Which structure initiates the heartbeat?

• A. The sino-atrial node (SA node) situated in the wall of the right atrium
• B. The atrioventricular node
• C. The left ventricle
• D. The aorta

Answer: (A)

Heartbeat starts with an electrical impulse generated by the heart’s natural pacemaker, the sinoatrial node, which sits in the wall of the right atrium. This impulse sets the pace for the heart and begins the heartbeat. It spreads through the atrial muscle to trigger atrial contraction, then reaches the atrioventricular node where a brief delay occurs to allow the ventricles to fill. From there the impulse travels down the conduction system to the ventricles, coordinating their contraction. The atrioventricular node mainly relays and times the signal rather than initiating it, while the left ventricle is a pumping chamber and the aorta is a major artery—not a source of the heartbeat.