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The heartbeat starts in the heart itself due to the sino-atrial node (SAN).

The SAN is found in the top of the right atrium and sets the rate at which the heart contracts. It sends out electrical impulses which are carried through the muscular walls of both atria. These impulses cause atrial systole.

The impulse is then passed to another node within the heart – the atrioventricular node (AVN). This node is in the lower part of the right atrium.

Once the impulse from the SAN reaches the AVN the impulse is passed to conducting fibres which travel down the central wall of the heart.

The impulse then splits and travels up the left and right ventricles – causing them to contract simultaneously (ventricular systole).