Sequential Atrial-Ventricular Contraction

Steps:

• Sinoatrial node impulse spreads to both atria, causing simultaneous contraction (atrial systole).
• Atrial contraction pumps blood into ventricles while AV valves are open.
• Impulse delays at AV node, then travels via bundle of His and Purkinje fibers to ventricles.
• Ventricles contract simultaneously (ventricular systole), ejecting blood through semilunar valves.

Why B is correct:

• Heart's conduction system ensures atrial systole precedes ventricular systole for efficient filling and pumping, per cardiac physiology.

Why the others are wrong:

• A: Simultaneous contraction prevents ventricles from filling properly before ejecting blood.
• C: Left and right chambers on each side contract together, not in sequence.
• D: Atria contract as a unit before ventricles, not right side before left.

Final answer: B