Overview

The small intestine, is the longest segment of the tubular digestive tract, measuring about 6 meters (20 feet) in length. It consists of three regions, duodenum (~25 cm), jejunum (~2.5 m) and ileum (~3.5 m) and functions in the completion of digestion and absorption of metabolites. Like the other regions of the digestive tract in general, the small intestine is composed of a mucosa, submucosa, muscularis externa and usually a serosa. Jejunum, 10x

The mucosa is the innermost layer of the small intestine and consists of three layers: a simple columnar epithelium with microvilli and goblet cells; a lamina propria of loose connective tissue in which intestinal glands are located; and a thin muscularis mucosae of smooth muscle.

Villi are finger-like projections from the surface of the small intestine into the lumen of the organ. Villi are composed of a central core of lamina propria covered by the intestinal epithelium. The presence of villi are diagnostic for the small intestine.

The submucosa consists of dense, irregular connective tissue containing larger blood vessels, nerves and lymphatics.

Plicae circulares are circular folds extending around the inner circumference of the small intestine. The central core of each plica, formed by the submucosa, pushes up all the overlying layers. Therefore, the entire mucosa (including villi and glands) overlying the submucosal core is included in each plica.

The muscularis externa of the small intestine is composed of inner circular and outer longitudinal layers of smooth muscle, similar to other regions of the digestive tract.

The outermost layer of the small intestine may be either an adventitia (in the part of the duodenum that is retroperitoneal) or a serosa, in the remainder of the duodenum and in the jejunum and ileum (shown here).

The mesentery suspends most of the small intestine from the dorsal body wall. It is composed of two serosal layers and an intervening layer of loose connective tissue.

Area shown in next image.