Which anatomical structure provides the delay between atrial and ventricular contractions in the heart's conducting system?

• A. Sinoatrial (SA) node
• B. Purkinje fibers
• C. Bundle of His
• D. Atrioventricular (AV) node

Answer: (D)

The key idea is that timing in the heart’s electrical system is controlled by slowing the signal at just the right place. After the signal starts in the SA node and makes the atria contract, it arrives at the atrioventricular (AV) node. Here conduction slows down significantly because of smaller-diameter fibers and fewer gap junctions, plus a short delay as the signal passes through a compact region. That slowdown produces a pause long enough for the atria to finish emptying into the ventricles, so ventricular filling is optimized before contraction begins.

Once the AV node passes the impulse onward, the signal races through the fast-conducting pathways—the bundle of His and the Purkinje fibers—to coordinate rapid, synchronized ventricular contraction. The AV node is specifically responsible for the delay; the Purkinje system and bundle of His quickly transmit the impulse to the ventricles, and the SA node mainly sets the pace rather than creating the delay.