In a typical heart, a tiny group of cells at the sinus node sends out an electrical signal. The signal travels through the heart's upper chambers to the atrioventricular (AV) node. The signal then passes into the lower heart chambers, causing them to squeeze and pump out blood. In atrial fibrillation, electrical signals fire from many locations in the upper chambers, causing them to beat chaotically. Since the AV node doesn't prevent all of these chaotic signals from entering the lower chambers, the heart beats faster and irregularly.
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