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An ischemic stroke happens when one of the blood vessels supplying part of your brain with blood gets blocked.

• The blockage is usually caused by a blood clot.
• Blood vessels can also be blocked up by a condition called atherosclerosis. This happens when fatty substances build up on the inside of the walls of your blood vessels and make them narrow and hard.
• Blood clots form as part of your body's normal defenses against damage. If you cut yourself, for example, they seal the vessel wall to help keep you from losing too much blood. Sticky cells in the blood (called platelets) help clots to form.
• But sometimes a blood clot forms inside a blood vessel. This usually happens when the inside wall of the blood vessel has been damaged. A clot inside a blood vessel can interrupt the flow of blood and cause serious problems.
• Platelets also release chemicals into the blood that make blood vessels narrower.

Platelets can be triggered into action by a number of things.¹ These include:

• Smoking
• Diabetes
• High blood pressure
• High levels of certain fats (known as lipids) in your blood, such as cholesterol and triglycerides.

There are two types of blood clots that can lead to an ischemic stroke.

• Sometimes a clot forms on the inside wall of a blood vessel, then breaks away and is carried around in your blood. This is called an embolus. These clots often come from the large blood vessels in your neck. They travel through the bloodstream, eventually getting stuck in an artery in your brain. A stroke caused by an embolus is called an embolic stroke.
• A clot that forms inside a blood vessel and stays connected to the blood vessel wall is called a thrombus. If this type of clot gets so big that it stops blood from flowing through to the brain, it can cause a thrombotic stroke.

Strokes can be large or small, depending on where in your brain the blockage is. A blockage in a big artery usually affects more of your brain than a blockage in a smaller one.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.