Coagulation (blood clotting) is a complicated series of physical reactions that transform liquid blood into a gel that provides a secure patch to the injured blood vessel. Thirteen coagulation factors are involved in this amazing sequence of events. Most of the factors are proteins released into the blood by the liver. Vitamin K is required for the synthesis of some of these factors. (Food sources of vitamin K include leafy vegetables, cheese, asparagus, bacon, coffee, and green tea.)
The coagulation process can be described in three major steps.
- Formation of factor X and prothrombinase. Prothrombinase can form either inside the blood vessels (intrinsically) or outside the blood vessels (extrinsically). In the intrinsic pathway, the collagen of the damaged blood vessel initiates a string of reactions that activates factor X. In the extrinsic pathway, damaged tissues release thromboplastin, which starts up a shorter, more rapid sequence of reactions to activate factor X. In both pathways, activated factor X combines with factor V (with vitamin K present) to form prothrombinase.
- Prothrombin is converted to thrombin. In this common pathway that follows both the intrinsic and extrinsic pathways, prothrombinase (with vitamin K) converts prothrombin to thrombin.
- Fibrinogen is converted to fibrin. The common pathway continues as thrombin (with vitamin K) converts fibrinogen to fibrin. Fibrin forms long strands that bind platelets together to form a dense web. Thrombin also activates factor XIII, which helps fibrin strands stick to one another. The result is a clot.
After its formation, a clot is further strengthened by a process called clot retraction. Platelets in the clot contract, pulling on the fibrin strands to which they're attached. The result is a more tightly sealed path.