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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.
All of these make your blood hypercoagulable, meaning it makes it thick and slow moving, which increases your risk of having a blood clot or stroke. Hypercoagulable blood contributes to inflammation, because when your blood does not flow well, oxygen can't get to your tissues.