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The stomach usually does not digest the proteins that make up its own cells, because it has three methods of protection. First, the stomach secretes little gastric juice until food is present. Second, some stomach cells secrete mucus, which prevents gastric juice from harming the cells of the stomach lining. Third, the stomach produces its protein-digesting enzyme, pepsin, in a form that remains inactive until hydrochloric acid is present. Once active, pepsin hydrolyzes proteins to yield polypeptides—a fi rst step in protein digestion in the digestive tract. Absorption in the Stomach Very few substances are absorbed from the chyme in the stomach because most substances in the chyme have not yet been broken down suffi ciently. The stomach does absorb some water and salts, however, as well as certain anti- infl ammatory medications such as AspirinTM, and alcohol. (This explains why AspirinTM can irritate the lining of the stomach and why many people feel alcohol’s intoxicating effects so quickly.)The small intestine is small only in terms of its diameter, compared with that of the large intestine. In terms of length, the small intestine is poorly named, because it is more than four times the length of the large intestine. It is, in fact, the longest part of the digestive tract. Some physical digestion occurs in the small intestine as a result of a process called segmentation. During this process, the chyme sloshes back and forth between segments of the small intestine that form when bands of circular muscle briefl y contract. Meanwhile, peristalsis pushes the food along the intestine. The main function of the small intestine is to complete the digestion of macromolecules and to absorb their component subunits. Although both digestion and absorption occur simultaneously throughout the small intestine, these processes and the structures associated with them will be discussed separately to help you understand how and where macromolecules are hydrolyzed so they can be absorbed into the bloodstream.The small intestine can be subdivided into three regions. The fi rst 25 cm of the small intestine is called the duodenum. The duodenum is generally U-shaped and is the shortest and widest of the three regions. Ducts (channels) from the liver and pancreas join to form one duct that enters the duodenum. Thus, the duodenum is an important site for the chemical digestion of the chyme received from the stomach. As you can see in Figure 6.17, the innermost surface of the duodenum, like the rest of the small intestine, is corrugated with circular ridges about 1.3 cm high. The surface of every “hill” and “valley” of these ridges has a velvety appearance due to additional folds—about 6 million tiny fi nger-like projections called villi (see Figure 6.18A). The surface of the villi bristle with thousands of microscopic extensions called microvilli. Because the microvilli give the villi a fuzzy, brush-like appearance in electron photomicrographs, they are often referred to as the “brush border” of the cells that line the intestinal wall.