What does the anticoagulant system do?
The anticoagulant system, like the fibrinolytic system, performs opposing functions to that of secondary haemostasis which ultimately forms a fibrin clot. Put more simply, as the diagram alongside shows, the anticoagulant system is the system responsible for your ability to bleed freely. Unlike the fibrinolytic system, however, the anticoagulant system chooses a different target: instead of using plasmin to attack the fibrin clot directly, the anticoagulant system chooses to inhibit factors V and VIII (5 and 8) within the coagulation (or clotting) cascade.
What components make up the anticoagulant system?
There are four key proteins involved in the anticoagulant system: tissue factor pathway inhibitor (TFPI), protein C, protein S and antithrombin III.
What does the Protein C anticoagulant pathway look like?
The Protein C anticoagulant pathway moves in two distinct stages: first protein C is activated, and then, in the presence of protein S, secondary haemostasis is inhibited. The first stage is demonstrated by the Smart Art Graphic alongside. Protein C binds to the complex of thrombin and thrombomodulin located on blood vessel walls to become activated by means of a section being cleaved off.
The second stage involves activated protein C inhibiting factors Va and VIIIa. Protein S must be bound to activated protein C in order for the process to work.
What does antithrombin III do?
Antithrombin III, as the name suggests, neutralizes thrombin, which is responsible for activating factors V and VIII. It is considered therefore the major inhibitor of the coagulation cascade. It does not do this alone, however: in order for antithrombin to have any real effect on the coagulaton cascade, it must be bound with heparin.