The semilunar valves are located between the ventricles and the major arteries leaving the heart.

• A. Between the ventricles and major arteries leaving the heart
• B. Between the atria and ventricles
• C. Between the ventricles and the atria
• D. Between the right atrium and right ventricle

Answer: (A)

Semilunar valves sit at the exits of the ventricles, where blood leaves the heart and enters the great arteries. There are two: one between the left ventricle and the aorta, and one between the right ventricle and the pulmonary trunk. This placement lets them regulate the flow of blood into the major arteries and prevent backflow when the heart relaxes. When the ventricles contract, pressure pushes the cusps open to eject blood; as the ventricles relax, arterial pressure pushes the cusps closed to stop blood from flowing back into the ventricles. The other descriptions refer to valves between the atria and ventricles (the atrioventricular valves), which are not semilunar.