Coagulation system

The blood coagulation system is shown in simplified form in Figure 28.1. It consists of glycoprotein components that circulate in (necessarily inactive) pro-enzyme or pro-cofactor (factors V and VIII) form. The activated enzymes are serine proteases.

Physiological coagulation (the 'extrinsic' pathway) begins when tissue factor (TF, tissue thromboplastin), exposed by vascular injury, activates and complexes with factor VII to activate factors IX and X which complex with Villa and Va respectively on membrane surfaces (which provide phospholipid, PL). The Xa/Va complex converts prothrombin to thrombin which converts fibrinogen to fibrin and also activates factors XI, VIII, V and XIII, both accelerating coagulation and cross-linking fibrin (-F-F-F-).

The 'intrinsic' pathway refers to coagulation in vitro. It is initiated when factor XII with the cofactor high molecular weight kininogen (HMWK) comes into contact with a foreign surface, e.g. glass, kaolin. Thus it has no physiological role (and patients lacking factor XII do not have a bleeding disorder).


Foreign surface

Damaged tissue

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