【正确答案】Stomach secretion are composed of four substances: hydrochloric acid, pepsin, mucus and intrinsic factor. (2`) (1) Hydrochloric acid is important because it kills bacteria contained within ingested food, begins the breakdown of proteins, converts the inactive pepsinogen into the active enzyme pepsin, helps Fe++, Ca++ absorption in the small intestine and promotes secretion of pancreatic fluid, bile and Small intestinal juice. (2`) (2) Pepsin breaks down protein to absorb amino acid in the small intestine. (2`) (3) Mucus serves a lubricative role as well as a protective function against the caustic effects of the hydrochloric acid to gastric wall. (2`) (4) Intrinsic factor is required for the absorption of vitamin B12 and is the only stomach secretion that is essential. (2`)