Coagulation
General
Extrinsic pathway

Author: Jeremy Parsons, M.D. (see Authors page)

Revised: 29 February 2016, last major update June 2012

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Extrinsic pathway [title] coagulation
Cite this page: Extrinsic pathway. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/coagulationextrinsicpathway.html. Accessed December 7th, 2016.
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
  • 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.:

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Coagulation extrinsic pathway