Atrial Fibrillation 3d medical animation. The animation identifies the normal anatomy - heart (Superior vena cava, aorta, atriums, ventricles, inferior vena cava, descending aorta) lungs, arteries and normal blood flow from the heart through the carotid arteries to the brain. A stroke occurs in the heart. The clot blocks a vessel in the brain, and the lack of oxygen causes tissue death (oxygen is not passed to the neurons - brain cells).