Atrial fibrillation is a fast and irregular heartbeat. It occurs in the heart’s two upper chambers, the right atrium and left atrium. Atrial fibrillation affects how the entire heart works.
The atria receive blood from the lungs and the rest of the body. They pump it into the ventricles, which are the lower two chambers of the heart. The ventricles then pump blood back to the lungs and the rest of the body. Normally, the atria contract and relax in a steady, synchronized rhythm. Each contraction pumps blood efficiently into the ventricles. When the heart is at rest, the atria beat between 60 and 100 times a minute.