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Coagulation

General

Extrinsic pathway

Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 9 June 2012, last major update June 2012
Copyright: (c) 2002-2012, PathologyOutlines.com, Inc.

General
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● 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
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Coagulation extrinsic pathway – by Jeremy Parsons, M.D.

Additional references
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● Kolde, Hans-Jurgen (2004). Haemostasis. Basel, Switzerland: Pentapharm Ltd.

End of Coagulation > General > Extrinsic pathway

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