Clotting

Clotting, also called coagulation, is the biological process that stops bleeding by forming a blood clot at the site of injury. It is a core part of hemostasis, which also includes vascular constriction and the formation of a platelet plug. The final clot is a fibrin-rich mesh that seals damaged vessels and allows tissue repair.

When a vessel is damaged, platelets adhere to exposed endothelium and become activated. They release chemical signals that recruit more platelets and form a temporary platelet plug. This platelet phase sets the stage for the coagulation cascade, a series of plasma protein reactions that amplify the response and stabilize the plug.

The coagulation cascade involves two pathways, the extrinsic and intrinsic, which converge on a common pathway. Thrombin is produced and then converts soluble fibrinogen into insoluble fibrin strands, creating a stable clot woven into a mesh that traps blood cells. Most clotting factors are produced in the liver, and several require vitamin K for activity.

Clot formation is tightly regulated and eventually balanced by anticoagulants and fibrinolysis. Anticoagulants such as antithrombin, protein C, and protein S limit the cascade, while plasminogen is converted to plasmin to dissolve fibrin through fibrinolysis as healing progresses.

Clinical aspects include bleeding disorders (e.g., hemophilia, von Willebrand disease) and pathological clotting (thrombosis, leading to deep vein thrombosis or pulmonary embolism). Laboratory tests assess clotting function (PT, aPTT, platelet count, fibrinogen). Treatments range from anticoagulants and antiplatelet drugs to thrombolytics or mechanical clot removal in specific cases.