The preservation of an intact vascular system requires not only that blood be capable of coagulating but also that there should be a mechanism for removing the products of coagulation when they have served their purpose of stopping a vascular leak. This is the function of the fibrinolytic system, the essential features of which are shown in Figure 28.2.
The system depends on the formation of the fibrinolytic enzyme plasmin from its precursor protein, plasminogen, in the blood. During the coagulation process, plasminogen binds to specific sites on fibrin. Simultaneously the natural activators of plasminogen, i.e. tissue plasminogen activator (tPA) and urokinase, are released from endothelial and other tissue cells and act on plasminogen to form plasmin. The result is that plasmin formation only takes place locally on the fibrin surface but not generally within the circulation where widespread defibrination would occur and the whole coagulation mechanism would be compromised. Since fibrin is the framework of the thrombus, its dissolution clears the clot away.
Fibrinolytics (thrombolytics) can remove established thrombi and emboli. Inhibitors of the fibrinolytic system (antifibrinolytics) can be of value in certain haemorrhagic states notably those characterised by excessive fibrinolysis.
Plasminogen (in the blood)
Fibrinolytics activators of plasminogen in the blood
An ist rep läse urokinase streptokinase
Antifibrinolytics inhibitors of plasminogen and plasmin
Tranexarnic acid blocks plasminogen binding to fibrin
Plasminogen binds to fibrin
Bound to fibrin alteplase prourokinase
Plasminogen formed on fibrin which it destroys
Aprotinin inhibits plasmin
Fibrin degradation products (FDP)
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