Which component of the conduction system introduces a delay between atrial and ventricular activation to facilitate ventricular filling?

• A. SA node
• B. AV node
• C. Purkinje fibers
• D. Bundle of His

Answer: (B)

Delaying the impulse between atrial and ventricular activation is achieved by the AV node acting as a gatekeeper. The atrial impulse travels from the SA node into the AV node, where conduction slows markedly due to cellular and electrical properties (smaller cells and fewer gap junctions). This pause gives the atria time to finish contracting and push additional blood into the ventricles—the atrial kick—so the ventricles fill more completely before they begin to contract.

After this delay, the impulse moves rapidly through the His-Purkinje system to coordinate synchronous ventricular contraction. The SA node sets the pace but doesn’t provide the interatrial–ventricular delay. The Purkinje fibers and bundle of His are built for fast conduction within the ventricles, not for creating a pause.