Ventricular relaxation initiates AV valve opening

Steps:

• Ventricular diastole begins with relaxation of ventricular myocardium.
• Ventricular pressure drops rapidly below atrial pressure.
• This pressure gradient causes atrioventricular (AV) valves to open.
• Blood begins passive flow from atria into ventricles.

Why B is correct:

• At the onset of ventricular diastole, falling ventricular pressure below atrial pressure opens AV valves, per the basic principle of valve function driven by pressure differences.

Why the others are wrong:

• A: Semilunar valves close at diastole start to prevent backflow from arteries.
• C: Atrial pressure exceeding ventricular pressure is a condition enabling flow, not the primary event.
• D: Left and right atrial pressures remain balanced at diastole start, with no disproportionate rise.

Final answer: B