The heart possesses two leaflet (flappes) valves and two semilunar valves.
The leaflet valves (tricuspid, mitral) devide the atria from the ventricles.
The semilunar pulmonary valve lies between the right ventricle and the pulmonary arteries.
The semilunar aortic valve lies between the left ventricle and the aorta.
Both valves prevent the backflow of blood into the ventricles during the diastole.