Definition

The aim of anticoagulants are to modify the body’s haemostatic mechanisms to prevent venous and or arterial thrombo-embolism.

There are three main phenomena in haemostasis, balanced by an opposing mechanism of fibrinolysis:

1. Platelet activation and aggregation
2. Vasoconstriction secondary to endothelial mediators
3. Formation of a fibrin mesh enhanced by clotting factors

Formation of a fibrin mesh is the end result of the clotting system, which is a cascade of enzymes and factors from I to XIII that leads to inactive precursors being activated.

There are two separate pathways; the intrinsic and the extrinsic pathways, which come together to form the common pathway. Prothrombin (Factor II) is activated to Thrombin (Factor IIa) which then converts soluble fibrinogen (Factor I) into the insoluble meshwork of fibrin, trapping blood cells to form a clot.

The intrinsic pathway is activated when blood comes in contact with the subendothelial tissues. It is the slower but most important of the two pathways, and includes factors XII, XI, IX, and X.

The extrinsic pathway responds rapidly to tissue injury, but its main function is to augment the intrinsic pathway through factors VII, II (tissue factor) and X.