434
Chapter 12
including the collagen fi bers underlying damaged endothe-
lium. The contact activation of factor XII to XIIa is a complex
process that requires the participation of several other plasma
proteins not shown in Figure 12–76. (Contact activation also
explains why blood coagulates when it is taken from the body
and put in a glass tube. This has nothing whatever to do with
exposure to air, but happens because the glass surface acts like
collagen and induces the same activation of factor XII and
aggregation of platelets as a damaged vessel surface. A silicone
coating delays clotting by reducing the activating effects of
the glass surface.)
Factor XIIa then catalyzes the activation of factor XI to
factor XIa, which activates factor IX to factor IXa. This last
factor then activates factor X to factor Xa, which is the enzyme
that converts prothrombin to thrombin. Note in Figure 12–76
that another plasma protein—factor VIIIa—serves as a cofac-
tor (not an enzyme) in the factor IXa-mediated activation of
Figure 12–76
Two clotting pathways—intrinsic and extrinsic—merge and can lead to the generation of thrombin. Under most physiological conditions,
however, factor XII and the contact activation step that begin the intrinsic pathway probably play little, if any, role in clotting. Rather, clotting
is initiated solely by the extrinsic pathway, as described in the text. You might think that factors IX and X were accidentally transposed in
the intrinsic pathway, but this is not the case; the order of activation really is XI, IX, and X. For the sake of clarity, the roles calcium plays in
clotting are not shown.
Subendothelial cells
exposed to blood
Tissue factor
Contact
activation
Xa
XX
Va
VIIa
VII
Activated
platelets
Activated
platelets
Intrinsic pathway
Extrinsic pathway
Vessel damage
Thrombin
Prothrombin
IX
XIIa
XII
IXa
IX
VIIIa
VIII
XIa
XI
Vessel damage
Exposed collagen
V
Table 12–13
Offi cial Designations for Clotting
Factors, Along with Synonyms More
Commonly Used
Factor I (fi brinogen)
Factor Ia (fi brin)
Factor II (prothrombin)
Factor IIa (thrombin)
Factor III (tissue factor, tissue thromboplastin)
Factor IV (Ca
2+
)
Factors V, VII, VIII, IX, X, XI, XII, and XIII are the inactive
forms of these factors; the active forms add an “a” (e.g., factor
XIIa). There is no factor VI.
Platelet factor (PF)
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