The gastrointestinal (GI) tract is a series of continuous, specialized tubular organs that begin at the mouth and extend to the anus. It is responsible for the digestion and absorption of nutrients from food, as well as the excretion of waste from the body, such as undigested food, excess cholesterol and drug metabolites. Because of its contact with the external environment, the GI tract also has a complex immune system.

The organs of the tract are generally separated by sphincters that control the passage of substances from one section to the next and prevent their retrograde movement. Each organ has anatomical specializations; however, the histology of the GI tract wall is fairly regular from the oesophagus to large intestine, in that it comprises an innermost layer of mucosa, surrounded by a submucosa, muscularis and serosa (intraperitoneal) or adventitia (retroperitoneal) (Figure 1). The most luminal component of the mucosa is an epithelial layer with characteristics appropriate to the organ, for example the abrasion suffered by the oesophagus warrants the presence of a stratified squamous epithelium, while a simple columnar epithelium is more suited to the absorptive functions of the small intestine. The epithelial layer is anchored by a basement membrane, deep to which is the lamina propria; a loose connective tissue layer where small blood and lymphatic vessels, immune cells and nerve endings reside. There is then a thin layer of smooth muscle known as the muscularis mucosae, which regulates the fine movement and folds of the mucosa. The mucosal layer is surrounded by a submucosa, composed of loose connective tissue. This provides support and flexibility to the mucosa and is the site of larger blood and lymphatic vessels and nerves, including the submucosal plexus of the enteric nervous system in most parts of the tract. The muscularis layer is a thicker layer of smooth muscle, usually comprising an inner circular layer and an outer longitudinal layer. These are responsible for motility in the GI tract. Between the two muscularis layers lies the myenteric plexus of the enteric nervous system. Either a serosal or advential connective tissue layer surrounds the outermost surface of the GI tract, depending on whether the organ is intra- or retroperitoneal. This layer helps to anchor the organ in place with allowance for some movement, and provides a conduit for large nerves, and blood and lymphatic vessels. Each part of the tract has specialisms in these four layers to help achieve its functions.