Upper Gastro-intestinal System

Structure and function of the GI tract

Overview

This section describes the general structure and function of the GI tract wall, as well as the anatomy and function of the upper GI regions – the mouth, oesophagus, stomach, and duodenum.

The function of the GI tract

The function of the GI tract is to carry out the digestive processes within the body. Large food molecules are metabolised into small, soluble molecules that can be absorbed into the blood stream and lymphatic system and incorporated into cells. Digestion occurs both mechanically by physical means, such as chewing, peristalsis and churning movements of the stomach and small intestine, and chemically through enzyme-mediated metabolic reactions.

General structure of the GI tract

The GI tract or alimentary canal forms a continuous ‘tube’ from the mouth to the anus, therefore, food is not actually inside the body until it has been metabolised and absorbed into the bloodstream. The wall of the GI tract is permeable to digested food molecules but impermeable to some potentially harmful organisms and other foreign particles, which remain outside the body.

The GI tract wall comprises four basic tissues surrounding the lumen:

General structure of the GI tract
FIGURE 1 – General structure
of the GI tract (click to enlarge)
  • Inner mucosa (secretion, absorption, protection)
  • Submucosa (support, blood supply, nerves controlling secretion)
  • Muscle layer (circular constriction, longitudinal contraction, nerves controlling motility)
  • Outer serosa (secretes lubricating fluid)

The inner mucosa

The mucosa is the innermost layer of the digestive tract, and its surface comes into direct contact with food particles in the GI tract. It is responsible for absorption of digested food. The mucosa lubricates solid contents to facilitate their movement along the digestive tract, and it contains glands and cells that produce digestive juices and mucus. It provides a degree of protection to the outer layers against harmful substances and pathogens present in the gut lumen. Some muscle fibres (the muscularis mucosa) are also present.

The submucosa

The submucosa lies under and around the mucosal layer. It consists of tough, fibrous tissues and acts as a supporting structure for the GI tract. The submucosa carries the major blood vessels into which digested food molecules are absorbed. It also contains the main lymph vessels of the GI tract, and the Meissner's plexus – a network of nerves that control digestive and hormonal secretions.

The muscle layer

The muscle layer is composed of two layers of smooth muscle – an inner layer of circular muscle fibres (which narrow the lumen when contracted) and an outer layer of longitudinal muscle fibres (which shorten and widen the lumen). The stomach has an additional internal layer of oblique muscles. Contractions of these muscles help to mechanically break down and mix food with digestive juices, and move food along the GI tract using peristalsis.

The major nerve supply of the GI tract, known as the myenteric or Auerbach’s plexus, is also contained in the muscle layer. It makes sensory connections through the layers, as well as along the length of the GI tract to control motility.

The outer serosa

The serosa, the outermost layer, is only found covering the abdominal digestive organs, and is an extension of the peritoneum, which lines the wall of the peritoneal cavity. In areas of the GI tract that lack epithelium, for example the oesophagus, this layer is known as the adventitia. The peritoneum itself is a membrane that holds the organs of the GI tract in place in the abdomen and encloses the vessels associated with absorption from the stomach and intestine. The serosa secretes a watery lubricant that allows parts of the gut to move smoothly over each other without friction.

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