Original Article: http://www.mayoclinic.com/health/medical/IM02485
A normal heartbeat begins when a tiny cluster of cells called the sinus node sends an electrical signal (1). The signal then travels through the atria and passes through another group of cells called the atrioventricular node (2). From the atrioventricular node, the signal travels through the ventricles (3), causing them to contract and pump out blood. After this, the process starts over (4).