What is the role of semilunar valves?

• A. They prevent backflow from arteries into the ventricles.
• B. They prevent backflow from the atria into the veins.
• C. They regulate the heartbeat.
• D. They connect the atria to the ventricles.

Answer: (A)

Semilunar valves sit at the outlets of the heart’s ventricles, between each ventricle and the great arteries (the aorta and the pulmonary artery). Their main job is to ensure one-way blood flow from the ventricles into these arteries and prevent backflow into the ventricles. When the ventricles contract, blood is ejected and the valves open; as the pressure in the arteries remains higher than in the ventricles during relaxation, the cusps fill and the valves snap shut, stopping blood from flowing backward into the ventricles. This is distinct from regulating heart rate, which is controlled by the heart’s electrical system, and from connecting the atria to the ventricles or preventing backflow into the veins—the latter functions involve the atrioventricular valves and other structures.