To summarize, these are the large intestine’s main
functions:
Peristalsis drives the contents of the colon into the
rectum.
Bacteria in the large intestine convert proteins to
amino acids, break down amino acids, and produce
some B vitamins and vitamin K.
The large intestine absorbs some water, ions, and
vitamins.
The large intestine forms feces.
The large intestine is involved in defecation (emptying
the rectum).
Numerous bacteria live within the lumen of the colon
and perform several digestive functions:
Ferment any remaining carbohydrates and
release gases (hydrogen, carbon dioxide,
methane) called
flatu s
.
Break down remaining proteins into amino
acids.
Convert
bilirubin
into
simpler
pigments,
including
stercobilin, which gives feces a characteristic brown color.
Produce essential vitamins (B, K) that are absorbed in
the colon.
The nutrients (carbohydrates and proteins) that are bro-
ken down in the large intestine are not able to be absorbed
and are passed out with the feces. The only nutrient mol-
ecules that are absorbed here are the vitamins that are
produced in the colon by the resident microorganisms.
Most of the food and materials secreted by the GI
tract during digestion is absorbed by the small intestine
(~90%), while the large intestine absorbs another 9% of
these substances. Therefore, only 1%
of the material en-
tering the GI tubing is lost in the
feces.
If chyme
moves
through
the
intestines too rapidly or if
enzymes
that
process
the
food
materials
are
missing,
absorp-
tion of fluid and nutrients by the
intestines decreases. Excess wa-
ter and nutrients remain in the
feces, causing
d iarrhea
. Frequent
diarrhea can lead to dehydration
and
electrolyte
imbalances.
In
contrast, if movement of chyme
through the intestines slows, ab-
sorption of fluid increases, result-
ing in
co n stip a tio n
.
The remains of your breakfast will take about 3 to
10 hours to journey through the large intestine. As water
gets absorbed from the chyme, it changes from a liquid
to a semi-solid or solid mass called feces. Feces consists
mainly of water, inorganic salts, epithelial cells that have
sloughed off the mucosa of the GI tract, bacteria, products
of bacterial decomposition, unabsorbed digested material,
and indigestible parts of food.
The only task that remains is elimination of the fecal
material from the body, which occurs during a process
called defecation. Defecation begins with waves of mass
peristalsis (strong peristalic wave) that occur periodi-
cally and drive material from the colon into the rectum.
This initiates the defecation reflex.
The defecation reflex involves stretch receptors in the
walls of the rectum, parasympathetic nerve
activity, and relaxation of the anal sphincters.
Sometimes contractions of the diaphragm and
abdominal muscles can help initiate defeca-
tion, while contraction of muscles in the anus
can stop the passage of feces out of the rectum.
Some muscle contractions are voluntary, while
others are involuntary.
The Three Phases of Digestion
Are Controlled by the Enteric
Nervous System
Digestion is controlled by the enteric nervous system (ENS),
a network of neurons within the walls of the GI tract that is
sometimes referred to as the “gut brain” (see Figure 14.2).
The ENS is influenced by the autonomic nervous system.
ENS motor neurons control muscle movements and secre-
tions within the GI tract, and sensory neurons detect the
presence of food, using chemical and mechanical stretch
receptors within the gastrointestinal organs.
Digestion occurs in three overlapping phases: the
cephalic, gastric, and intestinal phases. The cephalic
phase responds to the sensations of food (such as smell,
taste, and sight) and prepares the mouth and stomach
to receive it. This relatively short phase lasts only as
long as eating the meal. The gastric phase continues
gastric secretions and motility, mixing food and regu-
lating gastric emptying. The intestinal phase promotes
continued digestion through the small intestine. At the
same time, it inhibits gastric emptying, which prevents
the duodenum from becoming overloaded with chyme
from the stomach.
To summarize, digestion involves physical and chem-
ical processes in several organs. The physical processes
flatus
(FLA-tus) Gas
in the stomach or
intestines; commonly
used to denote expul-
sion of gas through
the anus.
diarrhea
(dT-a-RE-a)
Frequent defecation
of liquid feces, caused
by increased motility
of the intestines or
absence of enzymes
to process the food
nutrients.
con stip ation
(kon'-sti-PA-shun)
Infrequent or difficult
defecation caused by
decreased motility of
the intestines.
414 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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