Thromboplastin is released from damaged tissue and platelets, which activates an enzyme that catalyses the conversation of the protein prothrombin to thrombin. This occurs in the presence of Calcium and Vitamin K. Thrombin is an enzyme that then catalyses the conversion of the soluble plasma protein, fibrinogen, into the insoluble protein, fibrin. The fibrin forms a mesh that traps more platelets and red blood cells to form a clot.