Plasmatic coagulation - extrinsic system

Rapid coagulation: The extrinsic system is a phase of plasmatic coagulation and was given its name because it is activated directly after a tissue lesion (see hemostasis below).

In the case of a tissue lesion, tissue thromboplastin(factor III) is released. This in turn activates factor VII, the so-called proconvertin. Factor VII is activated together with Ca++ by proteolytic cleavage of F-VII.
In addition to the tissue factor, factor VII is also activated by an F-VII activating protease (FSAP).

When factor X is activated, the intrinsic and the extrinsic system merge into a common pathway. In addition, it could be shown that factor VIIa also activates factor IX of the intrinsic pathway.
The activated factor X activates together with the factor Va as coenzyme, phospholipids and Ca++ the prothrombin (factor II). The prothrombin is proteolytically cleaved to form thrombin and the prothrombin fragments F1 and F2.

The reaction in which the prothrombin is converted into thrombin depends on contact with the membrane surface. The prothrombin fragments F1 and 2 have a diagnostic significance, e.g. in the case of an AT-III deficiency, protein C deficiency. Finally, the conversion of fibrinogen into fibrin takes place via thrombin.

1. HA Neumann (2014) The coagulation system. ABW-Wissenschaftsverlag GmbH Berlin S. 74f.