The human heart has two upper chambers and two lower chambers. The upper chambers are called theleft atrium and the right atrium - the plural ofatrium is atria. The two lower chambers are the theleft ventricle and the right ventricle. When the two upper chambers - the atria - contract at an excessively high rate, and in an irregular way, the patient has atrial fibrillation.
The term atrial fibrillation comes from the Latin words atrium, meaning "hall", fibrilla, meaning "small fiber", and atio, meaning "process". According to Medilexicon's medical dictionary, atrial fibrillation is "fibrillation in which the normal rhythmic contractions of the cardiac atria are replaced by rapid irregular twitchings of the muscular wall the ventricles respond irregularly to the dysrhythmic bombardment from the atria."