Take a look at the stickies in this forum. Precisely what Coumadin does is told.
Blood thinner- Contrary to the oft heard term referring to what type of drug Coumadin is, it DOES NOT "thin" your blood. Your blood's viscosity (thickness) does not change. The drug blocks the action of vitamin K within the liver. The drug effectively competes for the same absorption sites within the liver needed for vitamin K uptake. Thus the levels of vitamin K within the liver decrease. Reduced amounts of clotting factors VII, IX, X, and II are produced, and the clotting action of the blood is progressively impaired. The term "blood thinner" leads people to think that their blood will run out of them if they are cut. If your blood is a bottle of ketchup, adding Coumadin to it is not like adding water to it.
The ketchup does not "thin out" with the addition of Coumadin. It's consistency is still the same, it just won't stick to your hamburger as easily.