36- Coagulation series part 2 of 7

KYJ Series part 2 on Coagulation Factors.

In series 1, we reviewed the initiation of the Tissue Factor (extrinsic) pathway of coagulation. It used to be called extrinsic meaning external stimuli, because the cascade is initiated when Tissue Factor (Factor III) is exposed through vessel wall injury.

This session carries on with where we left off.  Factor VII .

Factor VII circulates in the blood in an inactive form. It is manufactured by the liver using Vitamin K, a fat soluble nutrient found in leafy Green vegetables.

FVII is activated by tissue factor and vonWillebrand factor released by the endothelium and damaged vessel walls.  Here it becomes factor VIIa.

Now this activated FVIIa causes a conversion of the first factor of the Common Pathway.  So called because this is where the Tissue factor (extrinsic) initiation of coagulation converges with the contact activation (intrinsic pathway (another post)).

This first factor of the Common pathway is :
Factor X (Stuart-Prower factor), named after two people who suffered a form of factor X deficiency in the 20s and 30s.

Like FVII, Factor X is a liver synthesised protein enzyme also manufactured from Vit K.  It circulates in blood doing nothing until it comes into contact with activated FVII (FVIIa) , then it activates to form Factor Xa.

This activation now stimulates the conversion of prothrombin (Factor II) into Thrombin, it's activated form (FIIa).

Let's summarise :
Injury to vessel
FIII exposed to plasma
FVII activates
FVIIa converts FX to FXa
FXa activates FII (prothrombin)
Into Factor IIa (Thrombin).

Perhaps you wondered how Warfarin (Coumadin) works to
Inhibit coagulation.
Well this drug and others like it inhibit the Vit K initiated synthesis of factors VII, X and Prothrombin (FII).

We are almost there with this arm of the coagulation cascade and the Extrinsic pathway factors.
Stay tuned for our next edition of KnowingYourJargon .