During which phase are the ventricles contracting and emptying, and the semilunar valves are open?

• A. Isovolumetric contraction
• B. Ejection
• C. Isovolumetric relaxation
• D. End diastolic volume

Answer: (B)

This is the ejection phase of ventricular systole, when the ventricles actively contract and push blood into the arteries. As the ventricles contract, their pressure rises above the pressure in the aorta and pulmonary trunk, causing the semilunar valves to open. With the valves open, blood is ejected from the ventricles, so ventricular volume falls from the end-diastolic volume toward the end-systolic volume. The phase ends when ventricular pressure drops and the semilunar valves close, marking the transition to isovolumetric relaxation. The other phases don’t fit because isovolumetric contraction has the valves closed and no blood leaves the ventricle, and end diastolic volume is simply the volume in the ventricle at the end of filling, before contraction starts.