Valve Dynamics in Cardiac Cycle Phases

Steps:

• Examine ventricular volume graph: flat volume indicates isovolumetric phases; decreasing volume shows ejection; increasing volume shows filling.
• Locate P at start of systole: volume plateaus after atrial contraction, signaling AV valve closure to prevent backflow.
• Locate R at end of systole: volume plateaus after ejection, signaling semilunar valve closure to prevent regurgitation.
• Match to options: both points involve valve closure during pressure buildup or relaxation.

Why A is correct:

• AV valves close at ventricular systole onset (point P) for isovolumetric contraction; semilunar valves close at systole end (point R) for isovolumetric relaxation, per cardiac cycle physiology.

Why the others are wrong:

• B: Semilunar opens during ejection (not R's closure phase).
• C: Both open during diastole filling, not these isovolumetric points.
• D: AV opens in diastole; semilunar closes at R, but mismatch at P.

Final answer: A