Blood Clotting

The clotting process is initiated by a protein called tissue factor, which is bound to the membranes of most cells outside the bloodstream. Blood coming into contact with these membranes sets off a sequence of reactions involving various blood proteins called factors. Thus, a molecule of factor VII coming into contact with tissue factor converts to an active form that in turn converts ten molecules of factor X into thromboplastin, with the help of calcium ions. Thromboplastin in turn binds to factor Va, which then binds to the protein prothrombin to form two million molecules of the clotting enzyme thrombin. Thrombin acts as a catalyst to convert fibrinogen to billions of molecules of the insoluble protein fibrin, the basis of the final clot. The clot is made firm by factor XIII reinforced by blood platelets.

Because minor injuries often occur, the clotting process is almost constantly taking place somewhere in the body. It must be terminated quickly, however or blood would clot everywhere and death would ensue. To prevent this form happening, plasma protein known as fibrinolysins, or plasmins, dissolve old clots in the bloodstream.