The blood ?ow through the heart needs to be one-way and carefully regulated. Four one-way valves between the chambers ensure that the blood moves through the heart and lungs in sequence and never dams up or back-?ows. All the heart valves are constructed of overlapping ?aps (lea?ets or cusps) that open and close to control blood ?ow. The valves differ by structure and function.
The pulmonary and aortic valves between a ventricle and the great artery are called the semilunar valves because of their crescent-shaped lea?ets. The tricuspid and mitral valves between the right and left atria and a ventricle are also called atrioventricular valves. The lea?ets of the two atrioventricular valves are attached to the ventricular walls by ?brous cords. When the ventricle contracts and the valve closes, the cords secure the lea?ets in place so they are not blown backward by the force of the contraction.
The tricuspid valve, on the right side of the heart, is named for its three leaflets, or cusps. Returning, oxygen-depleted blood flows through this valve into the right ventricle.
As the blood ?ows out of the right ventricle and into the pulmonary circulation, it passes through the pulmonary valve. The pulmonary valve’s three lea?ets open as the right ventricle contracts and close again as it relaxes.
As the oxygen-enriched blood passes back into the left atrium, it passes through the mitral valve (named for its shape, which resembles a type of bishop’s hat called a miter). This valve has just two highly mobile cusps that can close rapidly when the powerful left ventricle contracts. This valve is attached by cords to muscles within the ventricle.
Finally, as the blood ?ows out of the left ventricle and into the aorta, the three-part aortic valve opens against the walls of the aorta. When the blood has passed into the aorta, the valve falls shut.