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Clotting Factors


Updated November 11, 2011

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.


The clotting factors (also known as coagulation factors or the thrombin system) are several types of specialized proteins in the bloodstream that are largely responsible for clotting blood.

The clotting factors are activated whenever an injury to a blood vessel causes bleeding. The activation of the clotting factors leads to a cascade of events that cause two things to occur:

  • Platelets clump together at the site of injury to form a platelet plug.
  • Strands of the clotting factor fibrin form a mesh of protein at the site of the injury, binding and strengthening the platelet plug.
The strengthened platelet plug is the blood clot. In this way the bleeding is stopped.

Abnormal clotting, or thrombosis, can occur in many medical conditions, including atherosclerosis or atrial fibrillation. In these conditions, anticoagulant drugs, such as Coumadin, are often used to inhibit the clotting factors and reduce the risk of thrombosis.

Also Known As: coagulation factors, thrombin system

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