Blood coagulation is a host defense process to
prevent the loss of blood from a high pressure circulatory system.
This system is highly evolved and complex and
includes circulating blood cells such as platelets,
endothelial cells, the wall of blood vessels,
and numerous soluble proteins in the plasma component of blood.
Blood coagulation is initiated by tissue factor,
a membrane protein in the cytokine receptor family.
This protein resides on most extra vascular cells and
can be induced under specific conditions on the surface of vascular cells,
including monocytes and the endothelial cells.
Furthermore, tissue factor is present on the surface of
cell derived microparticles that circulate in the blood.
Blood contains a number of blood clotting proteins that
circulate in their inactive zymogen form.
These proteins are present at low concentration
but play a critical functional role in translating
a signal of tissue injury into a major biological event,
the generation of thrombin and the formation of a fibrin clot.
When tissue factor comes in contact with a small amount of activated FVIIa,
were a connotes the activated enzyme form of the zymogen, the tissue factor.
FVIIa complex is formed.
This tissue FVIIa complex enzymatically converts FIX to its active form, FIXa.
FIXa binds to the active cofactor,
FVIIIa to form the FIXa-
FVIIIa complex on membrane surfaces.
This complex, also known as the Xase complex activates FX to FXa.
The tissue FVIIa complex also enzymatically converts FX to its active form, FXa.
FXa binds to the act of cofactor FVa
to form the FXa FVa complex assembled on a membrane surface.
This complex, also known as the prothrombinase complex,
converts prothrombin to thrombin.
Thrombin then acts on fibrinogen to generate fibrin.
Fibrin monomer polymerizes to form a fibrin clot.
The process of blood coagulation and