CBS.com; CBS All Access; CBS News; CBSN; Watching Live TV. All Access Live TV; TV Provider
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 is a blood cancer that begins in the marrow of your bones, the soft center where new blood cells grow. With polycythemia vera, your marrow makes too many red blood cells, which causes your blood to get too thick. That can make you more likely to have blood clots, a stroke, or a heart attack.