This question is asking about the role of HCl in the digestive system. HCl is secreted by the stomach as a component of gastric juice. It accounts for the low (acidic) pH of the stomach contents and helps begin protein digestion and protect the body from harmful bacteria. HCl converts pepsinogen to pepsin, not trypsinogen to trypsin; trypsinogen is converted in the small intestine. Pancreatic bicarbonate neutralizes the acidic contents of the stomach, and this also occurs in the small intestine. Mucus secreted by goblet cells in the stomach protects the walls of the stomach from digestive activity.