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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
Causes of a Thickened Vessel. Aneurysm. An aneurysm is the bulging of a portion of a blood vessel. It is an abnormal dilatation, like a balloon, and may affect an intact vessel wall (true aneurysm) or diseased wall (false aneurysm).It is not an actual thickening of the blood vessel wall but is rather a protrusion of the wall. Dissection.