What is the function of the semilunar valves?

• A. They prevent backflow from the atria into the ventricles
• B. They regulate flow from the ventricles to the great vessels
• C. They regulate flow from the vena cavae into the right atrium
• D. They regulate the flow within the ventricles

Answer: (B)

Semilunar valves—the aortic and pulmonary valves—control outflow from the heart into the great vessels. They open to let blood be ejected from the ventricles during systole and close to prevent it from flowing back into the ventricles during diastole. This unidirectional flow keeps circulation efficient. They don’t regulate movement from the vena cavae into the right atrium or flow within the ventricles—that’s the job of the atrioventricular valves.