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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.
Tweet. Thickening of a blood vessel may occur for a number of reasons. It is usually a result of changes within the vessel wall causing it to become thicker than normal. Alternatively it may arise from an enlargement of the vessel as a whole due to dilatation or birth defects.