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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.
Clog vessels: Way too thick, like with sickle cell disease, some cancer states or severe dehydration, can increase viscosity of blood so much that it clogs up smaller vessels--this stops blood flow beyond--can cause tissue to die, like a heart attack or stroke. ...Read more.