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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.
Conditions that can trigger excessive blood clotting in the heart and brain: 1  Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. 2  Vasculitis is a disorder that causes the bodyâs blood vessels to become inflamed.