The heart has its own blood supply which provides the energy needed for the heart muscle to pump about seventy times a minute when the body is at rest. The left and right coronary arteries, branches of the aorta, bring a constant supply of food and oxygen to the cardiac muscle cells. Carbon dioxide produced by muscle contractions is taken away by the coronary veins, which empty the blood into the right side of the heart, from where it is pumped to the lungs to be reoxygenated.