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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.
When the blood is thicker, it moves sluggishly; there is an increased risk for red cells to adhere to one another and form clots, and for there not to be enough oxygenation in a given time to areas such as the legs or the brain, and to vital organs.