The gastrointestinal tract

The gastrointestinal tract is shown in Figure 4.1. The major functions of each region are:

Mouth

  • starch hydrolysis catalysed by amylase, secreted by the salivary glands;
  • fat hydrolysis catalysed by lingual lipase, secreted by the tongue;
  • absorption of small amounts of vitamin C and a variety of non-nutrients (including nicotine).

Stomach

  • denaturation of dietary proteins (section 4.4.2) and the release of vitamin B, iron and other minerals from protein binding, for which gastric acid is important;
  • protein hydrolysis catalysed by pepsin;
  • fat hydrolysis catalysed by lipase.
  • secretion of intrinsic factor, required for the absorption of vitamin B (section 11.10.1).

Small intestine (duodenum, jejunum and ileum)

  • starch hydrolysis catalysed by amylase secreted by the pancreas;
  • hydrolysis of disaccharides within the brush border of the intestinal mucosa;
  • fat hydrolysis catalysed by lipase secreted by the pancreas;
  • protein hydrolysis catalysed by a variety of exo- and endopeptidases (section 4.4.3) secreted by the pancreas and small intestinal mucosa;
  • hydrolysis of di- and tripeptides within the brush border of the intestinal mucosa;
  • absorption of the products of digestion;
  • absorption of water (failure of water absorption, as in diarrhoea, can lead to serious dehydration).
Water Digestion And Absorption
  • Large intestine (caecum and colon)
  • bacterial metabolism of undigested carbohydrates and shed intestinal mucosal cells;
  • absorption of some of the products of bacterial metabolism;
  • absorption of water.
  • Rectum
  • storage of undigested gut contents prior to evacuation as faeces;

Throughout the gastrointestinal tract, and especially in the small intestine, the surface area of the mucosa is considerably greater than would appear from its superficial appearance. As shown in the inset in Figure 4.1, the intestinal mucosa is folded longitudinally into the lumen. The surface of these folds is covered with villi: finger like projections into the lumen, some 0.5—1.5 mm long. There are some 20— 40 villi per mm2, giving a total absorptive surface area of some 300 m2 in the small intestine.

As shown in Figure 4.2, each villus has both blood capillaries, which drain into the hepatic portal vein, and a lacteal, which drains into the lymphatic system. Water-soluble products of digestion (carbohydrates and amino acids) are absorbed into the blood capillaries, and the liver has a major role in controlling the availability of the products of carbohydrate and protein digestion to other tissues in the body. As discussed in section 4.3.3.2, lipids are absorbed into the lacteals; the lymphatic system joins the bloodstream at the thoracic duct, and extrahepatic tissues are exposed to the products of lipid digestion uncontrolled by the liver, which functions to clear the remnants from the circulation.

There is rapid turnover of the cells of the intestinal mucosa; epithelial cells proliferate in the crypts, alongside the cells that secrete digestive enzymes, and migrate to the tip of the villus, where they are shed into the lumen. The average life of an intestinal mucosal epithelial cell is about 48 hours. As discussed in section 4.5, this rapid turnover of epithelial cells is important in controlling the absorption of iron, and possible other minerals.

The rapid turnover of intestinal mucosal cells is also important for protection of the intestine against the digestive enzymes secreted into the lumen. Further protection is afforded by the secretion of mucus, a solution of proteins that are resistant to enzymic

Intestinal Crypt Blood Supply

absorptive enterocyte mucus secreting goblet cell cells shed at tip of villus absorptive enterocyte mucus secreting goblet cell cell proliferation in crypt lymphatic drainage venous drainage arterial blood supply

Figure 4.2 An intestinal villus.

hydrolysis and which coats the intestinal mucosa. The secretion of intestinal mucus explains a considerable part of an adult's continuing requirement for dietary protein (section 9.1.2).

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Responses

  • jorma
    What hydrolyses nutrients in the gi tract?
    5 years ago

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