Ventricular filling completes at end of diastole

Steps:

• Examine pressure curves: low ventricular pressure during diastole allows filling from atria.
• Note passive filling phase early in diastole as AV valves open.
• Identify atrial systole: atria contract, boosting ventricular volume.
• Confirm end of atrial systole: ventricle reaches full capacity before systole.

Why A is correct:

• A marks end of atrial systole, when ventricular end-diastolic volume is maximal per cardiac cycle physiology.

Why the others are wrong:

• B: Isovolumetric contraction begins systole; ventricle volume unchanged, not filling.
• C: Ventricular ejection expels blood; ventricle partially empties.
• D: Isovolumetric relaxation starts diastole; ventricle not yet full.

Final answer: A