The heart contains muscles called cardiac muscles. They have rythimic contractions(systole) and relaxation (distole).
At the beginning there is a brief distole where both atria and ventricles are relaxed and filling with blood . At this time the triscupid and Biscupid valves are open
The distole is followed by atrial systole, where atria contract pumping blood through triscupid valves to the ventricles which are relaxed and on filling blood they get into systole.
Contractions of the ventricles generates pressure which open the sem-lunar valves . This contractions pumps blood out of the heart.
Pressure falls below that of the aorta and pulmonary artery hence semi-lunar valves close to prevent back flow
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