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.

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