Valve Opening Sequence in Cardiac Cycle

Steps:

• Cardiac cycle begins with diastole: both atrioventricular (AV) valves open simultaneously for ventricular filling from atria.
• Atrial systole follows, with AV valves remaining open.
• Ventricular systole then occurs: AV valves close, and both semilunar valves open simultaneously for blood ejection.
• Opening order: left and right AV valves first, then aortic and pulmonary semilunar valves.

Why D is correct:

• Matches phases where AV valves open together during diastole for filling, and semilunar valves open together during systole for ejection, per heart physiology.

Why the others are wrong:

• A: Incorrectly places pulmonary semilunar before left AV, mixing diastole and systole phases.
• B: Opens aortic semilunar before right AV, interleaving valves from different cycle stages.
• C: Similar to B, opens aortic before right AV, violating simultaneous AV opening.

Final answer: D