Physiology of the Gastrointestinal Tract

The gastrointestinal tract stretches from the mouth to the anus and consists of our main anatomical areas; the esophagus, the stomach, the small intestine and the large intestine or colon.

The wall of the gastrointestinal tract is essentially similar in structure along its length, consisting of our principal histological layers:

  1. The serosa is an outer layer of epithelium with supporting connective tissues which are continuous with the peritoneum.
  2. The muscularis externa, which contains three layers of smooth muscle tissue, a thinner outer layer, which is longitudinal in orientation, and two inner layers, whose fibres are oriented in a circular pattern. Contractions o these muscles provide the forces or movement of gastrointestinal contents and physical breakdown of food.
  3. The submucosa, which is a connective tissue layer containing some secretory tissue and which is richly supplied with blood and lymphatic vessels. A network of nerve cells, known as the submucous plexus, is also located in this layer.
  4. The mucosa, which is essentially composed of three layers: the muscularis mucosa, which can alter the local confirmation of the mucosa, a layer of connective tissue known as the lamina propria and the epithelium.
Figure: Different layers of gastrointestinal tract.

The majority of the gastrointestinal epithelium is covered by a layer of mucus. This is a viscoelastic translucent aqueous gel that is secreted throughout the gastrointestinal tract, acting as a protective layer and a mechanical barrier.

Major Parts of Gastrointestinal Tract


The mouth is the point of entry or most drugs (so-called peroral – via the mouth – administration). At this point contact with the oral mucosa is usually brief. Linking the oral cavity to the stomach is the esophagus. The esophagus is composed of a thick muscular layer approximately 250 mm long and 20 mm in diameter. It joins the stomach at the gastroesophageal junction.


The next part of the gastrointestinal tract to be encountered by both food and pharmaceuticals is the stomach. The two major functions of the stomach are:

  • to act as a temporary reservoir or ingested food and to deliver it to the duodenum at a controlled rate
  • to reduce ingested solids to a uniform creamy consistency, known as chyme, by the action of acid and enzymatic digestion. This enables better contact o the ingested material with the mucous membrane o the intestines and thereby facilitates absorption.

Another, perhaps less obvious, function of the stomach is its protective role in reducing the risk of noxious agents reaching the intestine.

Figure: Anatomy of Stomach

Small intestine

The small intestine is the longest and most convoluted part of the gastrointestinal tract, extending from the pyloric sphincter o the stomach to the ileocecal junction where it joins the large intestine. Its main functions are:

  • Digestion – the process of enzymatic digestion, which began in the stomach, is completed in the small intestine
  • Absorption – the small intestine is the region where most nutrients and other materials are absorbed.

The small intestine is divided into the duodenum, the jejunum, and the ileum. The wall of the small intestine has a rich network of both blood and lymphatic vessels.

The sur ace area of the small intestine is increased enormously, by about 600 times that of a simple cylinder, to approximately 200 m2 in an adult, by several adaptations which make the small intestine such a good absorption site.


The colon is the final major part of the gastrointestinal tract. It stretches from the ileocecal junction to the anus. It is composed of the caecum, the ascending colon, the hepatic flexure, the transverse colon, the splenic flexure, the descending colon, the sigmoid colon and the rectum.

The main functions of the colon are:

  • the absorption of sodium ions, chloride ions and water from the lumen in exchange or bicarbonate and potassium ions. Thus the colon has a significant homeostatic role in the body.
  • the storage and compaction of feces.

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