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How do the pulmonary and aortic valves control blood leaving the heart?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

The pulmonary and aortic valves, known as semilunar valves, are pocketlike structures that regulate the exit of blood from the heart's ventricles.12

Function during contraction: During ventricular systole, the ventricles contract and force blood out of the heart. This pressure causes the pulmonary and aortic valves to open, permitting blood to flow into the pulmonary artery and the aorta, respectively.123
Function during relaxation: As the ventricles begin to relax (diastole), the pressure within the heart falls. The valves close immediately to prevent the backward flow of blood from the arteries into the ventricles.124

These valves are essential for maintaining one-way blood flow and the pressure required for effective circulation. Their closure at the end of systole is responsible for the second heart sound, often described as "dub".25