686
UNIT FIVE
triggering the gastroenteric reflex, which greatly increases
peristaltic activity in the small intestine. Another refl ex begins
when the duodenum F lls with chyme, stretching its wall. This
refl ex speeds movement through the small intestine.
If the small intestine wall becomes overdistended or
irritated, a strong
peristaltic rush
may pass along the entire
length of the organ, sweeping the contents into the large
intestine so quickly that water, nutrients, and electrolytes
that would normally be absorbed are not. The result is
diar-
rhea,
in which defecation becomes more frequent and the
stools become watery. Prolonged diarrhea causes imbalances
in water and electrolyte concentrations.
At the distal end of the small intestine, the
ileocecal
sphincter
joins the small intestine’s ileum to the large intes-
tine’s cecum. Normally, this sphincter remains constricted,
preventing the contents of the small intestine from entering
the large intestine, and at the same time keeping the contents
of the large intestine from backing up into the ileum. However,
eating a meal elicits a gastroileal refl ex that increases peristal-
sis in the ileum and relaxes the sphincter, forcing some of the
contents of the small intestine into the cecum.
PRACTICE
54
Describe the movements of the small intestine.
55
How are the movements of the small intestine initiated?
56
What is a peristaltic rush?
57
What stimulus relaxes the ileocecal sphincter?
17.10
LARGE INTESTINE
The
large intestine
is so named because its diameter is greater
than that of the small intestine. This part of the alimentary
canal is about 1.5 meters long, and it begins in the lower right
side of the abdominal cavity where the ileum joins the cecum.
±rom there, the large intestine ascends on the right side,
crosses obliquely to the left, and descends into the pelvis. At
its distal end, it opens to the outside of the body as the anus.
The large intestine absorbs ingested water and elec-
trolytes remaining in the alimentary canal. Additionally
it reabsorbs and recycles water and remnants of digestive
secretions. The large intestine also forms and stores feces.
In
malabsorption,
the small intestine digests, but does not absorb,
some nutrients. Symptoms of malabsorption include diarrhea, weight
loss, weakness, vitamin deF
ciencies, anemia, and bone demineraliza-
tion. Causes of malabsorption include surgical removal of a portion of
the small intestine, obstruction of lymphatic vessels due to a tumor,
or interference with the production and release of bile as a result of
liver disease.
Another cause of malabsorption is a reaction to
gluten,
which is
a protein found in certain grains, especially wheat and rye. This con-
dition is called
celiac disease.
Microvilli are damaged, and in severe
cases, villi are destroyed. Both of these e±
ects reduce the absorptive
surface of the small intestine, preventing absorption of some nutri-
ents. Health-food grocery stores sell gluten-free products, including
pasta and baked goods.
PRACTICE
50
Which substances resulting from digestion of carbohydrate,
protein, and fat molecules does the small intestine absorb?
51
Which ions does the small intestine absorb?
52
What transport mechanisms do intestinal villi use?
53
Describe how fatty acids are absorbed and transported.
Movements of the Small Intestine
Like the stomach, the small intestine carries on mixing
movements and peristalsis. The major mixing movement is
segmentation, in which small, ringlike, periodic contractions
cut the chyme into segments and move it back and forth.
Segmentation also slows the movement of chyme through
the small intestine.
Peristaltic waves propel chyme through the small intes-
tine. These waves are usually weak, and they stop after
pushing the chyme a short distance. Consequently, chyme
moves slowly through the small intestine, taking from three
to ten hours to travel its length.
As might be expected, parasympathetic impulses
enhance both mixing and peristaltic movements, and sym-
pathetic impulses inhibit them. Refl exes involving parasym-
pathetic impulses to the small intestine sometimes originate
in the stomach. ±or example, food distends the stomach wall,
TABLE
17.10
|
Intestinal Absorption of Nutrients
Nutrient
Absorption Mechanism
Means of Transport
Monosaccharides
²acilitated di±
usion and active transport
Blood in capillaries
Amino acids
Active transport
Blood in capillaries
²atty acids and glycerol
²acilitated di±
usion of glycerol; di±
usion of fatty acids into cells
(a)
Most fatty acids are resynthesized into fats and incorporated in chylomicrons for transport.
Lymph in lacteals
(b)
Some fatty acids with relatively short carbon chains are transported without being changed back into fats.
Blood in capillaries
Electrolytes
Di±
usion and active transport
Blood in capillaries
Water
Osmosis
Blood in capillaries
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