blodkoagulation

Blodkoagulation is the physiological process by which blood forms clots to stop bleeding after vascular injury. It arises from coordinated actions of platelets, coagulation factors, and the endothelium, producing a fibrin-rich clot that seals damaged vessels.

Coagulation proceeds in two stages: primary hemostasis, where platelets adhere to exposed subendothelium and aggregate to form a temporary plug; and secondary hemostasis, a proteolytic cascade that generates thrombin, which converts fibrinogen to fibrin and stabilizes the clot. The cascade is often described as intrinsic and extrinsic pathways that converge on a common pathway. Tissue factor from injured tissue initiates the extrinsic pathway, while contact activation triggers the intrinsic pathway. Most coagulation factors are produced by the liver, and several require vitamin K for maturation (notably factors II, VII, IX, and X).

Regulation and fibrinolysis: Anticoagulant mechanisms, including protein C and protein S, antithrombin, and endothelial factors, prevent excessive clot formation. After hemostasis, plasmin dissolves fibrin in a controlled manner, activated by tissue plasminogen activator (tPA). Endothelium also modulates coagulation and fibrinolysis through multiple mediators.

Clinical relevance: Abnormal coagulation underlies bleeding disorders such as hemophilia A (factor VIII deficiency), hemophilia B (factor IX deficiency), and von Willebrand disease, as well as thrombotic conditions like factor V Leiden. Diagnostic tests such as prothrombin time (PT) and activated partial thromboplastin time (aPTT) assess different cascade components. Severe coagulopathy can occur in disseminated intravascular coagulation (DIC) or advanced liver disease, reflecting impaired production or regulation of coagulation factors.

Management comprises replacement therapy for deficiencies and anticoagulant treatment to prevent pathological clotting, including heparin, warfarin, and direct oral anticoagulants. Antiplatelet drugs and, when indicated, fibrinolytic therapy are also used. Understanding blodkoagulation is essential in medicine, surgery, and transfusion practice.