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Blood reaches the brain from the heart through four major arterial systems. Two carotid arteries run up from the front of the neck, and two smaller vertebral arteries run the back of the neck. Inside the skull, these four arteries branch off into a web of tiny vessels to supply every area of the brain with blood at all times.
A stroke occurs when one of the arteries carrying nutrients and oxygen to the brain either becomes clogged or ruptures. Strokes caused by a lack of blood reaching the brain are called ischemic strokes; those caused by blood escaping from the vessels are called hemorrhagic strokes.
Ischemic strokes Most ischemic strokes occur when an artery is blocked by a blood clot that forms along the vessel wall (thrombosis) or a blood clot that has been carried from some other part of the bloodstream (embolism) until it hits an artery too small to get through. Thrombolic and embolic strokes make up about three of every four strokes. An ischemic stroke can be treated with the thrombolysis therapy, if caught early enough.
Hemorrhagic stroke Hemorrhagic stroke is less common but more likely to be fatal. It happens when a weakened blood vessel in the brain bursts. A hemorrhagic stroke is typically treated with surgery.