890
UNIT 4
Maintenance of the Body
23
enteric bacteria, no further food breakdown occurs in the large
intestine.
Te large intestine harvests vitamins made by the bacterial
flora and reclaims most of the remaining water and some of
the electrolytes (particularly sodium and chloride). However,
nutrient absorption is not its
major
function. As mentioned, the
primary concerns of the large intestine are propulsive activities
that force fecal material toward the anus and eliminate it from
the body (defecation).
Homeostatic Imbalance
23.13
Te large intestine is important for our comfort, but it is not es-
sential for life. If the colon is removed, as may be necessitated by
colon cancer, the terminal ileum is brought out to the abdomi-
nal wall in a procedure called an
ileostomy
(il
0
e-os
9
to-me). From
there food residues are eliminated into a sac attached to the
abdominal wall. Another surgical technique, ileoanal juncture,
links the ileum directly to the anal canal.
Motility of the Large Intestine
Te large intestine musculature is inactive much of the time, but
pressure in the ileum terminus opens the ileoceal sphincter and
then closes it, preventing backward movement of the chyme.
When presented with food residue the colon becomes motile,
but its contractions are sluggish or short-lived. Te movements
most seen in the colon are
haustral contractions
, slow seg-
menting movements lasting about one minute that occur every
30 minutes or so.
Tese contractions, which occur mainly in the ascending
and transverse colon, reflect local controls of smooth muscle
within the walls of the individual haustra. As a haustrum fills
with food residue, the distension stimulates its muscle to con-
tract, which propels the luminal contents into the next haus-
trum. Tese movements also mix the residue, which aids in
water absorption.
Mass movements
(also called mass peristalsis) are long,
slow-moving, but powerful contractile waves that move over
large areas of the colon three or four times daily and force the
contents toward the rectum. ±ypically, they occur during or just
a²er eating, which indicates that the presence of food in the
stomach activates the gastroileal reflex in the small intestine and
the propulsive
gastrocolic reflex
in the colon.
Segmenting movements in the descending and sigmoid co-
lon promote the final desiccation (drying out) of the feces. Tis
part of the colon also stores feces until mass movements propel
the feces into the rectum. Bulk, or fiber, in the diet strengthens
colon contractions and so²ens the feces, allowing the colon to
act like a well-oiled machine.
Homeostatic Imbalance
23.14
When the diet lacks bulk and the volume of residues in the
colon is small, the colon narrows and its contractions become
more powerful, increasing the pressure on its walls. Tis pro-
motes formation of
diverticula
(di
0
ver-tik
9
u-lah), small hernia-
tions of the mucosa through the colon walls.
itself. If these (hemorrhoidal) veins become inflamed, itchy var-
icosities called
hemorrhoids
result.
Te rectum and anal canal lack teniae coli and haustra. How-
ever, the rectum’s muscularis muscle layers are complete and
well developed, consistent with its role in generating strong con-
tractions to expel feces.
Bacterial Flora
Although most bacteria entering the cecum from the small
intestine are dead (killed by lysozyme, defensins, HCl, and
protein-digesting enzymes), some are still “alive and kicking.”
±ogether with bacteria that enter the GI tract via the anus, these
constitute the
bacterial flora
of the large intestine, an astonish-
ing 10 million discrete types. Tese bacteria:
Colonize the colon
Synthesize B complex vitamins and some of the vitamin K
the liver needs to produce several clotting proteins
Metabolize some host-derived molecules (mucin, heparin,
and hyaluronic acid)
Ferment some of the indigestible carbohydrates (cellulose,
xylan, and others), releasing irritating acids and a mixture of
gases (including dimethyl sulfide, H
2
, N
2
, CH
4
, and CO
2
)
Some of these gases, such as dimethyl sulfide, are quite odor-
ous (smelly). About 500 ml of gas (flatus) is produced each day,
much more when we eat certain carbohydrate-rich foods (such
as beans).
Although the huge intestinal population of bacteria would
seem to be enough inhabitants, the feces also have others, in-
cluding viruses and protozoans. Of these, at least 20 are known
pathogens.
Most enteric bacteria coexist peacefully with their host as long
as they remain in the gut lumen. An elegant system keeps the bac-
teria from breaching the mucosal barrier. Te epithelial cells of the
gut mucosa respond to specific bacterial components by releasing
chemicals that recruit immune cells, particularly dendritic cells, into
the mucosa. Te dendritic cells pry open tight junctions between
the epithelial cells and send extensions into the lumen to sample the
microbial antigens. Tey then migrate to the nearby lymphoid folli-
cles (MAL±) within the gut mucosa where they present the antigens
to ± cells. As a result, an IgA antibody–mediated response restricted
to the gut lumen is triggered that prevents the bacteria from stray-
ing into tissues deep to the mucosa where they might elicit a much
more widespread systemic response.
Tough beneficial in most ways, the coexistence of enteric
bacteria with our immune system does sometimes fail. When
that happens, the painful and debilitating conditions known as
inflammatory bowel disease (see Related Clinical ±erms, p. 905)
may result.
Digestive Processes in the Large Intestine
What is finally delivered to the large intestine contains few
nutrients, but it still has 12 to 24 hours more to spend there.
Except for a small amount of digestion of that residue by the
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