What is the role of the sinoatrial (SA) node?

• A. The natural pacemaker of the heart that initiates each heartbeat
• B. The atrioventricular node that delays conduction
• C. The primary muscle of the heart
• D. The valve regulating flow between atria and ventricles

Answer: (A)

The SA node acts as the heart’s natural pacemaker, generating electrical impulses that start each heartbeat. Its cells have automaticity, meaning they fire spontaneously at an intrinsic rate (about 60–100 beats per minute in a healthy adult) without needing a nerve stimulus. When the SA node fires, the impulse spreads through the atrial muscle, causing the atria to contract and push blood into the ventricles. The signal then reaches the AV node, where there’s a brief delay to ensure the atria finish contracting and the ventricles are filled before they contract. From there, the impulse travels through the His-Purkinje system to trigger ventricular contraction, coordinating the heartbeat and setting the overall pace.

Other parts have different roles: the AV node’s job is to delay conduction, not initiate it; the primary muscle of the heart is the myocardium that actually contracts; and the valves between the chambers regulate one-way blood flow rather than initiating electrical activity.