A stroke is caused when blood flow in the brain is blocked, depriving the brain of oxygen and causing the death of cells. Blood flow is disrupted in two ways.

Clot

In an ischemic stroke, a blood clot slows or stops blood flow to the brain. A clot can originate in the brain, or it can travel from another part of the body, such as from the heart. Clots are responsible for about 90 percent of all strokes.

Blood clots can be formed by atherosclerosis, a condition where the walls of arteries become hardened and clogged by deposits of plaques. Plaques are an accumulation of cholesterol, fat, inflammatory ells, and other substances in the blood. These are caused by high cholesterol, obesity, poor diet, and other reasons.

Blood clots compromised of platelets can also form in other parts of the body, typically the heart, and become lodged in the narrow vessels in the brain. These are formed in the heart as a result of abnormal heartbeats like atrial fibrillation or because of structural abnormalities in the valves or heart chambers that disrupt normal blood flow.

Hemorrhaging

Hemorrhaging involves leaking or ruptured vessels in the brain. The hemorrhaging can come from hypertension, or high blood pressure, which causes a weak wall of the artery to rupture. This weak spot is known as an aneurysm.