544
Chapter 15
As described in Chapter 13, the pressure in the thoracic
cavity is negative relative to atmospheric pressure, and this
subatmospheric pressure is transmitted across the thin wall
of the intrathoracic portion of the esophagus to the lumen.
In contrast, the luminal pressure in the pharynx at the open-
ing to the esophagus is equal to atmospheric pressure, and the
pressure at the opposite end of the esophagus in the stomach
is slightly greater than atmospheric pressure. Therefore, pres-
sure differences could tend to force both air (from above) and
gastric contents (from below) into the esophagus. This does
not occur, however, because both ends of the esophagus are
normally closed by the contraction of
sphincter
muscles. A
ring of skeletal muscles surrounds the esophagus just below
the pharynx and forms the
upper esophageal sphincter,
whereas the smooth muscles in the last portion of the esopha-
gus form the
lower esophageal sphincter
(
Figure 15–15
).
The esophageal phase of swallowing begins with relax-
ation of the upper esophageal sphincter. Immediately after the
food has passed, the sphincter closes, the glottis opens, and
breathing resumes. Once in the esophagus, the food moves
toward the stomach by a progressive wave of muscle contrac-
tions that proceeds along the esophagus, compressing the
lumen and forcing the food ahead. Such waves of contraction
in the muscle layers surrounding a tube are known as
peri-
staltic waves.
One esophageal peristaltic wave takes about
nine seconds to reach the stomach. Swallowing can occur even
when a person is upside down or in zero gravity (outer space)
Figure 15–14
Movements of food through the pharynx and upper esophagus during swallowing. (a) The tongue pushes the food bolus to the back of the
pharynx. (b) The soft palate elevates to prevent food from entering the nasal passages. (c) The epiglottis covers the glottis to prevent food or
liquid from entering the trachea (aspiration), and the upper esophageal sphincter relaxes. (d) Food descends into the esophagus.
Figure 15–14b
physiological
inquiry
What are some of the consequences of aspiration?
Answer can be found at end of chapter.
Soft palate
Hard palate
Tongue
Glottis
Trachea
Esophagus
(a)
(b)
(c)
Epiglottis
Food
Upper
esophageal
sphincter
(d)
Pharynx
Trachea
Upper
esophageal
sphincter
Lower
esophageal
sphincter
Diaphragm
Stomach
Esophagus
Figure 15–15
Location of upper and lower esophageal sphincters.
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