Two coagulation pathways lead to fibrin clot formation: the extrinsic and the intrinsic pathways. Contrary to previous thought, these pathways are not independent. However, this artificial distinction is retained in the following text to facilitate their description.

Initiation of fibrin clot formation in response to tissue injury is carried out by the extrinsic pathway. The intrinsic pathway can be activated by negatively charged surfaces in vitro, for example, glass. Both pathways lead to the proteolytic conversion of prothrombin to thrombin. Thrombin catalyzes cleavage of fibrinogen to initiate fibrin clot formation. The extrinsic and intrinsic pathways are complex and involve many different proteins (Figures 1 and 2; Table 1). The coagulation factors are another example of multidomain proteins sharing conserved domains. In general, as shown in Table 2, these proteins can be classified into five types: (1) zymogens of serine-dependent proteases that are activated during the process of coagulation; (2) cofactors; (3) fibrinogen; (4) a zymogen of a transglutaminase that covalently cross-links fibrin and stabilizes the fibrin clot; and (5) regulatory and other proteins.

Fig1. The pathways of blood coagulation, with the extrinsic pathway indicated at the top left and the intrinsic path way at the top right.The pathways converge in the formation of active factor X (ie, factor Xa) and culminate in the formation of cross linked fibrin (lower right). Complexes of tissue factor and factor VIIa activate not only factor X (extrinsic Xase [tenase]) but also factor IX in the intrinsic pathway (dotted arrow). In addition, thrombin feedback activates at the sites indicated (dashed arrows) and also activates fac tor XIII to factor XIIIa (dashed-dotted arrow) and activates factor VII to factor VIIa (not shown). The three predominant complexes, extrinsic Xase, intrinsic Xase, and prothrombinase, are indicated within the large arrows; these reactions require anionic procoagulant phospho lipid membrane and calcium. Activated proteases are in solid-out lined boxes; active cofactors are in dash-outlined boxes; and inactive factors are not in boxes. (HK, high-molecular-weight kininogen; PK, prekallikrein.)

Fig2. The structural domains of selected proteins involved in coagulation and fibrinolysis. Shared domains are a result of gene duplication and exon shuffling that contributed to the molecular evolution of the coagulation system. The domains are as identified at the bottom of the figure and include signal peptide, propeptide, Gla (γ-carboxyglutamate) domain, epidermal growth factor (EGF) domain, apple domain, kringle domain, fibronectin (types I and II) domain, the zymogen activation region, aromatic amino acid stack, and the catalytic domain. Interdomain disulfide bonds are indicated, but numerous intradomain disulfide bonds are not. Sites of proteolytic cleavage in synthesis or activation are indicated by arrows (dashed and solid, respectively). (FVII, factor VII; FIX, factor IX; FX, factor X, FXI; factor XI; FXII, factor XII; t-PA, tissue plasminogen activator.)

Table1. Numerical System for the Nomenclature of Blood Clotting Factors

Table2. The Functions of the Proteins Involved in Blood Coagulation

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