During which phase do the ventricles relax while all heart valves remain closed?

During which phase do the ventricles relax while all heart valves remain closed?

• A. Isovolumetric relaxation.
• B. Atrial systole.
• C. Ventricular systole.
• D. Atrial filling.

Answer: (A)

Isovolumetric relaxation is the moment in the cardiac cycle when the ventricles are relaxing but all valves are closed. With both the atrioventricular valves and the semilunar valves shut, no blood enters or leaves the ventricles, so ventricular volume stays constant even though the muscle fibers are relaxing and ventricular pressure is falling. This occurs right after the end of ventricular systole when the semilunar valves close, and it lasts until the ventricular pressure drops enough for the AV valves to open and filling to begin. The other phases involve at least one valve being open, so they do not fit the condition of all valves being closed and no volume change.

Isovolumetric relaxation is the moment in the cardiac cycle when the ventricles are relaxing but all valves are closed. With both the atrioventricular valves and the semilunar valves shut, no blood enters or leaves the ventricles, so ventricular volume stays constant even though the muscle fibers are relaxing and ventricular pressure is falling. This occurs right after the end of ventricular systole when the semilunar valves close, and it lasts until the ventricular pressure drops enough for the AV valves to open and filling to begin. The other phases involve at least one valve being open, so they do not fit the condition of all valves being closed and no volume change.