Secretions of the Small Intestine

In addition to the mucus-secreting goblet cells, which are abundant throughout the mucosa of the small intestine, many specialized mucus-secreting glands (Brunner's glands) are in the submucosa within the proximal portion of the duodenum. These glands secrete large quantities of viscid, alkaline mucus in response to certain stimuli.

The epithelial cells that form the lining of the small intestine are continually replaced. New cells form within the intestinal glands by mitosis and migrate outward onto the villus surface. When the migrating cells reach the tip of the villus, they are shed. This cellular turnover renews the small intestine's epithelial lining every three to six days.

The intestinal glands at the bases of the villi secrete a great amount of watery fluid (see fig. 17.37). The villi rapidly reabsorb this fluid, which carries digestive

Microvilli

Cell membrane

Mitochondrion

Microvilli

Cell membrane

Mitochondrion

Golgi apparatus

Nucleus

Epithelial Cell Small Intestine

Nucleolus

Golgi apparatus

Nucleus

Rough endoplasmic reticulum

Nucleolus

Cytoplasm of epithelial cell Microvilli

Lumen

Cytoplasm of epithelial cell Microvilli

Lumen

Villa And Microvilli Secretions

Figure 17.39

(a) Microvilli increase the surface area of intestinal epithelial cells.

(b) Transmission electron micrograph of microvilli (16,000x).

Figure 17.39

(a) Microvilli increase the surface area of intestinal epithelial cells.

(b) Transmission electron micrograph of microvilli (16,000x).

Villi

Plicae circulares

Villi

Intestine Longitudinal

Plicae circulares

Figure 17.40

(a) The inner lining of the small intestine contains many circular folds, the plicae circulares. (b) A longitudinal section through some of these folds.

Submucosa

-Muscular layer

Circular muscle Longitudinal muscle _ _ Serosa

Figure 17.40

(a) The inner lining of the small intestine contains many circular folds, the plicae circulares. (b) A longitudinal section through some of these folds.

products into the villi. The fluid the intestinal glands secrete has a pH that is nearly neutral (6.5-7.5), and it lacks digestive enzymes. However, the epithelial cells of the intestinal mucosa have digestive enzymes embedded in the membranes of the microvilli on their luminal surfaces. These enzymes break down food molecules just before absorption takes place. They include peptidases, which split peptides into their constituent amino acids; sucrase, maltase, and lactase, which split the double sugars (disaccharides) sucrose, maltose, and lactose into the simple sugars (monosaccharides) glucose, fructose, and galactose; and intestinal lipase, which splits fats into fatty acids and glycerol. Table 17.9 summarizes the sources and actions of the major digestive enzymes.

Many adults do not produce sufficient lactase to adequately digest lactose, or milk sugar. In this lactose intolerance, lactose remains undigested, increasing osmotic pressure of the intestinal contents and drawing water into the intestines. At the same time, intestinal bacteria metabolize undigested sugar, producing organic acids and gases. The overall result of lactose intolerance is bloating, intestinal cramps, and diarrhea. To avoid these unpleasant symptoms, people with lactose intolerance can take lactase in pill form before eating dairy products. Infants with lactose intolerance can drink formula based on soybeans rather than milk.

Shier-Butler-Lewis: Human Anatomy and Physiology, Ninth Edition

V. Absorption and Excretion

17. Digestive System

© The McGraw-Hill Companies, 2001

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Responses

  • david
    Which cell in the small intestine's mucosa is a mucussecreting cell goblet?
    5 years ago
  • KIM ROSS
    Which cells in the small intestine's mucosa secrete mucus?
    1 year ago

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