What Happens When Blood Is Too Thick?

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When blood is too thick, it clots more easily, and the potential exists for blockage of the blood flow through the arteries and veins, according to the National Heart, Lung and Blood Institute. This may trigger a heart attack or stroke. Polycythemia vera is a condition in which the body produces too many red blood cells, causing thickening of the blood and increasing the possibility that clotting occurs.