The human heart is a muscular organ that is formed into four chambers with an interconnecting vascular system. Venous blood from the body enters the right atrium, is pumped into the right ventricle and from there is pumped to the lungs for reoxygenation. Oxygenated blood from the lungs accumulates in the left atrium and is pumped into the left ventricle and out to the body. Myocytes require large amounts of energy and oxygen to accomplish this. The capacity of the heart to pump adequately is regulated by the volume of blood, systemic blood pressure, and the force of contraction achieved in the left ventricular wall.