Chapter 14. GI Tract

A. Introduction

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The human digestive tract is a true and complete digestive tract. A true digestive tract is defined as one in which digestion is extracellular, occurring within the tract's lumen or on the surface of its lining. A complete digestive tract is defined as one that is a tubular, compartmentalized organ, with a lumen that lies outside of the body and with an entrance (a mouth) that is separate from its exit (the anus).

The human digestive tract has five major compartments: the esophagus, the stomach, the small intestine, the large intestine, and the rectum.

The wall of the digestive has the same basic structure throughout its length. It consists of four basic tissue layers. Listed from the lining epithelium to the covering connective tissue, they are

  • the mucosa,
  • submucosa,
  • muscularis externae,
  • the serosa or adventita.

The mucosa is the innermost major tissue layer. It contains the following sublayers:

  • the lining epithelium,
  • mucosal glands (in most parts of the alimentary canal)
  • the lamina propria, which consists of a loose connective tissue supporting the epithelium,
  • muscularis mucosa, which is a double-layer of smooth muscle surrounding the lamina propria. The inner layer is a layer of circularly-arranged smooth muscle cells, and the outer layer is a layer of longitudinally-arranged smooth muscle cells.

The submucosa is a relatively thick layer of dense irregular connective tissue. It not only binds and supports the other layers of the canal wall, but also supports vascular, lymphatic, and nerve supplies of the wall. In most parts of the digestive tract, the submucosa folds to create large submucosal folds taht can be seen from the lumen with the unaided eye.

The muscularis externae consists of multiple layers of muscle. Contractions of the muscularis externae provide the movements that mix the luminal contents of the canal and move the contents along its length. Peristalsis, the common waves of contraction that move lumenal contents, come from the coordinated contractions and relaxations of the muscularis externae.

The serosa is the outermost tissue layer of those portions of the alimentary canal that are suspended in the peritoneal cavity by mesentery. It is the visceral peritoneum of gross anatomy. It is continuous with mesentery and consists of a thin layer of loose connective tissue covered by mesothelium. The loose connective tissue layer supports large blood vessels, lymphatic vessels, and contains variable numbers of adipocytes.

Regions of the digestive tract that are not suspended by mesentery, but rather are attached directly to the body wall, are surrounded by an adventitia rather than a serosa. These regions include the esophagus, which is actually physically attached to the aorta, the duodenum, the ascending colon, and the descending colon. The adventitia is a thicker layer of loose connective tissue that is continuous with the loose connective tissue of other structures.