L07: Haemostasis 2 – Coagulation

The coagulation cascade is a series of enzymatic reactions that culminate in the generation of thrombin and, ultimately, a stable fibrin clot. Coagulation factors are plasma proteins, most synthesised by the liver, that circulate as inactive zymogens (serine protease precursors) until activated by cleavage.

The extrinsic pathway is triggered by tissue factor (TF, factor III), a transmembrane protein expressed by subendothelial cells, smooth muscle, and fibroblasts normally hidden from circulating blood. After vascular injury, TF is exposed and forms a complex with factor VIIa. The TF:VIIa complex activates factor X to Xa, which then combines with its cofactor factor Va (the prothrombinase complex) on phosphatidylserine-exposing platelet surfaces to convert prothrombin (factor II) to thrombin (factor IIa).

The intrinsic (contact activation) pathway is initiated by factor XII binding to negatively charged surfaces (e.g., collagen, kaolin). Factor XII activates XI, which activates IX. Factor IXa combines with its cofactor factor VIIIa to form the tenase complex, which activates factor X. Both pathways converge at factor X, sharing a common pathway to thrombin generation.

Thrombin is the central effector enzyme of coagulation. It cleaves fibrinopeptides A and B from fibrinogen, generating fibrin monomers that spontaneously polymerise into a loose fibrin mesh. Thrombin also activates factor XIII (transglutaminase), which covalently cross-links fibrin polymers, creating a mechanically stable clot resistant to fibrinolysis. Additionally, thrombin amplifies its own generation by activating factors V and VIII (positive feedback).

Several natural anticoagulant systems limit clot formation. Antithrombin (AT) is a serine protease inhibitor that neutralises thrombin, Xa, IXa, and XIa; its activity is increased 300-fold by heparin. Protein C, activated by thrombin bound to thrombomodulin on endothelial cells, inactivates factors Va and VIIIa (with cofactor Protein S). Tissue factor pathway inhibitor (TFPI) inhibits the TF:VIIa:Xa complex, limiting extrinsic pathway activity.

Vitamin K is required for the gamma-carboxylation of glutamate residues on factors II (prothrombin), VII, IX, X, and proteins C and S. Gamma-carboxylation enables these factors to bind calcium ions and assemble on phospholipid surfaces. Warfarin inhibits vitamin K epoxide reductase, blocking recycling of vitamin K to its active form and thereby depleting gamma-carboxylated factors. Because factor VII has the shortest half-life (~6 hours), warfarin prolongs the PT first.

Fibrinolysis dissolves the clot once healing is complete. Tissue plasminogen activator (tPA) converts plasminogen to plasmin, which cleaves fibrin. Plasmin generates fibrin degradation products, including D-dimers (cross-linked fibrin fragments unique to fibrin, not fibrinogen). D-dimers are elevated in DVT, PE, DIC, and any state of heightened coagulation/fibrinolysis.

Laboratory tests of coagulation assess the two pathways separately. The prothrombin time (PT) measures the extrinsic and common pathways by adding tissue factor, calcium, and phospholipid to citrated plasma; normal PT is approximately 10–14 seconds. The activated partial thromboplastin time (APTT) measures the intrinsic and common pathways using a contact activator (kaolin or silica) and phospholipid without TF; normal APTT is approximately 22–34 seconds. The international normalised ratio (INR) standardises PT measurement across laboratories and reagents, used to monitor warfarin therapy.