Digestion in the Large intestine

Processing in the Large Intestine The alimentary canal ends with the large intestine, which includes the colon, cecum, and rectum. The
small intestine connects to the large intestine at a
T-shaped junction. One arm of the T is the 1.5-m-long
colon, which leads to the rectum and anus. The
other arm is a pouch called the cecum. The cecum is
important for fermenting ingested material, especially
in animals that eat large amounts of plant material.
Compared with many other mammals, humans have a small
cecum. The appendix, a finger-like extension of the human
cecum, has a minor and dispensable role in immunity.
The colon completes the reabsorption of water that
began in the small intestine. What remain are the feces, the
wastes of the digestive system, which become increasingly
solid as they are moved along the colon by peristalsis. It
takes approximately 12–24 hours for material to travel the
length of the colon. If the lining of the colon is irritated—by
a viral or bacterial infection, for instance—less water than
normal may be reabsorbed, resulting in diarrhea. The opposite
problem, constipation, occurs when the feces move
along the colon too slowly. Too much water is reabsorbed,
and the feces become compacted.
The undigested material in feces includes cellulose fiber.
Although it provides no caloric value (energy) to humans,
fiber helps move food along the alimentary canal.
A rich community of mostly harmless bacteria lives on
the unabsorbed organic material in the human colon, contributing
approximately one-third of the dry weight of feces.
As by-products of their metabolism, many colon bacteria
generate gases, including methane and hydrogen sulfide, the
latter of which has an offensive odor. These gases and ingested
air are expelled through the anus.
The terminal portion of the large intestine is the rectum,
where the feces are stored until they can be eliminated. Between
the rectum and the anus are two sphincters, the inner
one being involuntary and the outer one being voluntary.
Periodically, strong contractions of the colon create an urge to defecate. Because filling of the stomach triggers a reflex
that increases the rate of contractions in the colon, the urge
to defecate often follows a meal.
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