Digestion and absorption

Digestion and absorption are the ultimate functions of the gastrointestinal tract. Digestion is the chemical breakdown of ingested foods into absorbable molecules. The digestive enzymes are secreted in salivary, gastric, and pancreatic juices as well as the mucosa of the small intestine.

Absorption is the movement of nutrients, water and electrolytes from the lumen of the intestine into the blood system. There are two distinct paths for absorption, a cellular path and a paracellular path. In the cellular path, the substance must cross the gastrointestinal luminal membrane, enter the intestinal epithelial cell, and then be extruded from the cell into the blood. Paracellular absorption involves the movement of substances across the tight junctions between the intestinal epithelial cells, through the lateral intercellular spaces, and then into the blood.

The structure of the intestinal mucosa is ideally suited for absorption of large quantities of nutrients. Structural features called villi and microvilli increase the surface area of the small intestine, maximising the exposure of nutrients to digestive enzymes and to the absorptive surface.

The epithelial cells of the small intestine have the highest turnover rate of any cells in the body. They are replaced every 3-6 days.

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