When do the semilunar valves open?

• A. When atrial pressure exceeds ventricular pressure
• B. When the ventricles are relaxing
• C. When the pressure in the ventricles exceeds the pressure in the arteries
• D. When the ventricles fill and relax

Answer: (C)

Semilunar valves open when ventricular pressure becomes greater than arterial pressure. This happens during ventricular systole as the ventricles contract, raising the pressure inside them above the pressure in the aorta and pulmonary artery, which pushes the valve cusps open to eject blood. They close when ventricular pressure falls below arterial pressure during relaxation, preventing backflow. The other statements describe conditions for the atrioventricular valves or for diastole/filling, not when the semilunar valves open.