Atrial fibrillation

In a typical heart, a tiny cluster of cells at the sinus node sends out an electrical signal. The signal then travels through the atria to the atrioventricular (AV) node and passes into the ventricles, causing them to contract and pump out blood. In atrial fibrillation, electrical signals fire from multiple locations in the atria (typically pulmonary veins), causing them to beat chaotically. Since the atrioventricular (AV) node doesn't prevent all of these chaotic signals from entering the ventricles, the heart beats faster and irregularly.