During which phase does pressure decrease rapidly, the ventricles begin to relax, and the semilunar valves close with the second heart sound?

During which phase does pressure decrease rapidly, the ventricles begin to relax, and the semilunar valves close with the second heart sound?

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

Answer: (C)

During ventricular relaxation the heart goes through a phase where the ventricles are relaxing and all valves are momentarily closed. This is when pressure in the ventricles falls rapidly as the myocardium relaxes. The semilunar valves close at the start of this phase, producing the second heart sound. Because both sets of valves are closed, the ventricular volume stays the same for a brief moment (isovolumetric relaxation). This differs from the contraction phase, where pressure rises with all valves closed but blood isn’t yet leaving the heart, and from the ejection phase, where the semilunar valves are open and blood is actively being pumped out. End systolic volume isn’t a phase of the cardiac cycle; it’s the volume remaining in the ventricle at the end of systole.

During ventricular relaxation the heart goes through a phase where the ventricles are relaxing and all valves are momentarily closed. This is when pressure in the ventricles falls rapidly as the myocardium relaxes. The semilunar valves close at the start of this phase, producing the second heart sound. Because both sets of valves are closed, the ventricular volume stays the same for a brief moment (isovolumetric relaxation).

This differs from the contraction phase, where pressure rises with all valves closed but blood isn’t yet leaving the heart, and from the ejection phase, where the semilunar valves are open and blood is actively being pumped out. End systolic volume isn’t a phase of the cardiac cycle; it’s the volume remaining in the ventricle at the end of systole.