During isovolumetric contraction, which valves are closed?

• A. AV valves closed; semilunar valves closed
• B. AV valves open; semilunar valves closed
• C. AV valves open; semilunar valves open
• D. AV valves closed; semilunar valves open

Answer: (A)

Isovolumetric contraction is the phase when the ventricles contract but no blood is ejected because both sets of valves are closed. The AV valves shut as soon as ventricular pressure rises above atrial pressure, preventing backflow into the atria. The semilunar valves remain closed because the ventricular pressure has not yet exceeded the pressure in the aorta and pulmonary trunk. With both valve pairs closed, the ventricular volume stays constant while the muscle contracts. When the ventricular pressure eventually exceeds arterial pressure, the semilunar valves open and ejection begins.