540
UNIT FOUR
stored in a glass container. Activated factor XII activates fac-
tor XI, which activates factor IX. Factor IX then joins with
factor VIII and platelet phospholipids to activate factor X.
These reactions, which also require calcium ions, lead to the
production of prothrombin activator. The subsequent steps
of blood clot formation are the same as those described for
the extrinsic mechanism
(f
g. 14.19)
.
Table 14.8
compares
Intrinsic Clotting Mechanism
Unlike extrinsic clotting, all of the components necessary for
intrinsic clotting are in the blood. Activation of a substance
called the
Hageman factor
(factor XII) initiates intrinsic clot-
ting. This happens when blood is exposed to a foreign surface
such as collagen in connective tissue instead of the smooth
endothelial lining of intact blood vessels or when blood is
Extrinsic Clotting
Mechanism
Tissue damage
Releases
Factor Vll
Factor X
(Ca
+2
)
(Ca
+2
)
Activates
(Ca
+2
)
Converts
Converts
(Ca
+2
)
(Ca
+2
)
Activates
Activates
Factor V
Tissue thromboplastin
(Factor lll)
Prothrombin
activator
Fibrin
Fibrin
clot
Intrinsic Clotting
Mechanism
Blood contacts
foreign surface
Factor lX
Factor Xl
Factor X
Factor Vlll
platelet phospholipids
Prothrombin
(Factor ll)
Thrombin
(Factor lla)
Fibrinogen
(Factor l)
Activates
Activates
Activates
Activates
Activates
Hageman Factor Xll
Factor V
Stabilizes
Factor Xlll
FIGURE 14.19
Schematic of blood clotting mechanisms.
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