Coumarins as Anticoagulants

When a coumarin, such as warfarin, is given to a patient, the plasma levels of prothrombin and Factors VII, IX, and X, all formed by the liver, begin to fall, indicating that warfarin has a potent depressant effect on liver formation of these compounds. Warfarin causes this effect by competing with vitamin K for reactive sites in the enzymatic processes for formation of prothrombin and the other three clotting factors, thereby blocking the action of vitamin K.

After administration of an effective dose of warfarin, the coagulant activity of the blood decreases to about 50 per cent of normal by the end of 12 hours and to about 20 per cent of normal by the end of 24 hours. In other words, the coagulation process is not blocked immediately but must await the natural consumption of the prothrombin and the other affected coagulation factors already present in the plasma. Normal coagulation usually returns 1 to 3 days after discontinuing coumarin therapy.

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Essentials of Human Physiology

Essentials of Human Physiology

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