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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.
Polycythemia Vera. This slow-developing disease occurs when bone marrow produces too many red blood cells, which can lead to thickening of the blood. The condition is thought to be caused in part by a mutation in a gene called JAK2, which regulates the production of various types of blood cells, keeping them in proper balance.