689
CHAPTER SEVENTEEN
Digestive System
Intestinal gas is 99% nitrogen and oxygen taken in while breathing
and eating, plus methane (CH
4
), carbon dioxide (CO
2
), and hydrogen
contributed from the bacterial fermentation of undigested food. The
characteristic odor comes from bacterial action on the nitrogen and
sulfur in proteins, which yields pungent-smelling ammonia (NH
3
) and
foul hydrogen sulF
de (H
2
S). Most people release a half liter of intes-
tinal gas a day. ±oods rich in sulfur-containing amino acids make
intestinal gas more foul. These include beans, broccoli, bran, brussels
sprouts, cabbage, cauli²
ower, and onions.
PRACTICE
61
How does the structure of the large intestine di³
er from that of
the small intestine?
62
What substances does the large intestine absorb?
63
What useful substances do bacteria inhabiting the large intestine
produce?
Movements of the Large Intestine
The movements of the large intestine—mixing and peri-
stalsis—are similar to those of the small intestine, although
usually slower. The mixing movements break the fecal matter
into segments and turn it so that all portions are exposed to the
intestinal mucosa. This helps absorb water and electrolytes.
The peristaltic waves of the large intestine are differ-
ent from those of the small intestine. Instead of occurring
frequently, they happen only two or three times each day.
These waves produce
mass movements
in which a large sec-
tion of the intestinal wall constricts vigorously, forcing the
intestinal contents to move toward the rectum. Typically,
mass movements follow a meal, as a result of the gastrocolic
refl ex initiated in the small intestine. Irritation of the intesti-
nal mucosa can also trigger such movements. For instance, a
person suffering from an infl
amed colon (colitis) may expe-
rience frequent mass movements. Clinical Application 17.5
examines conditions affecting the large intestine.
Chyme entering the large intestine usually has few
nutrients remaining in it and mostly consists of materials
not digested or absorbed in the small intestine. It also con-
tains water, electrolytes, mucus, and bacteria.
Absorption in the large intestine is normally limited to
water and electrolytes, and this usually occurs in the proxi-
mal half of the tube. Electrolytes such as sodium ions can
be absorbed by active transport, while the water follows
passively, entering the mucosa by osmosis. About 90% of
the water that enters the large intestine is absorbed, and
little sodium or water is lost in the feces.
The many bacteria that normally inhabit the large intes-
tine, called
intestinal fl
ora,
break down some of the mol-
ecules that escape the actions of human digestive enzymes.
For instance, cellulose, a complex carbohydrate in food of
plant origin, passes through the alimentary canal almost
unchanged, but colon bacteria can break down cellulose
and use it as an energy source. These bacteria, in turn, syn-
thesize vitamins, such as K, B
12
, thiamine, and ribofl
avin,
which the intestinal mucosa absorbs. Bacterial actions in
the large intestine may produce intestinal gas (fl
atus).
FIGURE 17.47
Light micrograph of the large intestinal mucosa (560×).
Lumen
Submucosa
Muscular layer
Serosa
Mucosa
FIGURE 17.46
Light micrograph of the large intestinal wall (64×).
Lumen of
large
intestine
Goblet
cells
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