Secondary coagulation is comprised of plasma proteins commonly called “coagulation factors.” There are 13 coagulation factors, designated by Roman numerals I-XIII. The numbering does NOT correspond to the order in which they are involved in the coagulation cascade, so do not be led to believe that factor I leads to factor II and so on. Coagulation factors that are actively involved in clotting are written with an “a” after the Roman numeral (eg, factor Ia for fibrin), while coagulation factors that are circulating inactive in the vasculature are written without an “a” and may have a different name (eg, factor I for fibrinogen).
While primary coagulation may seem rather straightforward, secondary coagulation is often viewed as more complex. To help “keep your eye on the prize,” keep the final goal of secondary coagulation in mind: The formation of fibrin (factor Ia) from the activation of fibrinogen (factor I), with fibrin functioning to reinforce the platelet plug at the site of bleeding.