The Sino-Atrial (S-A) node, located in the right atrium near the superior vena cava opening, is the heart's natural pacemaker. It spontaneously generates electrical impulses at ~60–100 beats/min, initiating each heartbeat. The impulse travels to the A-V node, then down the Bundle of His and Purkinje fibres to the ventricles. The A-V node acts as a secondary pacemaker (~40–60 bpm) if the S-A node fails.