Throughout its length, the inner wall of the small intestine has a velvety appearance. This is due to many tiny projections of mucous membrane called intestinal villi (figs. 17.37 and 17.38). These structures are most numerous in the duodenum and the proximal portion of the jejunum. They project into the passageway, or lumen, of the alimentary canal, contacting the intestinal contents. Villi greatly increase the surface area of the intestinal lining, aiding absorption of digestive products.
Each villus consists of a layer of simple columnar epithelium and a core of connective tissue containing blood capillaries, a lymphatic capillary called a lacteal, and nerve fibers. At their free surfaces, the epithelial cells have many fine extensions called microvilli that form a brushlike border and greatly increase the surface area of the intestinal cells, enhancing absorption further (see figs. 17.3 and 17.39). The blood and lymph capillaries carry away absorbed nutrients, and impulses transmitted by the nerve fibers can stimulate or inhibit activities of the villus.
Light micrograph of intestinal villi from the wall of the duodenum (63x).
Between the bases of adjacent villi are tubular intestinal glands (crypts of Lieberkühn) that extend downward into the mucous membrane. The deeper layers of the small intestinal wall are much like those of other parts of the alimentary canal in that they include a sub-mucosa, a muscular layer, and a serosa.
The lining of the small intestine has many circular folds of mucosa, called plicae circulares, that are especially well developed in the lower duodenum and upper jejunum. Together with the villi and microvilli, these folds help increase the surface area of the intestinal lining (fig. 17.40).
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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.