The Digestive System and the Liver - The small intestine
By the time food-turned-chyme gets through the pyloric sphincter, it has already traveled about two-and-a-half feet: about 6 inches from lips to epiglottis; 10 or 12 inches down the esophagus; and about a foot through the stomach. But at this point it has actually traveled less than one-tenth of the gastrointestinal (GI) tract, and the longest stretch lies just ahead: the 20-plus feet of the small intestine, so named because of its relatively small one-to-two inch diameter. The preparation of food particles to pass through the walls of the GI tract is completed in the small intestine—almost completed, in fact, before the chyme has traveled the first foot of the small intestine. By the time it leaves the small intestine, chyme has given up virtually all its nutrients. In other words, the process called absorption or assimilation has taken place: the nutrients have left the GI tract for other parts of the body via the circulating blood and lymph. What passes on to the large intestine is principally waste and water.
The small intestine is somewhat arbitrarily divided into three sections: the duodenum , the jejunum , and the ileum .
Within this horseshoe loop, eight to ten inches long and about two inches in diameter, more chemical interactions are concentrated than in any other section of the GI tract. One of the first jobs in the duodenum is to neutralize the acidity of the chyme. The final steps of digestion, and the absorption of food through the intestinal lining, proceed best in a slightly alkaline environment.
The alkaline juices needed to neutralize the acidity of the chyme come mainly from the liver in the form of bile. Bile produced by the liver but not needed immediately in the duodenum is stored in concentrated form in the gallbladder, a pouchlike, three-inch-long organ. On signal from the autonomic nervous system, the membranous muscular walls of the gallbladder contract, squeezing concentrated, highly alkaline bile into a short duct that leads to the duodenum. Bile components are indispensable for the digestion and absorption of stubborn fatty materials.
Through a duct from the pancreas, a host of pancreatic enzymes, capable of splitting apart large, tough molecules of carbohydrate, protein, and fat, enters the duodenum. These digestive enzymes manufactured by the pancreas are the most powerful in the GI tract.
What triggers the production of bile and pancreatic juice for the duodenum? Apparently, it is a two-step process involving hormones. When the stomach walls secrete hydrochloric acid on the arrival of food, hormones are released; they travel to the liver and pancreas with instructions to step up their production of digestive juices.
Still other strong enzymes are secreted by the walls of the duodenum and join the bile and pancreatic enzymes in the duodenum.
Thus, in the not quite foot-long tube of the duodenum, the final breakdown of food—digestion—reaches a dramatic climax. The nutrients in the food eaten some hours ago have almost all been reduced to molecules small enough to be absorbed through the intestinal walls into the bloodstream. Carbohydrates are reduced to simpler sugars; proteins to amino acids; and fats to fatty acids and glycerol.
Some absorption of these nutrients occurs in the duodenum, but the far greater proportion takes place in the next two, longer sections of the small intestine: the eight- to ten-foot jejunum and the twelve-foot ileum. Likewise, some oversize food molecules that get past the duodenum may be digested further along in their passage through the small intestine.
As peristalsis pushes the nutrient broth out of the duodenum and into the first reaches of the jejunum, a gradual change in the appearance of the intestinal lining is evident. Greater and greater number of villi —microscopic, hairlike structures—sprout from the already bumpy walls of the intestinal lining into the GI tube.
The villi (singular, villus ) have the primary responsibility for absorbing amino acids (from protein), sugars (from carbohydrates), and fatty acids and glycerol (from fats) from the digested contents of the small intestine, and starting them on their way to other parts of the body. What the villi do not remove from the chyme—such as the cellulose fragments of fruits and vegetables—passes on to the large intestine in a thin, watery soup almost completely lacking in nutritional value.
Gland cells near the bottom of a villus secrete various enzymes, mucus, and other substances that perform digestive “mop-up operations” along the whole length of the small intestine.
In this third and final 12-foot section of the small intestine, villi line the walls in such profusion that the intestinal lining resembles, under moderate magnification, nothing so much as a plush, velvety carpet. The greatest numbers of the estimated five or six million villi in the small intestine are found along the lining of the ileum, making it the primary absorption site of the GI tract.
Also adding to the ileum's absorption efficiency is its gradually narrowing diameter (just one inch at its junction with the large intestine), which helps to keep the chyme always in close contact with the swishing villi. The end of the ileum is marked by the ileocecal valve , beyond which lies the first bulge of the large intestine, the cecum .
Principally because of vigorous peristaltic contractions and relaxations, the walls of the small intestine are always moving like the walls within some spasmodically flexing, nightmarish tunnel. Attached to the intestinal walls, the villi, too, are always in restless motion: waving and thrashing, protracting and retracting, even growing thinner or fatter.
Although the entire distance through the small intestine, from the pyloric sphincter to the ileocecal valve at the junction with the large intestine, is only a bit over 20 feet, the villi give the small intestine's internal lining a relatively gigantic surface area—over 100 square feet. This is about five times the surface area of our body's skin. Of course, the greatly enlarged surface area gives the small intestine lining that much more space in which to absorb nutrients.
The small intestine is supported in the abdomen by a fan-shaped web of tissues called the mesentery . Attached at the back of the abdomen, the mesentery connects to the small intestine at various points, and yet allows it some freedom to squirm and sway—much like the V network of ropes that attaches either end of a hammock to a tree. Nerve fibers and blood vessels also reach the small intestine via the mesentery.