Mechanisms of blood coagulation
The mechanism of coagulation involves a complex series of interactions between blood cells, clotting factors, and the endothelium (the inner lining of blood vessels). There are two main pathways involved in the coagulation process: the extrinsic pathway and the intrinsic pathway. These pathways eventually converge to form the common pathway, which leads to the formation of a fibrin clot.
- Tissue factor is released from damaged endothelial cells or from tissue outside the blood vessel.
- Tissue factor binds to and activates factor VII.
- Activated factor VII, along with factor III (tissue factor), activates factor X.
- Factor XII, also known as Hageman factor, is activated by contact with exposed collagen and other substances in the blood.
- Activated factor XII activates factor XI.
- Activated factor XI activates factor IX.
- Activated factor IX, along with factor VIII, calcium ions, and phospholipids, activates factor X.
- Activated factor X combines with factor V, calcium ions, and phospholipids to form prothrombinase.
- Prothrombinase converts prothrombin into thrombin.
- Thrombin then converts fibrinogen into fibrin.
- Fibrin strands combine to form a fibrin clot, which stabilizes the platelet plug and prevents further bleeding.
The coagulation process is carefully regulated to prevent excessive clotting, which can lead to thrombosis or blockage of blood vessels. Anticoagulant proteins like antithrombin, protein C, and protein S help to balance the coagulation process and prevent unwanted clotting.
Disorders of coagulation can lead to bleeding disorders, such as hemophilia, or clotting disorders, such as thrombophilia. These conditions can increase the risk of bleeding or thrombosis and may require medical intervention to manage.