The body requires oxygen to carry on the process of life. A network of arteries and veins transport oxygen-rich blood to the body and return oxygen-poor blood to the lungs. At the center of this continuous process is the heart, a beating muscle about the size of your fist. The heart pumps approximately 5 quarts of blood every minute, and each heartbeat circulates blood to both the lungs and the body. This is possible because of the heart's complex internal structure. The heart is divided into the right side and the left side. The right side comprises the right atrium and right ventricle; these chambers collect oxygen-poor blood and pump it to the lungs, where oxygen is replenished. The left side of the heart comprises the left atrium and left ventricle; these chambers collect and pump the oxygen-rich blood to the body. Four valves keep the blood moving in the appropriate direction. Two of the heart's valves separate the right and left atrium from the right and left ventricles. Two other valves separate the right ventricle from the pulmonary artery and the left ventricle from the aorta. During a normal heartbeat, oxygen-poor blood returning from the body enters the right atrium through the vena cavae. The right atrium contracts, pushing blood through the tricuspid valve and into the right ventricle. Next, the right ventricle contracts to pump blood through the pulmonic valve and into the pulmonary artery, which connects to the lungs. At the same time, oxygen-rich blood returning from the lungs is delivered to the heart through the pulmonary veins. The pulmonary veins empty into the left atrium, which contracts to push oxygen-rich blood through the mitral valve and into the left ventricle. The left ventricle contracts, pushing the blood through the aortic valve and into the aorta, which distributes blood to arteries throughout the body. The heart is supplied with blood through the coronary arteries, which branch off the aorta.