695
CHAPTER SEVENTEEN
Digestive System
6. Movements of the small intestine
a. Movements include mixing by segmentation and
peristalsis.
b. Overdistension or irritation may stimulate a
peristaltic rush and result in diarrhea.
c. The ileocecal sphincter controls movement of the
intestinal contents from the small intestine into the
large intestine.
17.10
LARGE INTESTINE (PAGE 686)
The large intestine absorbs water and electrolytes and
forms and stores feces.
1. Parts of the large intestine
a. The large intestine consists of the cecum, colon,
rectum, and anal canal.
b. The colon is divided into ascending, transverse,
descending, and sigmoid portions.
2. Structure of the large intestinal wall
a. The large intestinal wall resembles the wall in
other parts of the alimentary canal.
b. The large intestinal wall has a unique layer of
longitudinal muscle F
bers, arranged in distinct
bands, that extend the entire length of the colon.
3. ±unctions of the large intestine
a. The large intestine has little or no digestive
function, although it secretes mucus.
b. Mechanical stimulation and parasympathetic
impulses control the rate of mucous secretion.
c. The large intestine absorbs water and electrolytes.
d. Many bacteria inhabit the large intestine, where
they break down some undigestible substances,
such as cellulose, and synthesize vitamins K, B
12
,
thiamine, and ribofl avin.
4. Movements of the large intestine
a. Movements are similar to those in the small
intestine.
b. Mass movements occur two to three times each day.
c. A refl
ex stimulates defecation.
5. ±eces
a. The large intestine forms and stores feces.
b. ±eces consist of water, undigested material, mucus,
and bacteria.
c. The color of feces is due to bile pigments that have
been altered by bacterial actions.
17.11
LIFE-SPAN CHANGES (PAGE 690)
1. Older people sometimes do not chew food thoroughly
because thinning enamel makes teeth more sensitive
to hot and cold foods, gums recede, and teeth may
loosen.
2. Slowing peristalsis in the digestive tract may cause
heartburn and constipation.
3. Aging affects nutrient absorption in the small intestine.
4. Accessory organs to digestion also age, but not
necessarily in ways that affect health.
4. Gallbladder
a. The gallbladder stores bile between meals.
b. A sphincter muscle controls release of bile from
the bile duct.
c. Gallstones may form within the gallbladder.
5. Regulation of bile release
a. Cholecystokinin from the small intestine stimulates
bile release.
b. The sphincter muscle at the base of the bile duct
relaxes as a peristaltic wave in the duodenal wall
approaches.
6. ±unctions of bile salts
a. Bile salts emulsify fats and aid in the absorption of
fatty acids, cholesterol, and certain vitamins.
b. Bile salts are reabsorbed in the small intestine.
17.9
SMALL INTESTINE (PAGE 678)
The small intestine extends from the pyloric sphincter to
the large intestine. It receives secretions from the pancreas
and liver, completes digestion of nutrients, absorbs the
products of digestion, and transports the residues to the
large intestine.
1. Parts of the small intestine
a. The small intestine consists of the duodenum,
jejunum, and ileum.
b. The small intestine is suspended from the posterior
abdominal wall by mesentery.
2. Structure of the small intestinal wall
a. The wall is lined with villi that greatly increase the
surface area and aid in mixing and absorption.
b. Microvilli on the free ends of epithelial cells
increase the surface area even more.
c. Intestinal glands are located between the villi.
d. Circular folds in the lining of the intestinal wall
also increase its surface area.
3. Secretions of the small intestine
a. Intestinal glands secrete a watery fl
uid that lacks
digestive enzymes but provides a vehicle for
moving chyme to the villi.
b. Digestive enzymes embedded in the surfaces of
microvilli split molecules of sugars, proteins, and
fats.
4. Regulation of small intestinal secretions
a. Secretion is stimulated by gastric juice, chyme,
and refl
exes stimulated by distension of the small
intestinal wall.
5. Absorption in the small intestine
a. Blood capillaries in the villi absorb mono-
saccharides, amino acids, fatty acids, and glycerol.
b. Blood capillaries in the villi also absorb water and
electrolytes.
c. ±at molecules with longer chains of carbon atoms
enter the lacteals of the villi; fatty acids with short
carbon chains enter the blood capillaries of the villi.
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