The gastrointestinal tract is shown in Figure 4.1. The major functions of each region are:
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 18.104.22.168, 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
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
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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