Clotting factors are proteins found in the plasma, which are essential for the clotting cascade necessary to form the cross‐linked fibrin network. One of these clotting factors is ionized calcium (Factor IV); the only factor which is not a protein.
Coagulation of blood involves two vital steps:
Conversion of prothrombin to thrombin.
Conversion of fibrinogen to fibrin by thrombin.
In the beginning, when a blood vessel is injured, Factor XII (Hageman's factor) is activated and in a cascade fashion, it forms a prothrombin activator. The prothrombin activator is also released from Factor VII (proconvertin) in the extrinsic pathway. This activator in the presence of Ca2+ converts prothrombin to thrombin. Thrombin and Factor XIII (fibrin stabilizing factor) form the fibrin (mesh-like) network, in which RBCs get entangled and a clot is formed.