Coagulation Cascade

Two pathways make up the system: intrinsic and extrinsic. The pathways are named based on in vitro experiments that used "extrinsic" source of tissue thrombo-plastin (usually brain tissue) to induce the cascade. The second system was initiated in the test tube without and external stimulus (i.e., the "intrinsic" system).

The intrinsic pathway is initiated by exposure of coagulation factors to suben-dothelial collagen. Tissue factor combined with various other factors to converge on factor X which cleaves prothrombin to thrombin. The extrinsic pathway is activated by tissue factors or glycoproteins.

Table 3.1. Coagulation factors

Factor

Name

I

Fibrinogen

II

Prothrombin

III

Tissue thromboplastin

IV

Calcium

V

Proaccelerin

VI

VII

Proconnectin

VIII

Antihemophilic factor

IX

Christmas factor

X

Stuart-Prower factor

XI

Plasma thromboplastin

XII

Hageman factor

XIII

Fibrin stabilizing factor

All of the coagulation factors are synthesized in the liver except thromboplastin, factor VIII and calcium. Factors II, VII, XI, X, protein C and S are vitamin K dependent enzymes and therefore effected by coumadin (Table 3.1).

Calcium plays an integral role in the cascade and is required as a cofactor in many of the steps. Hypocalcemia can lead to coagulopathy and should be monitored in patients with coagulation defects especially following significant transfusions (Table 3.2).

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