Dr. Luis Agote (2nd from right) overseeing one of the first safe and effective blood transfusion in 1914 Direct transfusions were still not practiced at the beginning of the 20th century because it was impossible to keep unaltered blood outside the body for later use. After 6–12 minutes, coagulation begins manifested initially by a gradual increase of viscosity that terminates with almost complete solidification. Coagulation is the defense of an organism to staunch wounds and minimize hemorrhages. Now we know that clotting is almost totally formed by platelets fastened by a network of filaments of fibrin. Fibrin do not normally exist in blood and are created starting from protein plasma by the action of the thrombin enzyme. Similarly, thrombin are not naturally present in blood and are created by the precursor substance prothombrin, in a process that involves platelets, some exiting from calcium and substances produced by lesioned materials. Since clots are not created if there is a lack of some of these elements, the addition of sodium citrate (which eliminates calcium ions from blood) prevents its formation.