Atrial fibrillation is a heart arrhythmia.
Each contraction represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract. This exact route must be followed to ensure the heart pumps properly. In atrial fibrillation, the two small upper chambers of the heart, the atria, quiver instead of beat, and blood is not pumped completely out of them when the heart beats. Then, the blood pools and clots in the atria.