Thromboplastin reacts with calcium and another substance,
prothrombin, to form thrombin. Vitamin K is necessary for the proper
formation of prothrombin.
The thrombin formed acts as an enzyme to convert fibrinogen to
fibrin threads that eventually form the blood clot.
NOTE: For a more in-depth discussion of blood clotting you should locate and read a
physiology text that is appropriate to your level of understanding.
Figure 5-1. The blood clotting process.
Clot breakdown. Plasminogen binds to fibrin as the clot forms. In
response to thrombin formation and venous stasis (clot), plasminogen
activators convert plasminogen to plasmin. Plasmin digests fibrin and
dissolves the clot.
5-4. COAGULANTS (PROMOTING CLOT FORMATION)
There are several drugs that affect the clotting process at different stages to
promote coagulation. Vitamin K derivatives and coagulation factors work by enhancing
the formation or increasing the amount of circulating clotting factors and promoting the
coagulation process (steps 2 and 3 above). Drugs that inhibit plasminogen or plasmin
result in coagulation by preventing the breakdown of clots (step 4 above).
a. Phytonadione (Mephyton, Aqua-Mephyton, Vitamin K1). Phytonadione
or vitamin K is the most commonly prescribed coagulant and antidote for warfarin
overdose. As a coagulant, the usual dose is 0.5 to 1.0 milligram given intramuscularly