The atrioventricular valves are the tricuspid (on the right side of the heart) and mitral (on the left side of the heart). These valves regulate the flow of blood from the atria to the ventricles. Their leaflets are delicate flaps that open during ventricular diastole and close during systole. The leaflets are larger than needed to close the opening. Thus, some overlap and puckering of the leaflet tissue occur, ensuring a good seal when the valve is closed.
The leaflets are anchored and supported by chordae tendineae, strong cords that stretch from the valve edges to the myocardium and restrict how far the valve swings when it closes. They prevent the leaflets from flapping back into the atria during ventricular contraction. These chordae are similar to the strings of a parachute in function and appearance.
The chordae insert into mounds of myocardium, known as papillary muscles, inside the ventricle.
The tricuspid valve, separating the right atrium and right ventricle, is named for its three tooth-shaped leaflets. Tricuspid leaflets tend to be thinner and more translucent than mitral leaflets, and tricuspid chordae tend to be thinner than mitral chordae.
The mitral valve (resembling the pointed shape of a bishop's miter) separates the left atrium and left ventricle. In contrast to the three other cardiac valves, which each have three leaflets, the mitral valve has only two.
During diastole when the ventricles relax and expand, the tricuspid and mitral valves open, allowing blood to flow into and refill the ventricles. When the ventricles contract again, blood pushes on the undersides of the leaflets and forces them to close. At the same time, the tiny tendons pull on the edges of the leaflets to keep them from bulging back into the atria and letting blood leak backward.