Prothrombin activator can be formed outside of a blood vessel when there is tissue damage associated with the break in a vessel wall. Damaged tissues are represented by the ruptured cell releasing its contents (stars).
Active factor X and its cofactors factor V, calcium ions, and tissue factor constitute 'prothrombin activator' (encircled in red). The ruptured cell membrane with its attached tissue factor (yellow complex) provides the starting point of the pathway.
Blood from a nearby broken vessel supplies the necessary coagulation factors (hexagons). Factor VII is activated when it binds to tissue factor. In the presence of calcium ions and factor V, factor X becomes activated.
Prothrombin activator is the enzyme that converts prothrombin to thrombin. This enzymatic activity is shown by the heavy arrow passing through the enzyme. A positive feedback loop is established as thrombin interacts with factor V in the enzyme complex (solid arrow); this interaction increases the efficiency of the enzyme complex and thrombin production quickens.
This pathway to prothrombin activator is more rapid than the intrinsic pathway because fewer factors are involved. The intrinsic pathway will be occurring simultaneously and the clots resulting from both will merge.
Last update: 7/19/2005