Small Intestine

The mucosa of the small intestine is folded into villi that project into the lumen. In addition, the cells that line these villi have foldings of their plasma membrane called microvilli. This arrangement greatly increases the surface area for absorption. It also improves digestion, since the digestive enzymes of the small intestine are embedded within the plasma membrane of the microvilli.

The small intestine (fig. 18.10) is that portion of the GI tract between the pyloric sphincter of the stomach and the ileo-cecal valve opening into the large intestine. It is called "small" because of its relatively small diameter compared to that of the large intestine. The small intestine is the longest part of the GI tract, however. It is approximately 3 m (12 ft) long in a living person, but it will measure nearly twice this length in a cadaver when the muscle wall is relaxed. The first 20 to 30 cm (10 in.)

Fox: Human Physiology, I 18. The Digestive System I Text I I © The McGraw-Hill

Eighth Edition Companies, 2003

The Digestive System

Duodenum Stomach

Plicae circulares

Duodenum Stomach

Plicae circulares

Ascending colon

Ascending colon

Mesentery

Cecum Appendix

Mucosa-

Circular muscle

Longitudinal muscle

Serosa

■ Figure 18.10 The small intestine. (a) The regions of the small intestine. (b) A section of the intestinal wall showing the tissue layers, plicae circulates, and villi.

extending from the pyloric sphincter is the duodenum. The next two-fifths of the small intestine is the jejunum, and the last three-fifths is the ileum. The ileum empties into the large intestine through the ileocecal valve.

The products of digestion are absorbed across the epithelial lining of intestinal mucosa. Absorption of carbohydrates, lipids, amino acids, calcium, and iron occurs primarily in the duodenum and jejunum. Bile salts, vitamin B12, water, and electrolytes are absorbed primarily in the ileum. Absorption occurs at a rapid rate as a result of extensive foldings of the intestinal mucosa, which greatly increase its absorptive surface area. The mucosa and submucosa form large folds, called plicae circulares, which can be observed with the unaided eye. The surface area is further increased by microscopic folds of mucosa, called villi, and by foldings of the apical plasma membrane of epithelial cells (which can be seen only with an electron microscope), called microvilli.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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