Cite this page: Parsons J. Extrinsic pathway. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/coagulationextrinsicpathway.html. Accessed May 29th, 2020.
Definition / general
- Involves tissue factor (TF), originally considered "extrinsic" to blood since it is present on cell surfaces not normally in contact with (i.e. extrinsic to) the circulatory system
- The primary mechanism of the coagulation pathway in vivo is tissue factor binding to activated factor VII (factor VIIa)
- TF-Factor VIIa complex activates factors X and IX (though in vivo it appears to first involve factors VIII and V from the intrinsic pathway, which then activate factors X and IX)
- Activated factor IX activates more factor X, with cofactors activated factor VIII, anionic phospholipids (from activated platelets) and calcium
- Activated factor X converts prothrombin to thrombin, with activated factor V, anionic phospholipids (from activated platelets) and calcium as cofactors
- Prothrombin factor 1.2 is released (see common pathway)
- After initial activation, pathway is inhibited by the binding of tissue factor pathway inhibitor (TFPI) to factor Xa, which inhibits TF-Factor VIIa complex, and further coagulation is dependent on the intrinsic pathway
- Merges with extrinsic pathway into common pathway
- The prothrombin time (PT) measures the extrinsic and common pathways
Diagrams / tables
Flickr image courtesy of Jeremey Parsons, M.D.:
Coagulation extrinsic pathway
Additional references
