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.
