The HCL begins breaking apart connective tissue of the ingested food and the pepsin begins digesting protein. An ulcer results when HCL penetrates the mucous layer and begins breaking down the stomach lining. If too much acid is secreted in response to stress, this can lead to an ulcer. A bacterium can also lead to ulcers by causing a disintigraiton of the stomach lining. Fibrinogen and and prothrombrin are blood proteins produced by the liver.
Platelets arrive at the site of a rupture of a blood vessel and initially plug the leak.
Platelets release a protein called prothrombrin activator which causes prothrombrin to be converted into thrombrin. Thrombrin is an enzyme that chops off some amino acids from fibrinogen and turns it into fibrin. Fibrin fragments join into a mesh that traps platelets and red blood cells which forms a clot. The clot seals off the ruptured area until it can repair itself. Then the clot is dissolved by another protein called plasmin.