How does the small intestine know how to absorb the good stuff and leave the rest behind to pass along as faeces?

oldstuff28

It absorbs nutrients as well as fats (which also are nutrients.) http://wiki.answers.com/Q/Why_is_the_small_intestine_good_at_its_job Apr 26 12, 11:46 AM

queproblema

The intestine isn't actually smart or discerning; absorption is based on mechanics, we could say--some stuff is able to pass through the intestinal wall and some isn't. Getting through is complicated; nutrients need "transporters," which we could grossly oversimplify and call chemical "passwords." Cut and paste: "The bottom line is that as chyme (the mixture of broken down food and digestive juices) travels through the small intestine, it is exposed to a wide variety of absorption sites, each with very different characteristics. These absorption/receptor sites differ in the number and type of transporter molecules found in the plasma membranes of each individual cell." Also: "The key to the absorption of most nutrients in the small intestine is the electrochemical pump powered by electrolytes (primarily sodium) which works across the epithelial cell boundary of the villi. . ." The page goes on and on, explaining how the various nutrients--carbohydrates, proteins, lipids, vitamins, minerals, and enzymes(?) are absorbed, each in its own way. http://www.jonbarron.org/enzymes/digestive-health-physiology-small-intestine-2 Apr 29 12, 12:41 AM