The brain depends on blood for its supply of oxygen and nutrients. When the flow of blood to an area of the brain is blocked or interrupted, a stroke occurs. This block may be caused by a blood clot clogging a blood vessel or artery, or by a broken blood vessel. A stroke immediately begins killing the brain cells in the area in which the blockage occurs (this area is called the infarct). It can take anywhere from a few minutes to a few hours for the cells to die. As the brain cells die, the areas of the body controlled by them can no longer function.
Infarct cells release chemicals that can potentially harm other parts of the brain. This is why it is important for someone who suspects they have suffered a stroke to get to a hospital immediately. A CT scan can help doctors determine the extent of damage, and to decide on a course of action to prevent further damage.