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Animation of a beating heart, showing the electrical waves that produce the beats. The beats are the contractions of the atria and ventricles that pump blood around the body. The blue wave that spreads throughout the heart is the wave of depolarisation, which arises from the sinoatrial node, or SA node. The SA node is found in a groove located at the back of the heart, in the wall of the right atrium. From here, the wave of depolarisation spreads through the heart's muscle sequentially. The heart muscle contracts shortly after a wave of depolarisation passes through it. After contraction, the heart muscle has to recharge itself for the next wave. This recharge generates the wave of repolarisation (red wave). It is this cycle of depolarisation and repolarisation that is recorded by an electrocardiogram. The SA node is known as the heart's natural pacemaker