Thrombin is a component of some snake venoms and Russell's viper venom has been used to stop bleeding from tooth sockets. In fact, the fibrin is the end product of a long series of these reactions, which is illustrated below solely for your interest. PL stands for phopholipids from platelets. All the factors were originally given names, but some factors were discovered independently by more than one person and were subsequently found to be the same factor, so some factors had several names. To keep the system as simple as possible, the names have been replaced by roman numerals, given in the order in which the factors were discovered, not in the order in which the factors work, so that new factors can be discovered without having to alter the numbers of all the other factors.
Some factors have to be activated and the active form is given a suffix a.
The order in which the factors act was worked out by Macfarlane and the scheme has been likened unto a series of waterfalls, so it is called Macfarlane's cascade. It is not known what starts the intrinsic pathway working. This vitamin is formed by bacteria within our intestines, so an occasional complication of antibiotic treatment is that the bacteria that produce vitamin K are killed and the patients are unable to make the clotting factors and their blood temporarily is unable to clot normally. Defects of the genes responsible for producing the clotting factors can stop blood clotting. The activation of factor VII in the extrinsic pathway is promoted by some of the factors in the intrinsic pathway, so that the extrinsic pathway cannot produce normal blood clotting without a functional intrinsic pathway.