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. • 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.
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