Digestion in the Small Intestine As chyme is pushed through the pyloric valve, it enters the duodenum (doo oh DEE num). The duodenum is the first part of the small intestine, and it is where almost all of the digestive enzymes enter the intestine. Most of the chemical digestion and absorption of the food you eat occurs in the small intestine. As chyme enters the duodenum from the stomach, it mixes with enzymes and digestive fluids from the pancreas, the liver, and even the lining of the duodenum itself. The pancreas and liver are shown in Figure 30–15.

▸ Pancreas Just behind the stomach is the pancreas, a gland that serves three important functions. One function is to produce hormones that regulate blood sugar levels. Within the digestive system, the pancreas has two other roles. It produces enzymes that break down carbohydrates, proteins, lipids, and nucleic acids. The pancreas also produces sodium bicarbonate, a base that quickly neutralizes stomach acid as chyme enters the duodenum. The enzymes produced by the pancreas, unlike those produced in the stomach, would be destroyed by strong acid, and therefore the sodium bicarbonate is necessary for digestion to proceed.

▸ The Liver and Gallbladder Assisting the pancreas in fat digestion is the liver. The liver produces bile, a fluid loaded with lipids and salts. Bile is stored in a small, pouchlike organ called the gallbladder. When fat is present in the duodenum, the gallbladder releases bile through a duct into the small intestine. Fats tend to glob together, which makes fat digestion by enzymes such as lipase difficult. Bile breaks up the globs of fat into smaller droplets that disperse in the watery environment of the small intestine. This action makes it possible for enzymes to reach the smaller fat droplets and break them down.

In Your Notebook Summarize the two roles of the pancreas in fat digestion.

FIGURE 30–14 Effects of Digestive Enzymes Digestive enzymes hasten the breakdown of foods and make nutrients available to the body. Interpret Tables Where in the body does the digestion of carbohydrates begin?

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