A Look at Digestion and Absorption

The basic purpose of digestion and absorption is to deliver essential nutrients to the cells in order to sustain life. To break food down into these essential nutrients, the body sends it through various mechanical and chemical processes in the GI tract, or alimentary canal. Successful digestion and absorption depend on the coordinated function of the GI tract wall’s muscles and nerves, the GI tract organs and the accessory organs of digestion.

GI tract wall structures
The wall of the GI tract consists of four major layers:

* visceral peritoneum
* tunica muscularis
* submucosa
* mucosa

Visceral peritoneum
The visceral peritoneum is the GI tract’s outer covering. It covers most of the abdominal organs and lies next to an identical layer, the parietal peritoneum, which lines the abdominal cavity.

To serve and protect
The main job of this outer layer of the GI tract wall is to protect the blood vessels, nerves and lymphatics. It also attaches the jejunum, ileum and transverse colon to the posterior abdominal wall to prevent twisting.

Many names, one layer
The visceral peritoneum has many names. In the oesophagus and rectum, it’s called the tunica adventitia. Elsewhere, in the GI tract, it’s called the tunica serosa.

Tunica muscularis
The tunica muscularis, which lies within the visceral peritoneum, is a layer composed of skeletal muscle in the mouth, pharynx and upper oesophagus.

Elsewhere in the tract…
The tunica muscularis is made up of longitudinal and circular smooth-muscle fibres. At points along the tract, the circular fi bres thicken to form sphincters.

Pucker pouches
In the large intestine, these fibres gather into three narrow bands (taeniae coli) down the middle of the colon and pucker the intestine into characteristic pouches (haustra). Between the two muscle layers lies a nerve network - the myenteric plexus, also known as Auerbach’s plexus. The stomach wall contains a third muscle layer made up of oblique fibres.

Submucosa
The submucosa, also called the tunica submucosa, lies under the tunica muscularis. It’s composed of loose connective tissue, blood and lymphatic vessels and another nerve network called the submucosal plexus, or Meissner’s plexus.

Mucosa
The mucosa, the innermost layer of the GI tract wall, is also called the tunica mucosa. This layer consists of epithelial and surface cells and loose connective tissue. Villi, from surface cells, secrete gastric and protective juices and absorb nutrients.

GI tract wall functions
The nerves and muscles of the GI tract wall work jointly to ensure that food moves spontaneously through the digestive system (motility). GI tract functions include innervation and secretion.

GI tract innervation
Distention of the submucosal plexus in the submucosa or myenteric plexus in the tunica muscularis stimulates transmission of nerve signals to the smooth muscle, which initiates contraction and relaxation of these muscles, called peristalsis. During peristalsis, longitudinal fi bres of the tunica muscularis shorten the lumen length and circular fibres reduce the lumen diameter.

GI tract secretion
Five major substances secreted by the GI tract contribute to the chemical process of digestion:

* Mucus protects the lining of the GI tract and aids in motility.
* Enzymes are proteins that break down nutrients.
* Acid and various buffer ions contribute to the level of alkalinity or acidity (pH) needed to activate digestive enzymes.
* Electrolytes and water carry nutrients through the GI tract and aid in the absorption process.
* Bile emulsifies fat to promote intestinal absorption of fatty acids, cholesterol and other lipids.

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