Isovolumetric relaxation phase of the cardiac cycle

Steps:

• Cardiac cycle duration is ~0.8 seconds; systole ends ~0.4 seconds with semilunar valve closure.
• AV valves closed since ventricular systole onset remain shut as ventricular pressure falls.
• Semilunar valves stay closed post-ejection until pressure equilibrates.
• Both valve sets closed during brief isovolumetric relaxation at 0.7 seconds.

Why C is correct:

• At 0.7 seconds, isovolumetric relaxation occurs: ventricular pressure drops below aortic pressure (semilunar closed) but above atrial pressure (AV closed), per pressure-volume dynamics.

Why the others are wrong:

• A. 0.3s: Ventricular ejection phase; semilunar valves open for blood outflow.
• B. 0.4s: Onset of ventricular filling; AV valves open as pressure equalizes.
• D. 0.2s: Mid-ejection; semilunar valves open, AV valves closed.

Final answer: C