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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.
High blood pressure causes thickening of the walls of blood vessels and also makes them more likely to develop hardening of the arteries (atherosclerosis). People with thickened blood vessel walls and atherosclerosis are at higher risk of stroke, heart attack, and kidney failure.