ of the blood circulation. Its muscle, the myocardium, contracting under the nerve influx, propulse the blood in the body. It is composed of 4 cavities, 2 atria and 2 ventricles. Each ventricle has its corresponding atrium. They communicate through valvular orifices. The heart is divided in 2 parts, the blood depart from the right heart, goes through the lungs and comes back towards the left heart.