Chapter 23
The Digestive System
863
23
swallowing, and (4) starts the chemical breakdown (digestion)
of polysaccharides.
Except for a few drugs that are absorbed through the oral
mucosa (for example, nitroglycerine used to alleviate the pain of
angina), essentially no absorption occurs in the mouth.
In contrast to the multifunctional mouth, the pharynx and
esophagus merely serve as conduits to pass food from the
mouth to the stomach. Teir single digestive system function is
food propulsion, accomplished by swallowing.
Since we cover digestion in a special physiology section later
in the chapter, we will discuss only the mechanical processes of
chewing and swallowing food here.
Mastication (Chewing)
As food enters the mouth, its mechanical breakdown begins
with
mastication
, or chewing. Te cheeks and closed lips hold
food between the teeth, the tongue mixes food with saliva to
soFen it, and the teeth cut and grind solid foods into smaller
morsels.
Mastication is partly voluntary and partly reflexive. We vol-
untarily put food into our mouths and contract the muscles
that close our jaws. Te pattern and rhythm of continued jaw
movements are controlled mainly by stretch reflexes and in re-
sponse to pressure inputs from receptors in the cheeks, gums,
and tongue, but they can also be voluntary if desired.
Deglutition (Swallowing)
±o send food on its way from the mouth, it is first compacted
by the tongue into a bolus and is then swallowed.
Deglutition
(deg
0
loo-tish
9
un), or swallowing, is a complicated process
that involves coordinated activity of over 22 separate mus-
cle groups. It has two major phases, the buccal and the
pharyngeal-esophageal.
Te
buccal phase
occurs in the mouth and is voluntary. In
the buccal phase, we place the tip of the tongue against the hard
palate, and then contract the tongue to force the bolus into the
oropharynx (
Figure 23.13
1
). As food enters the pharynx
and stimulates tactile receptors there, it passes out of our control
and into the realm of involuntary reflex activity.
±riggered by that “bit of saliva” or food reaching receptors in
the posterior pharynx, the involuntary
pharyngeal-esophageal
phase
of swallowing is controlled by the swallowing center lo-
cated in the brain stem (medulla and lower pons). Various cra-
nial nerves, most importantly the vagus nerves, transmit motor
impulses from the swallowing center to the muscles of the phar-
ynx and esophagus. Once food enters the pharynx, respiration
is momentarily inhibited and all routes except the desired one
into the digestive tract are blocked off (²igure 23.13
2
): Te
tongue blocks off the mouth. Te soF palate rises to close off
the nasopharynx. Te larynx rises so that the epiglottis cov-
ers its opening into the respiratory passageways, and the upper
esophageal sphincter relaxes.
Wavelike peristaltic contractions create pressure waves that
propel food through the pharynx and into the esophagus (²ig-
ure 23.13
3
5
). Solid foods pass from the oropharynx to the
stomach in about 8 seconds, and fluids, aided by gravity, pass
is most likely when a person has eaten or drunk to excess, and in
conditions that force abdominal contents superiorly, such as ex-
treme obesity, pregnancy, and running, which splashes stomach
contents upward with each step (runner’s reflux).
Heartburn is also common in those with a
hiatal
hernia
, a
structural abnormality (most oFen due to abnormal relaxation
or weakening of the gastroesophageal sphincter) in which the
superior part of the stomach protrudes slightly above the dia-
phragm. Since the diaphragm no longer reinforces the sphinc-
ter, gastric juice may enter the esophagus, particularly when
lying down. If the episodes are frequent and prolonged,
esoph-
agitis
(inflammation of the esophagus) and
esophageal ulcers
may result. An even more threatening sequel is esophageal can-
cer. Tese consequences can usually be prevented or managed
by avoiding late-night snacks and using antacid preparations.
Unlike the mouth and pharynx, the esophagus wall has all
four of the basic alimentary canal layers described earlier. Some
features of interest:
Te esophageal mucosa contains a
nonkeratinized
stratified
squamous epithelium. At the esophagus-stomach junction,
that abrasion-resistant epithelium changes abruptly to the
simple columnar epithelium of the stomach, which is spe-
cialized for secretion (²igure 23.12b).
When the esophagus is empty, its mucosa and submucosa are
thrown into longitudinal folds (²igure 23.12a). When food is
in transit in the esophagus, these folds flatten out.
Te submucosa contains mucus-secreting
esophageal glands
.
As a bolus moves through the esophagus, it compresses these
glands, causing them to secrete mucus that “greases” the
esophageal walls and aids food passage.
Te muscularis externa is skeletal muscle in its superior
third, a mixture of skeletal and smooth muscle in its middle
third, and entirely smooth muscle in its inferior third.
Instead of a serosa, the esophagus has a fibrous adventitia
composed entirely of connective tissue, which blends with
surrounding structures along its route.
Check Your Understanding
19.
To which two organ systems does the pharynx belong?
20.
How is the muscularis externa of the esophagus unique in
the body?
21.
What is the functional significance of the epithelial change
seen at the esophagus-stomach junction?
For answers, see Appendix H.
Digestive Processes:
Mouth to Esophagus
Describe the mechanisms of chewing and swallowing.
Te mouth and its accessory digestive organs are involved in
most digestive processes. Te mouth (1) ingests, (2) begins
mechanical breakdown by chewing, (3) initiates propulsion by
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