Strokes occur when there is an interruption of blood flow to part of the brain. The most common cause of the interruption of blood flow is the plugging of an artery inside or leading to the brain. The plug is most often a blood clot or an embolus. An embolus is a piece of clot or other material broken off from somewhere else in the circulatory system. This kind of stroke, caused by interruption of blood flow, is an ischemic stroke and is the major cause of stroke in elderly patients. TIAs are small, reversible strokes.