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In adults, the resting heart rate is 60-80 beats a minute. The heart has its own pacemaker, the sinoatrial node, in the wall of the upper right atrium. Each burst of electricity from the sinoatrial node spreads to the heart's muscle fibres which contract and cause the heart to pump. The ECG graphically follows the signals (left to right) as they travel to each part of the heart causing it to fill and empty its chambers of blood.