Atrial fibrillation is a common heart condition in older persons. Knowing how the heart pumps blood is helpful in understanding atrial fibrillation. The heart has 4 chambers: 2 on the right side and 2 on the left side. The right side squeezes the blood through the lungs, and the left side squeezes it into the rest of the body. Most of the squeezing is done by the ventricles. The job of the atria is to squeeze the blood into the ventricles. Normally, the atria of the heart contract like an open hand making a fist. In atrial fibrillation, the atria do not contract. Because they don't contract, blood moves slowly through the atria and into the ventricle. Blood that moves slowly is more likely to form a clot. When pieces of clot break off, the heart pumps them into the body. If a piece of clot goes through the blood vessels to the brain, it will stick in the blood vessel and prevent further blood flow. When a part of the brain doesn't get blood, it dies, and the part of the body controlled by that part of the brain cannot work. This condition is called a stroke. Strokes can cause serious disability, making it impossible to walk or to talk.