A stroke is the death of brain tissue that occurs when the brain does not get enough blood and oxygen. Strokes are classified as ischemic strokes or hemorrhagic strokes.
Ischemic strokes happen when the blood supply to the brain is interrupted.
The brain cells then die from lack of oxygen. Hemorrhagic strokes occur when blood vessels in the brain burst and release blood into the area around the brain cells. The blood then damages the brain cells. The products released when cells die cause swelling in the brain. Since the skull doesn't allow much room for expansion, this swelling can damage the brain tissue even further.