Atrial fibrillation: When the heart loses its rhythm
You may not even feel it happening, but if the atria of the heart don't beat correctly you could be in danger of a stroke.
Your heart has four compartments. The upper two are the atria; the larger two below are the ventricles. The atria are the anterooms of the cardiac pump house, each capable of holding just a few ounces of blood at a time.
The right atrium receives oxygen-depleted blood on its trip back to the heart from the rest of the body. The left one receives blood returning from the lungs after it has been freshly oxygenated and sends it to the powerful left ventricle. The left ventricle pumps blood out through the aorta to supply the whole body.