This module explores the structure and function of the four valves of the heart.

There are two types of valves. The pulmonary and aortic valves are called outflow valves. They are also called semilunar valves because they are made up of three crescent-shaped flaps called cusps. These two valves open and close to allow blood to flow out when the ventricles contract and to prevent backflow between contractions.

The atrioventricular (AV) valves separate the atria from the ventricles. They open when the atria pump blood into the ventricles. And they close when the ventricles contract to prevent backflow into the atria.

The cusps of these valves are usually called leaflets. They are attached by strings called chordae tendonae to papillary muscles on the inside of the ventricle walls. The right AV valve has three leaflets and is therefore called the tricuspid valve. The left AV valve, with two leaflets is most commonly called the mitral valve.