22.21: Coagulation
The coagulation phase is a critical part of the body's process to prevent blood loss following injury to blood vessels. It involves chemical reactions that form a clot to seal the injured area. The clotting process begins shortly after injury, within 15-20 seconds for severe damage and 1-2 minutes for minor injuries.
During the coagulation phase, clotting factors, or procoagulants, play a vital role in initiating and progressing the coagulation cascade. This cascade is a series of reactions that ultimately lead to the formation of a blood clot. Key factors involved include fibrinogen, prothrombin, tissue factor, calcium, and thrombin, each playing a unique role in the clotting process.
Platelets also play a crucial role. These small cell fragments respond to vascular damage by becoming sticky and adhering to the damaged vessel wall. Once activated, they release thromboxane A2 and adenosine diphosphate (ADP), substances that activate nearby platelets, contributing to the formation of a platelet plug. This plug acts as a temporary seal while the coagulation cascade is initiated.
A significant step during the coagulation phase is the transformation of prothrombin, a protein in the blood, into thrombin. The prothrombin activator complex facilitates this transformation. Thrombin is essential in the clotting process, acting as an enzyme to convert fibrinogen into fibrin fibers.
Thrombin then converts fibrinogen into fibrin fibers, creating a mesh-like structure that captures blood cells and other components. This structure forms the blood clot as a permanent seal for the injured vessel, preventing further blood loss. Typically, the clot takes 3 to 6 minutes to completely seal the injury, assuming the vessel opening is not excessively large.
