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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.
Thick Blood: Common Causes. Certain ethnic races (particularly Caucasian) may have a genetic predisposition towards thick blood. Defects in the coagulation genes adversely affect the capability of the body to turn off the switch that activates the clotting process.