Why do patients have to take anticoagulants when they receive and artificial valve in their heart?

Avoid strokes: Mechanical valves cause some turbulence in the blood flow. This can cause clots to form and float up to the brain where they become lodged in the smaller vessels and block the blood flow to a portion of the brain resulting in a stroke. A blood thinner helps prevent this from occurring.

Blood clot: The layer of cells lining the heart and all blood vessels allows blood to pass without clotting. A break in the layer from injury recruits platelets and clotting factors to form clot and seal the injury. A mechanical valve will encourage clot by having areas of slow flow and places for clot to stick.

Stop clotting: Certain types (often metallic) of heart valves are prone to forming blood clots. Such clots can shoot out to the lungs or the brain.