Which statement about the sinus node is true?

• A. It is a small group of cells in the upper right atrium near the tricuspid valve and has the slowest rate of automaticity.
• B. It is a small group of cells in the upper right atrium and functions as the normal pacemaker of the heart because it has the highest rate of automaticity.
• C. It is not involved in initiating the cardiac impulse.
• D. It can function as a backup pacemaker with a rate of 120-140 times/min.

Answer: (B)

The sinus node serves as the heart’s normal pacemaker because its cells have the fastest rate of automaticity, so it sets the pace of the heartbeat under normal conditions. It’s located in the upper right atrium near the origin of the superior vena cava, not near the tricuspid valve. Because it automatically fires the impulses quickest, it initiates the cardiac cycle and directs rhythm around 60–100 beats per minute in a healthy adult. If the SA node fails, other pacemaker areas like the AV node can take over but at a slower rate (roughly 40–60 bpm), not the 120–140/min speed described for a backup pacemaker. Thus, this statement correctly identifies the SA node as the normal pacemaker due to its highest rate of automaticity.