Summary
Highlights
Coagulation is the process where soluble fibrinogen in plasma is converted into insoluble fibrin strands. This process involves two main pathways: the intrinsic and extrinsic pathways.
The intrinsic pathway was identified when early doctors observed blood coagulating in a clean container without any external additives. This led them to hypothesize an 'intrinsic' mechanism within the blood itself. This mechanism involves Factor 12 (Hageman Factor), which activates upon contact with non-endothelial surfaces, initiating coagulation.
The extrinsic pathway was discovered when scientists found that blood, even with its intrinsic pathway blocked (e.g., by citrates), would coagulate upon the addition of 'tissue juice' (later identified as tissue factor or thromboplastin). This demonstrated an 'extrinsic' mechanism initiated by external tissue components.
In a real injury scenario, both intrinsic and extrinsic pathways are activated. The intrinsic pathway is triggered when blood contacts injured endothelial cells, exposed sub-endothelial collagen, or activated platelets. The extrinsic pathway is activated by tissue factor released from injured cells, which accelerates the coagulation process.
The intrinsic pathway starts with Factor 12 being activated at the injury site. Activated Factor 12 then activates Factor 11, which in turn activates Factor 9. Activated Factor 9, along with phospholipids, calcium, and Factor 8, activates Factor 10. Activated Factor 10 converts prothrombin into thrombin.
Thrombin is a crucial enzyme. It breaks down fibrinogen into fibrin monomers. These monomers deposit on the platelet plug. Thrombin also activates Factor 13 (fibrin stabilizing factor), which cross-links the fibrin monomers, creating a strong, covalent meshwork that converts the primary platelet plug into a secondary, more stable hemostatic plug.
Concurrently, tissue factors produced at the injury site activate Factor 7. Activated Factor 7 can directly activate Factor 9 and Factor 10. This extrinsic pathway is shorter and faster than the intrinsic pathway, providing a rapid initial burst of coagulation.
Injured endothelium releases von Willebrand factor, leading to platelet adhesion, activation, and aggregation, forming a primary platelet plug. Subsequently, both intrinsic and extrinsic coagulation pathways are activated, leading to fibrin deposition and cross-linking to fortify this plug, transforming it into a robust secondary hemostatic plug.