Coagulation

Coagulation is the process of formation of a fibrin clot. Coagulation is an important part of hemostasis, the process of blood clotting and then the subsequent dissolution of the clot, following repair of the injured tissue. Coagulation begins almost instantly after an injury to the blood vessel has damaged the endothelium. When the endothelium is damaged, the von Willebrand factor (vWF) is exposed to blood and recruits Factor VIII, collagen, and other coagulation factors. Circulating platelets adhere to exposed collagen, release the contents of their granules, and aggregate at the site of injury, forming a temporary, loose platelet plug. To insure stability of the initially loose platelet plug, proteins in the blood plasma, called coagulation factors or clotting factors, respond in a complex cascade to form fibrin clots.The coagulation cascade has two pathways, the intrinsic pathway (also known as the contact activation pathway), and the extrinsic pathway (also known as the tissue factor pathway), which lead to fibrin formation. The coagulation system overlaps with the immune system. Coagulation can physically trap invading microbes in blood clots. Also, some products of the coagulation system can contribute to the innate immune system by their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells. In addition, some of the products of the coagulation system are directly antimicrobial. Disorders of coagulation can lead to an increased risk of bleeding (hemorrhage) or clotting (thrombosis).