The bicuspid valve, also known as the mitral valve and the tricuspid valve, guard the atrioventricular openings between the upper (atria) and the lower (ventricles) chambers of the heart, and are therefore referred to as the atrioventricular valves.
The mitral valve, made up of two flaps, guards the opening between the left atrium and the left ventricle, while the tricuspid valve, made up of three irregularly shaped flaps, guards the opening between the right atrium and the right ventricle.
The pulmonary and the aortic valves make up the semilunar valves. The pulmonary valve, made up of two leaflets, guards the opening between the pulmonary artery and the right ventricle, while the aortic valve, made up of three leaflets, guards the opening between the left ventricle and the aorta.
When the atria contract, the atrioventricular valves (the bicuspid and the tricuspid valves) allow blood to flow through them into the ventricles, but when the ventricles contract, the atrioventricular valves close to prevent blood from flowing backwards into the atria. The valves have been equipped with papillary muscles and tendinous chords of dense tissue called chodae tendineae, which ensure that these valves are firmly closed when the ventricles contract.
The semilunar valves open to allow blood to flow through them when the ventricles contract to propel blood into the aorta and the pulmonary artery but prevent the backward flow of blood into the ventricles. The leaflike structures making up the semilunar valves are similar to those of the atrioventricular valves, but they are thinner than those of the atrioventricular valves and they do not have chodae tendineae. The first heart sound is associated with the closure of the atrioventricular valves, while the second heart sound is associated with the closure of the semilunar valves.