Which phase of the cardiac cycle is characterized by isovolumetric relaxation?

• A. Systole
• B. Isovolumetric relaxation
• C. Atrial systole
• D. Rapid filling

Answer: (B)

Isovolumetric relaxation is the part of the cardiac cycle where the ventricles relax but no blood enters or leaves them. After the ventricles finish contracting and ejecting blood, the aortic (and pulmonary) valves close, and both the aortic and mitral valves remain closed for a short period. During this interval, ventricular pressure falls but the volume stays the same, because no valves are open to allow flow. This defines the isovolumetric relaxation phase and marks the start of early diastole, before the mitral valve opens to begin filling. Understanding the other phases helps place this in the sequence: systole involves contraction and ejection; rapid filling occurs once the mitral valve opens; atrial systole is the atrial kick adding more blood to the ventricle. The key feature here is the closed valves with constant ventricular volume as the ventricles relax.