690
UNIT FIVE
Despite regular dental care, some signs of aging may
affect the teeth. The enamel often thins from years of brush-
ing, teeth grinding, and eating acidic foods. Thinning enamel
may make the teeth more sensitive to hot and cold foods. At
the same time, the cementum may thicken. The dentin heals
more slowly and enlarges as the pulp shrinks. Loss of neu-
rons in the pulp may make it more difF
cult to notice tooth
decay. The gums recede, creating more pockets to harbor the
bacteria whose activity contributes to periodontal disease.
The teeth may loosen as the bones of the jaw weaken. On a
functional level, older people sometimes do not chew their
food thoroughly, swallowing larger chunks of food that may
present a choking hazard.
A common complaint of older individuals is “dry mouth,”
or xerostomia. This condition is not a normal part of aging—
studies have shown that the oldest healthy people make just
as much saliva as healthy younger people. Dry mouth is com-
mon, however, because it is a side effect of more than 400
medications, many of which are more likely to be taken by
older persons. These include antidepressants, antihistamines,
and drugs that treat cancer or hypertension. In addition, radia-
tion and chemotherapy used to treat cancer can cause mouth
sores and tooth decay. It is a good idea for cancer patients to
coordinate dental visits with other aspects of their care.
Once past the mouth, food travels through a gastroin-
testinal tract that declines gradually in efF ciency with age.
Slowing peristalsis may cause frequent heartburn as food
backs up into the esophagus. The stomach lining thins with
age, and secretion of hydrochloric acid, pepsin, and intrin-
sic factor decline. Exit of chyme from the stomach slows.
Overall, these changes may affect the rate at which certain
medications are absorbed.
The small intestine is the site of absorption of nutrients,
so it is here that noticeable signs of aging on digestion arise.
Subtle shifts in the microbial species that inhabit the small
intestine alter the rates of absorption of particular nutrients.
With age, the small intestine becomes less efF
cient at absorb-
ing vitamins A, D, and K and the mineral zinc. This raises
the risk of deF
ciency symptoms—effects on skin and vision
due to a lack of vitamin A; weakened bones from inadequate
vitamin D; impaired blood clotting seen in vitamin K deF -
ciency; and slowed healing, decreased immunity, and altered
taste evidenced in zinc deF
ciency.
Many people who have inherited lactose intolerance
begin to notice the telltale cramping after eating dairy foods
in the middle years. They must be careful that by avoiding
dairy products, they do not also lower their calcium intake.
Less hydrochloric acid also adversely affects the absorption
of calcium, as well as iron. Too little intrinsic factor may lead
to vitamin B
12
deF ciency anemia.
The lining of the large intestine changes too, thinning
and containing less smooth muscle and mucus. A dampen-
ing of the responsiveness of the smooth muscle to neural
When it is appropriate to defecate, a person usually can
initiate a
defecation refl
ex
by holding a deep breath and con-
tracting the abdominal wall muscles. This action increases the
internal abdominal pressure and forces feces into the rectum.
As the rectum F lls its wall distends, triggering the defecation
refl ex, which stimulates peristaltic waves in the descending
colon. The internal anal sphincter relaxes. At the same time,
other refl exes involving the sacral region of the spinal cord
strengthen the peristaltic waves, lower the diaphragm, close
the glottis, and contract the abdominal wall muscles. These
actions further increase the internal abdominal pressure and
squeeze the rectum. The external anal sphincter is signaled
to relax, and the feces are forced to the outside. A person can
voluntarily inhibit defecation by contracting the external anal
sphincter.
Hirschsprung disease causes extreme, chronic constipation and
abdominal distension. The part of the large intestine distal to the
distension lacks innervation. The person does not feel the urge to
defecate. The problem begins in the embryo, when a mutant gene
prevents neurons from migrating to this portion of the gastrointes-
tinal tract. Surgery may be used to treat the once lethal Hirschsprung
disease.
Feces
Feces
(fe
se
¯z) are composed of materials not digested or
absorbed, along with water, electrolytes, mucus, and bacte-
ria. Usually the feces are about 75% water, and their color
derives from bile pigments altered by bacterial action.
The pungent odor of the feces results from a variety of
compounds that bacteria produce. These compounds include
phenol, hydrogen sulF
de, indole, skatole, and ammonia.
PRACTICE
64
How does peristalsis in the large intestine diF
er from peristalsis in
the small intestine?
65
List the major events of defecation.
66
Describe the composition of feces.
17.11
LIFE-SPAN CHANGES
Changes to the digestive system associated with the passing
years are slow and slight, so most people can enjoy eating
a variety of foods as they grow older. Maintaining healthy
teeth is vital to obtaining adequate nutrition. This requires
frequent dental checkups, cleanings, and plaque removal,
plus care of the gums. Tooth loss due to periodontal disease
becomes more likely after age thirty-F
ve.
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