What are semilunar valves?

• A. Valves located at the exits of the heart that prevent backflow of blood
• B. Valves between the atria and ventricles
• C. Valves within veins
• D. Valves controlling blood flow into the heart

Answer: (A)

Semilunar valves are the valves at the exits of the heart—the aortic valve where the left ventricle times blood into the aorta and the pulmonic valve where the right ventricle sends blood into the pulmonary artery. Their job is to prevent backflow of blood into the ventricles after they contract. When the ventricle contracts, pressure pushes blood through these valves and they open; as the ventricle relaxes and pressure falls, the cusps fill and snap shut to stop blood from flowing backward. They’re not located between the atria and ventricles (that’s the atrioventricular valves), not inside veins (vein valves prevent backflow in the vessels, but semilunar valves regulate flow out of the heart), and they don’t control blood flow into the heart (that’s governed by venous return and the atrioventricular valves).