A heartbeat is caused by an electrical impulse traveling through the heart. This electrical impulse originates in the heart's natural pacemaker called the sinus node.
The sinus node is usually located in the right atrium, though this location can differ in some cases. When the sinus node fires, a wave of electricity sweeps across the atria, causing the upper chambers to squeeze or contract.
The electrical impulse travels from the atria to the ventricles by way of the atrioventricular (AV) node, located in the center of the heart.
From the AV node, the electrical impulse travels through the specialized bundle of His and through bundle branches to transmit the impulse to the ventricular muscle, resulting in ventricular contraction.
Contraction of the atria, followed by the ventricles, causes blood to be pumped to the body.