Blood returning from the body enters the heart through the right atrium, where it collects and is then pumped to the right ventricle. Each time the right ventricle contracts, it propels this blood, which is low in oxygen content, into the lungs, where it is enriched with oxygen. Pulmonary veins return the blood to the left atrium, which contracts and sends it to the left ventricle.
The left ventricle, the main pumping chamber of the heart, ejects the blood through the aorta into the major circulatory network. Because it delivers blood to the entire body, this ventricle works harder than all other chambers. Its walls can be two to three times thicker than the walls of the right ventricle.
The ventricular muscle itself receives a fraction of the large volume of blood flowing through the atria and ventricles. A system of arteries and veins supplies the muscle with oxygen-rich blood and then returns oxygen-depleted blood to the right atrium. The right coronary artery and the left coronary artery branch off the aorta to deliver blood.