Define isovolumetric relaxation.

• A. A period in early diastole when all heart valves are closed and ventricular volume remains constant as the ventricle relaxes
• B. A period in late diastole when the atria contract to top off the ventricles
• C. The phase during which the ventricle empties
• D. A period during systole when the heart sounds occur

Answer: (D)

Isovolumetric relaxation is the brief interval in early diastole when the ventricles stop contracting and begin to relax, but all the heart valves are closed. Because no valves are open, the ventricular volume stays the same even though the pressure inside the ventricle is dropping. This phase occurs after the semilunar valves have closed at the start of diastole (the period marked by the second heart sound) and ends when the AV valves finally open, allowing rapid filling to begin.

The other ideas describe different parts of the cardiac cycle: atrial contraction to top off the ventricles, the phase when the ventricle actually ejects blood, and a systolic period that includes heart sounds.