Coagulation is a commonly physiological and pathological phenomenon. The occurrence of coagulation is a cascade of chemical reactions. Firstly

the coagulation factors generate the activator of prothrombin under the trigger of internal or external stimuli

so that the prothrombin is converted into thrombin. Secondly

the polypeptide chain is removed from fibrinogen with the activation of thrombin and fibrinogen is converted into the monomers of fibrin

which are polymerized to form the polymer of fibrin. Thirdly

fibrin combines with platelets

red blood cells

and endothelial cells in the blood and finally they form a network of blood clot. In this study

the molecular reactions and the structural changes of thrombin and fibrin in the coagulation have been analyzed from the perspective of macromolecular chemistry and physics.