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(b) Some fatty acids with relatively short carbon chains are transported without being changed back into fats.

Blood in capillaries

Electrolytes

Diffusion and active transport

Blood in capillaries

Water

Osmosis

Blood in capillaries

Electrolytes are usually absorbed by active transport, and water by osmosis. Thus, even though the intestinal contents may be hypertonic to the epithelial cells at first, as nutrients and electrolytes are absorbed, they become slightly hypotonic to the cells. Then, water follows the nutrients and electrolytes into the villi by osmosis. The absorption process is summarized in table 17.10.

In malabsorption, the small intestine digests, but does not absorb, some nutrients. Causes of malabsorption include surgical removal of a portion of the small intestine (see chapter opener), obstruction of lym phatic vessels due to a tumor, or interference with the production and release of bile as a result of liver disease.

Another cause of malabsorption is a reaction to gluten, found in certain grains, especially wheat and rye. This condition is called celiac disease. Microvilli are damaged and, in severe cases, villi may be destroyed. Both of these effects reduce the absorptive surface of the small intestine, preventing absorption of some nutrients. Symptoms of malabsorption include diarrhea, weight loss, weakness, vitamin deficiencies, anemia, and bone demineralization.

U Which substances resulting from digestion of carbohydrate, protein, and fat molecules does the small intestine absorb?

Which ions does the small intestine absorb?

What transport mechanisms do intestinal villi use?

Describe how fatty acids are absorbed and transported.

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