1. Digestion in the Stomach Food, once it is swallowed, may remain in the stomach for up to 4 hours. Embedded in the folds of the stomach wall are glands that secrete HCl, pepsin, and mucus. HCl activates pepsin (enzyme), which breaks down proteins. Carbohydrates and fats continue to be mechanically broken down by muscular contractions in the stomach, but are not acted upon chemically.

2. Mucus, secreted by special cells in the stomach, help protect it from gastric juices (combination of HCl and pepsin). If the mucus levels become low, the gastric juices begin to digest the stomach wall, which leads to an ulcer. Eventually, the stomach contents turn into a thick, uniform liquid called chyme.

3. Review 1.How long might food remain in the stomach? 2.List three substances secreted by the folds of the stomach wall. 3.What are the roles of both pepsin and hydrochloric acid? 4.Why doesn’t the HCl digest the stomach wall itself? 5.Describe chyme.