Where is the sinus node located and what is its function?

• A. A group of cells in the left atrium that inhibits the heartbeat
• B. The AV node in the right atrium that delays conduction
• C. The ventricular tissue that conducts impulse to the apex
• D. A group of cells in the right atrium where the electrical signal is generated that establishes the heartbeat

Answer: (D)

The main idea is that the sinus (sinoatrial) node acts as the heart’s natural pacemaker. It’s located in the right atrium, near where the superior vena cava enters. Its cells spontaneously generate electrical impulses at a regular rate, and this impulse sets the pace of the heartbeat. From there, the signal spreads through the atria to trigger contraction, then moves on to the AV node and the rest of the conduction system to coordinate ventricular rhythm. This is why the sinus node is described as the source that establishes the heartbeat. The other descriptions describe the wrong location or a different function (such as delaying conduction or being in the ventricles), which don’t match what the sinus node does.