In Vitro and in Vivo Hemostasis

In vitro (test tube):

• The clotting cascade in plasma is activated by the addition of a negative charge, which activates the intrinsic pathway (aPTT), or tissue factor (TF), which activates the extrinsic pathway (PT) in the presence of Ca²⁺ and phospholipid (PL).
• Once the ingredients are added together, the “stop watch” is started and time to clot formation at 37^oC is measured (the assays do not discriminate between crosslinked and uncrosslinked fibrin).

In vivo (the body):

• The clotting cascade is always activated by TF-mediated activation of factor VII (FVII). TF ‘hides’ in the subendothelial layer of the blood vessel wall (where it becomes exposed to blood upon injury to the endothelial lining), and is expressed on the surface of activated monocytes.
• Activated FVII (FVIIa) not only activates FX of the common pathway, but also activates FIX of the intrinsic pathway (cross-talk).
• Thrombin activates FIX and FXI of the intrinsic pathway (feedback).
• FXII plays no role in in vivo coagulation!

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