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Updated: 03/11/08 02:32 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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E. Absorption - General |
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Know your osmolality |
The human intestine absorbs an incredible variety of food molecules, many of which are dissolved in water. The solutions presented to the small bowel from the stomach may be iso-, hyper- or hypotonic compared with plasma. The isotonicity of the luminal contents, the extracellular fluid and the intravascular fluid, are nevertheless, maintained; this is accomplished by bidirectional flux of fluid, electrolytes and more complex molecules across the mucosa. Remember that osmolality depends on the number of particles in solution and that isotonic solutions are about 290 mosmols/kg, the same as plasma. To perform this homeostatic miracle, each portion of the gut must function differently: The stomach secretes HC1 but absorbs minimally. More importantly, the stomach assures slow delivery of the chyme to the small bowel so that the osmotic load is not overwhelming. The NaHCO3 secreted into the lumen by the pancreas and by Brunner's glands help to neutralize gastric acid.
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Regional differences |
The chyme which enters the small bowel is mostly composed of macromolecules such as large polypeptides, polysaccharides and triglycerides. Within the jejunum, each macromolecule may be rapidly digested into many smaller molecules, many of which are active osmotically. To maintain isotonicity, the jejunum must therefore quickly absorb most of these products of digestion. This requires special mechanisms in the jejunum for simultaneous absorption of nutrients, electrolytes and water. By the time the chyme reaches the ileum, there are few nutrients but much electrolyte and water left to be absorbed. Thus, the ileum's main task is to resorb electrolyte solutions which are iso-osmolar with plasma. Both the ileum and colon have the ability to absorb water and electrolytes actively against large concentration gradients. Other specific functions of the ileum are absorption of vitamin B12 and retrieval of most of the bile salts secreted into the duodenal lumen by the liver.
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