To ensure that the blood circulating in the heart adheres to a strict "one-way" system, i.e. to prevent a backflow of blood, the heart contains numerous valves. One set of valves is located between the atrium and the ventricle on either side to prevent backflow from the ventricle to the atrium during systole. Collectively, these are known as the atrioventricular valves: on the right side the tricuspid valve with 3 flaps and on the left side the bicuspid or mitral valve with 2 flaps.

The other valves, known as the semilunar valves because of their half-moon shaped flaps, are found on either side of the heart between the ventricle and the large blood vessel into which blood is expelled: on the right the pulmonary artery with the pulmonary valve, on the left the aorta with the aortic valve. These valves prevent the blood flowing back into the ventricles during diastole.