Oxygen-rich blood leaves the lungs and returns to the heart through the left atrium. At the exit to each heart chamber lies a valve that prevents backflow , or flow of blood back into the chamber from which it came. An atrioventricular valve separates the atrium from the ventricle. On the right side, this valve is called the tricuspid valve because it has three flaps of tissue, or cusps. The left atrioventricular valve is called the mitral valve, but is sometimes referred to as the bicuspid valve because it has two cusps.
Semilunar valves separate the ventricles from the outgoing blood vessels.
When the ventricle expands, the atrioventricular valve allows blood to flow forward from the atrium into the ventricle while the semilunar valve keeps blood that has already been pumped out of the heart from flowing back in. Conversely, when the ventricle contracts the atrioventricular valve closes to prevent backflow while the semilunar valve opens to allow blood to be pumped either to the body or the lungs. The prevention of backflow ensures the proper direction of flow through the circulatory system and reduces the amount of work the heart must do to pump blood through the system. Stenotic valves cannot open fully, requiring more work to push the liquid through the valve. By contrast, incompetent valves waste work by allowing blood to flow backward.