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 arteries, the vertebral arteries, are found at the back of the neck. Once inside the skull, these four major arteries branch off into a spidery 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, while those caused by blood escaping from the vessels are called hemorrhagic strokes.
Most ischemic strokes, which translates literally to "withholding blood," 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 is less common but is more likely to be fatal. It happens when a weakened blood vessel in the brain bursts. A hemorrhagic stroke is treated in a different fashion, usually surgically.
