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The normal pattern of blood supply is that large arteries leave the heart carrying oxygen-rich blood. They branch into smaller and smaller arteries reaching every point in the body. Capillaries, or very thin-walled blood vessels, slow down the flow of blood and perform the crucial step of oxygen and nutrient exchange with the body's tissues. The depleted blood moves into veins which gather together like streams into a river, carrying the blood back to the heart and lungs where it is replenished with oxygen and the cycle begins again.
In arteriovenous malformation, a tangle of large arteries and veins forms, bypassing capillaries and sending blood directly back into the venous system. This prevents the blood from slowing down and releasing its load of oxygen -- starving nearby tissues. Arteriovenous malformations can form throughout the body, but they are especially problematic when they are located in the brain or spinal cord.
In areas near the malformation, brain tissue deteriorates and sometimes dies for lack of oxygen. The defect also alters the normal flow of blood and the balance of fluids throughout the brain. The fistula, or area where arteries and veins connect, has very high rates of blood flow along with unusually high blood pressure. Arteries leading to the AVM can become enlarged and veins leading away may become tightened and narrowed (stenosis). Blood vessel walls become weakened and distended and may burst (aneurysm). The size of the AVM can also cause problems, putting pressure on nearby structures or blocking the normal flow of cerebrospinal fluid through and around the brain.
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