Coagulation Theories Hematology Review 350
CASCADE MODEL* CELL-BASED OR PHYSIOLOGICAL MODEL
Overview
Steps
Focuses on role of coagulation factors. Sees coagulation
as chain rxn in which each coag factor is converted to
active form by preceding factor. Intrinsic & extrinsic
pathways converge on common pathway.
Extrinsic pathway (TF, factor VII):
- TF from injured blood vessel wall activates factor VII.
- TF:VIIa activate factor X.
Intrinsic pathway (factors XII, XI, IX, VIII):
- Factor XII activated by exposure to collagen.
- Factor XIIa, HMWK, & PK activate factor XI.
- Factor XIa activates factor IX.
- IXa:VIIIa activates factor X.
Focuses on role of receptors for coagulation factors on surface
of tissue factor (TF)–bearing cells (e.g., fibroblast or monocyte)
& platelets. Sees coagulation as 3 overlapping phases that
begin with small amount of thrombin formation on surface of
TF-bearing cells, followed by large-scale thrombin production
on platelet surface.
Initiation (on surface of TF-bearing cell):
- Break in vessel wall exposes extravascular TF-bearing cell to
plasma.
- Factor VII binds to TF on cell membrane.
- TF:VIIa activates factors IX & X.
- Factor Xa combines with factor Va.
- Xa:Va generates small amount of thrombin, but no fibrin
formed at this point.
Amplification:
- Thrombin & collagen activate platelets.
- Platelets release factor V from granules.
- Thrombin activates factors V, VIII, & XI.
- Factor XIa supplements activation of factor IX.
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