From left to right:

The heart relaxes between beats during the diastolic phase. Blood flows into the heart from the veins, filling both atria. The tricuspid and mitral valves are closed.

The atria contract, forcing blood through the mitral and tricuspid valves and into the ventricles. This action is called atrial systole. The ventricles are still in diastole.

The ventricles contract in ventricular systole. Blood forces the mitral and tricuspid valves shut and opens the semilunar valves. It enters the aorta and pulmonary artery.

The heart again relaxes. The semilunar valves close, and the atria expand and fill with blood. These steps mark the beginning of diastole, and the cycle is repeated.

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Aortic Valve