(1) Mechanical digestion. Remember that the large intestine is composed
of the cecum, the colon, and the rectum. After an individual eats a meal, the peristaltic
wave-like motions in the ileum (the lower part of the small intestine) become stronger
forcing chyme from the ileum into the cecum (the first part of the large intestine).
Chyme continues its journey filling the cecum and then moving into the colon. In the
colon, mass peristalsis, a stronger peristaltic wave, drives the contents of the colon into
the rectum. When the rectum is full, pressure receptors in its walls activate the
defecation reflex which causes the contents of the rectum to be eliminated.
(2) Chemical digestion. Bacterial action rather than the action of enzymes
completes the last stage of digestion, a stage which is completed in the large intestine.
Glands in the large intestine secrete mucus, and bacteria prepare the undigested
remainder of chyme for eventual elimination. Bacteria causes any remaining
carbohydrates to ferment releasing hydrogen, carbon dioxide, and methane gas in the
fermentation process. These gases contribute to gas in the colon. Bacteria also act on
any proteins which remain changing them to amino acids and breaking down amino
acids into simpler substances. Some of these simpler substances are carried off in the
feces and contribute to fecal odor. The brown color of feces is the result of bacteria
decomposing bilirubin (an orange pigment) to a simpler pigment. Several vitamins
necessary for normal metabolism including some B vitamins and vitamin K are the
result of bacterial actions in the large intestine.
a. General. Absorption is the passage of substances (water, salts, vitamins,
carbohydrates, proteins, and fats) through the intestinal mucosa of the villi into the blood
or lymph. The chemical and mechanical phases of digestion are focused on changing
food into forms that can go through the epithelial cells which line the mucosa and into
the blood and lymph vessels underneath. Most absorption takes place in the small
intestine; actually, 90 percent of nutrients are absorbed in the small intestine. The other
10 percent of absorption takes place in the stomach and the large intestine.
b. Proteins. Most proteins are absorbed in the form of amino acids, and
absorption takes place mainly in the duodenum and the jejunum. As amino acids,
proteins move into the epithelial cells of the villi. Amino acids move out of these cells
and enter the bloodstream.
c. Carbohydrates. Most carbohydrates are absorbed as simple sugars
(monosaccharides). They move into the epithelial cells of the villi, then to the capillaries
of the villi, next to the bloodstream and into the liver, through the heart, and into general
circulation in the body.