Opening of the semilunar valves allows the ventricles to:

• A. Eject blood into the great arteries
• B. Fill with blood
• C. Return blood to the atria
• D. Relax and rest

Answer: (A)

Opening of the semilunar valves happens when the ventricles are contracting. As the ventricle pressure rises above the pressure in the aorta and pulmonary trunk, the cusps of the semilunar valves are forced open, allowing blood to be ejected into the great arteries—the aorta on the left and the pulmonary trunk on the right. This is how the heart delivers blood to circulation. After ejection, ventricular pressure falls and the semilunar valves close to prevent backflow, while filling occurs mainly through the atrioventricular valves during diastole and relaxation follows systole.