Sequence of electrical and mechanical events in the cardiac cycle Steps:

• Wave of excitation sweeps across atria (2), initiating the cycle.
• Impulse reaches atrioventricular node for delay (3), allowing atrial filling.
• Atria contract (7) during this delay, pumping blood into ventricles.
• Impulse travels through Purkinje tissue (1) to ventricles.
• Excitation sweeps through ventricles (5), starting ventricular systole.

Why D is correct:

• Atrial contraction (7) follows AV node delay per the heart's conduction system, ensuring complete atrial emptying before ventricular contraction.

Why the others are wrong:

• A. 1: Purkinje conduction occurs after atrial events, initiating ventricular depolarization.
• B. 3: AV delay is second, immediately after atrial excitation.
• C. 4: No statement 4 provided; insufficient information.

Final answer: D