THE SMALL INTESTINE

The intestines, small and large combined, form a continuous tube from stomach to anus. The small intestine, so called because of its lesser diameter, plays important roles in both digestion and absorption. It is about 20 feet long, roughly four times as long as the large intestine, and resembles a coiled tube about one inch in diameter. The first part of the small intestine, the duodenum, begins at the pylorus of the stomach and extends about 12 inches, making several bends in the upper region of the abdominal cavity.

The duodenum joins the jejunum, a second region extending about 8 feet and less coiled than the other regions. The final, much coiled, region of the small intestine is the 12-foot-Iong ileum. As enzyme enters the duodenum, more digestive juices are added, not only by the intestine itself but also by the pancreas and the liver. 

Several enzymes in pancreatic fluid act upon all types of food-breaking down proteins into their amino acid constituents, converting large and complex sugar molecules into simple sugars, and changing fats to fatty acids. (Insulin is also secreted by the pancreas, but it goes directly into the bloodstream rather than into the intestine, where it would be destroyed by digestive juices; this is the reason why insulin, when required by diabetics, cannot be taken by mouth but must be injected.)

A discussion of oral drugs (not insulin) for use in the treatment of diabetes can be found on page 569. Bile enters the duodenum either directly from the liver where it is produced or from the gallbladder where it is stored. It serves to emulsify fatty foods, so they become easier to absorb, and to reduce the acidity of the chyme. Bile also functions as a carrier for the pigments of red blood cells that have been destroyed. 

These pigments, which undergo chemical changes in the intestinal tract, give the characteristic yellow-brown color to feces. While a limited amount of actual absorption of digested materials occurs in the stomach, it is in the small intestine that the bulk of absorption takes place. For this purpose, the walls of the small intestine are lined with tiny, fingerlike projections, the villi. Millions of villi give the mucous membrane lining the small intestine a velvety appearance. In constant motion, the villi keep the chyme mixed with digestive juices while carrying on absorption through two types of vessels.

Tiny, branching capillaries, smallest of blood vessels, form a network in each villus to absorb sugars and amino acids

 Other vessels, lacteals, contain lymph, a fluid present in the spaces between body cells. Fat products generally are absorbed by the lacteals. Indigestible food remains in the intestine.