Food assimilation takes place primarily in the small intestine and it is optimized by the increased surface area produced by Kerkring's folds, villi and microvilli. The chyme presented to the duodenum from the stomach consists of a mixture of coarsely emulsified fat, protein and some metabolites produced by the action of pepsin, and carbohydrates including starch, the majority of which would have escaped the action of the salivary amylase.
The chyme is acidic and this is buffered by bile and the bicarbonate present in the pancreatic juice to between pH 6.5 and 7.6. The digestive enzymes are located in the brush border of the glycocalyx and they can be altered by changes in diet, especially by the proportion of ingested disaccharides. The protein content of the diet does not affect the proteases, but a diet deficient in protein leads to a reduction in all enzymes.
It is important to recognize that the epithelium of the gut is not a monotonous sheet of functionally identical cells. As chyme travels through the intestine, it is sequentially exposed to regions having epithelia with very different characteristics. This diversity in function results from differences in the number and type of transporter molecules expressed in the epithelial plasma membrane, and the structure of the tight junctions. Even within a given segment there are major differences in the type of transport that occurs, for example, cells in the crypts have different transporter systems than cells on the tips of villi.
Blood passing through the minute veins of the capillaries is brought into close proximity with the intestinal contents in an area estimated to be about 10m2. The capillaries are fenestrated, hence allowing a very rapid exchange of absorbed materials. During digestion and absorption the villi contract fairly quickly at regular intervals and relax slowly. The contraction probably serves to pump lymph into the lacteals of the submucosa and stir the intestinal contents. The veins in the villi ultimately open into the portal vein, which leads directly to the liver and hence all materials carried from the small intestine undergo "firstpass" metabolism.
The site of absorption of the small intestine depends upon the relationship between the rate of transit to that of absorption. This is more apparent for drugs than for food, since excipients may control the rate of drug release. For example the duodenum can be demonstrated to have a high rate of absorption, however the passage through this region is extremely rapid and so the net absorption in this region is probably quite low. The function of the duodenum is to sample the chyme which is delivered from the stomach and thus regulate the delivery of the food according to its calorific value by a feedback process. Virtually all nutrients from the diet are absorbed into blood across the mucosa of the small intestine. The absorption of water and electrolytes plays a critical role in maintenance of body water and acid-base balance.
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