Atrial fibrillation is also the result of intra-atrial re-entry, usually occurring in the left atrium. In this arrhythmia there are multiple small re-entry circuits rather than the single large reentry circuit that characterizes atrial flutter or the AV junctional re-entrant rhythms. These small circuits frequently involve the ostia of the pulmonary veins. As a result, the atrial rate in atrial fibrillation is more rapid and more chaotic than in atrial flutter.