Chapter 23
The Digestive System
859
23
(
xerostomia
;
xero
5
dry). Dehydration also inhibits salivation
because low blood volume reduces filtration pressure at capil-
lary beds.
Homeostatic Imbalance
23.4
Anything that inhibits saliva secretion promotes tooth decay
and makes it difficult to talk and eat. Decomposing food par-
ticles accumulate and bacteria flourish, resulting in
halitosis
(hal
0
ĭ-to
9
sis; “bad breath”). Te odor is caused mainly by the
metabolic activity of anaerobic protein-digesting bacteria at
the back of the tongue that yields hydrogen sulfide (rotten egg
smell), and methyl mercaptan (also in feces), cadaverine (as-
sociated with rotting corpses), and other smelly chemicals.
Check Your Understanding
14.
What is the importance of the serous portion of saliva?
15.
Name three antimicrobial substances found in saliva.
For answers, see Appendix H.
The Teeth
Te
teeth
lie in sockets (alveoli) in the gum-covered margins of
the mandible and maxilla. Te role of the teeth in food process-
ing needs little introduction. We
masticate
, or chew, by opening
and closing our jaws and moving them from side to side while
using our tongue to move the food between our teeth. In the
process, the teeth tear and grind the food, physically breaking it
down into smaller fragments.
Dentition and the Dental Formula
Ordinarily by age 21, two sets of teeth, the
primary
and
per-
manent dentitions
, have formed
(Figure 23.10)
. Te primary
dentition consists of the
deciduous teeth
(de-sid
9
u-us;
decid
5
falling off), also called
milk
or
baby teeth
. Te first teeth to ap-
pear, at about age 6 months, are the lower central incisors. Ad-
ditional pairs of teeth erupt at one- to two-month intervals until
about 24 months, when all 20 milk teeth have emerged.
As the deep-lying
permanent teeth
enlarge and develop, the
roots of the milk teeth are resorbed from below, causing them to
loosen and fall out between ages 6 and 12. Generally, all the perma-
nent teeth but the third molars have erupted by the end of adoles-
cence. Te third molars, also called
wisdom teeth
, emerge between
ages 17 and 25. Tere are usually 32 permanent teeth in a full set, but
sometimes the wisdom teeth never erupt or are completely absent.
Homeostatic Imbalance
23.5
When a tooth remains embedded in the jawbone, it is said to be
impacted
. Impacted teeth can cause a good deal of pressure and
pain and must be removed surgically. Wisdom teeth are most
commonly involved.
±eeth are classified according to their shape and function
as incisors, canines, premolars, and molars (Figure 23.10). Te
chisel-shaped
incisors
are adapted for cutting or nipping off
Composition of Saliva
Saliva is largely water—97 to 99.5%—and therefore is hypo-
osmotic. Its osmolarity depends on the specific glands that are
active and the stimulus for salivation. As a rule, saliva is slightly
acidic (pH 6.75 to 7.00), but its pH may vary. Its solutes include
electrolytes (Na
1
, K
1
, Cl
2
, PO
4
3
2
, and HCO
3
2
); the digestive
enzymes salivary amylase and lingual lipase (both optimally ac-
tive at an acid pH); the proteins mucin, lysozyme, and IgA; and
metabolic wastes (urea and uric acid). When dissolved in water,
the glycoprotein
mucin
forms thick mucus that lubricates the
oral cavity and hydrates foodstuffs.
Saliva protects against microorganisms because it con-
tains (1)
IgA antibodies
; (2)
lysozyme
, a bactericidal enzyme
that inhibits bacterial growth in the mouth and may help
prevent tooth decay; and (3)
defensins
(see p. 641). Besides
acting as a local antibiotic, defensins function as cytokines to
call defensive cells (lymphocytes, neutrophils, etc.) into the
mouth for battle.
In addition to these three protectors, the friendly bacteria
that live on the back of the tongue promote the conversion of
food-derived nitrates in saliva into
nitric oxide
(NO) in an acid
environment. Tis transformation occurs around the gums,
where acid-producing bacteria tend to cluster, and in the hy-
drochloric acid–rich secretions of the stomach. Te highly toxic
nitric oxide is believed to be bactericidal in these locations.
Control of Salivation
Te minor salivary glands secrete saliva continuously in
amounts just sufficient to keep the mouth moist. But when food
enters the mouth, the major glands are activated and copious
amounts of saliva pour out. Te average output of saliva is about
1500 ml per day, but can be much higher when salivary glands
are appropriately stimulated.
Salivation is controlled primarily by the parasympathetic di-
vision of the autonomic nervous system. When we ingest food,
chemoreceptors and mechanoreceptors in the mouth send sig-
nals to the
salivatory nuclei
in the brain stem (pons and me-
dulla). As a result, parasympathetic nervous system activity
increases. Impulses sent via motor fibers in the
facial
(VII) and
glossopharyngeal
(IX)
nerves
dramatically increase the output of
watery (serous), enzyme-rich saliva.
Te chemoreceptors are activated most strongly by acidic
substances such as vinegar and citrus juice. Te mechanorecep-
tors are activated by virtually any mechanical stimulus in the
mouth—even chewing rubber bands.
Sometimes just the sight or smell of food is enough to get the
juices flowing. Te mere thought of hot fudge sauce on pepper-
mint ice cream will make many a mouth water! Irritation of the
lower GI tract by bacterial toxins, spicy foods, or hyperacidity
also increases salivation. Tis response may help wash away or
neutralize the irritants.
In contrast to parasympathetic controls, the sympathetic
division (specifically fibers in ±
1
–±
3
) causes release of a thick,
mucin-rich saliva. Strong activation of the sympathetic divi-
sion constricts blood vessels serving the salivary glands and
almost completely inhibits saliva release, causing a dry mouth
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