The blood coagulation cascade involves three pathways that ultimately lead to the formation of a fibrin clot. The intrinsic pathway is activated by disturbances in blood and platelet phospholipids and involves multiple coagulation factors leading to the activation of factor X. The extrinsic pathway is initiated when tissue factor is released from injured endothelium, also activating factor X. The common pathway follows, where factor Xa converts prothrombin to thrombin which then cleaves fibrinogen to form fibrin, an essential component of a clot.

1. BLOOD COAGULATION CASCADE
• .
1
Intrinsic pathway
Blood disturbances and platelet
phospholipids activate factor XII
to initiate the intrinsic
coagulation pathway, which
leads to the activation of
(factor X).X
Extrinsic pathway
When a vessel is injured,
(tissue factor, or
thromboplastin) is released
from the endothelium
initiating the extrinsic
coagulation pathway, which
leads to the activation of
(factor X).
TF
X
Common pathway
(factor II, or prothrombin) is converted by
factor Xa into factor IIa, or thrombin.
Thrombin cleaves (factor I, or fibrinogen) to
form fibrin (factor Ia), a key component of clot
formation.
II
I
Adapted from Ansell J. J Thromb Haemost. 2007;5(suppl1):60-64.
Guyton AC, Hall JE. Hemostasis and blood coagulation. In: Textbook of Medical Physiology. 10th ed. Philadelphia, PA: WB Saunders Co;
2000:419-429.
Moake JL. Hemostasis. In: Porter RS, Kaplan JL, Homeier BP, eds. The Merck Manual Online. http://www.merck.com/mmpe/sec11/ch134/ch134a.html. Accessed March
4, 2008.
Fibrin Clot
Xll
Xl
lX
VIIIa VII
TF
Va
II
I
X

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