The opening and closing of the heart valves is caused by

action potentials within the cusps of the valves.

contraction of the smooth muscle in the walls of the great vessels leaving the heart.

contraction and relaxation of papillary muscles that pull on heart strings.

pressure changes of alternating contraction and relaxation during the cardiac cycle.

pressure changes of alternating contraction and relaxation during the cardiac cycle.

Explanation:

Opening and closing of heart valves are regulated by changes in the blood pressure of various heart chambers during the cardiac cycle. During atrial systole, the higher blood pressure in the atria opens both AV valves and forces the blood to enter the ventricles. The semilunar valves are closed. During ventricular systole, the higher blood pressure in the ventricles opens both semilunar valves to allow the blood to enter the pulmonary trunk and aorta. At this time, both the atrioventricular valves are closed.