The Sinoatrial node is the impulse-generating tissue that acts as the heart's natural pacemaker located in which structure?

• A. SA node
• B. Vein
• C. Pleurisy
• D. PND

Answer: (A)

Location of the heart's natural pacemaker. The sinoatrial node is a small cluster of pacemaker tissue located in the wall of the right atrium, near where the superior vena cava enters. Its cells generate electrical impulses automatically, setting the heart’s rhythm and rate. From there, the impulse spreads through the conduction system to coordinate atrial and then ventricular contraction. Other options don’t fit because a vein is a blood vessel, pleurisy is inflammation of the pleura around the lungs, and PND (paroxysmal nocturnal dyspnea) is a symptom, not a tissue structure.