Blood clotting (coagulation) is a complex cascade of reactions that stops bleeding (haemostasis) after blood vessel injury. It involves three overlapping phases: Vascular spasm: immediate vasoconstriction reduces blood flow to injured area. Primary haemostasis: platelet plug formation — platelets adhere to exposed collagen at injury site (via von Willebrand factor), become activated, and aggregate, forming a loose platelet plug. Secondary haemostasis / coagulation cascade: series of proteolytic reactions (clotting cascade) produces thrombin which converts fibrinogen to fibrin, forming a mesh that reinforces and stabilises the platelet plug into a firm blood clot. The clotting cascade involves approximately 13 clotting factors (numbered I-XIII, with VI not actually existing as a separate factor), most produced by the liver, many requiring Vitamin K for synthesis.

Fibrinogen (Factor I) is a large (340 kDa), soluble glycoprotein consisting of two sets of three polypeptide chains (Aα, Bβ, and γ, held together by disulfide bonds) synthesised by hepatocytes (liver cells) and present in blood plasma at concentrations of approximately 2-4 g/L. It is an acute phase protein whose levels increase during inflammation. During clotting, thrombin cleaves short peptides (fibrinopeptides A from Aα chains, fibrinopeptides B from Bβ chains) from fibrinogen, converting it to fibrin monomers. These fibrin monomers spontaneously self-assemble into fibrin polymers (protofibrils), which then form a loose, soluble fibrin mesh. Factor XIIIa (activated by thrombin in the presence of Ca2+) then cross-links adjacent fibrin molecules through isopeptide bonds between glutamine and lysine residues, creating a stable, insoluble, cross-linked fibrin clot that can withstand mechanical forces.

The clotting cascade is typically described as having two initiation pathways converging on a common final pathway. Extrinsic pathway: begins when tissue factor (TF, also called Factor III or thromboplastin) is exposed after vessel injury. TF + Factor VIIa complex activates Factors X and IX. This is the physiologically dominant pathway. Intrinsic pathway: begins with contact activation of Factor XII (Hageman factor) when blood contacts a negatively charged surface (collagen, glass, kaolin). Activated XII → XI → IX → X. This pathway is important for amplification but less critical for initial clot formation in vivo (Factor XII deficiency doesn't cause bleeding). Common pathway: Factor Xa + Factor Va + phospholipid + Ca2+ = prothrombin activator complex → cleaves prothrombin (Factor II) to thrombin (Factor IIa) → thrombin cleaves fibrinogen to fibrin → Factor XIIIa cross-links fibrin. The cascade is designed with amplification — each activated factor is an enzyme that activates many molecules of the next factor, creating rapid, explosive thrombin generation once initiated.

Vitamin K is essential for blood clotting because it serves as a cofactor for carboxylase enzymes that add gamma-carboxyl groups to glutamate residues on several clotting factors (II, VII, IX, X, and the anticoagulant proteins C and S). These gamma-carboxyglutamate (Gla) residues are required for calcium-dependent binding of these clotting factors to phospholipid surfaces where the clotting cascade reactions take place. Without Vitamin K, these clotting factors are synthesised but non-functional, leading to impaired coagulation and bleeding tendency. Warfarin (a widely used anticoagulant drug) works by inhibiting Vitamin K epoxide reductase, the enzyme that recycles the oxidised form of Vitamin K back to its active reduced form, effectively depleting active Vitamin K and impairing synthesis of functional clotting factors. Other anticoagulants: Heparin activates antithrombin III, which inhibits thrombin and Factor Xa. Direct oral anticoagulants (DOACs) like rivaroxaban and dabigatran directly inhibit Factor Xa and thrombin respectively. EDTA and citrate chelate calcium ions, used for in vitro blood preservation.