862
UNIT 4
Maintenance of the Body
23
the abdomen. It joins the stomach at the
cardial orifice
within
the abdominal cavity. Te cardial orifice is surrounded by the
gastroesophageal
or
cardiac sphincter
(gas
0
tro-ĕ-sof
0
ah-je
9
al),
which is a
physiological
sphincter (see Figure 23.13). Tat is,
it acts as a sphincter, but the only structural evidence of this
sphincter is a slight thickening of the circular smooth muscle
at that point. Te muscular diaphragm, which surrounds this
sphincter, helps keep it closed when food is not being swal-
lowed. Mucous cells on both sides of the sphincter help protect
the esophagus from reflux of stomach acid.
Homeostatic Imbalance
23.7
Heartburn
, the first symptom of
gastroesophageal reflux disease
(
GERD
), is the burning, radiating substernal pain that occurs
when stomach acid regurgitates into the esophagus. Symptoms
are so similar to those of a heart attack that many first-time suffer-
ers of heartburn are rushed to the emergency room. Heartburn
squamous epithelium well supplied with mucus-producing
glands. Te external muscle layer consists of two
skeletal muscle
layers. Te cells of the inner layer run longitudinally. Tose of
the outer layer, the
pharyngeal constrictor
muscles, encircle the
wall like three stacked fists (see Figure 10.9c). Contractions of
these muscles propel food into the esophagus below.
The Esophagus
Te
esophagus
(ĕ-sof
9
ah-gus; “carry food”) is a muscular tube
about 25 cm (10 inches) long and is collapsed when not involved
in food propulsion
(Figure 23.12)
. A±er food moves through
the laryngopharynx, it is routed into the esophagus posteriorly
as the epiglottis closes off the larynx to incoming food.
As shown in Figure 23.1, the esophagus takes a fairly straight
course through the mediastinum of the thorax. It pierces the
diaphragm at the
esophageal hiatus
(hi-a
9
tus; “gap”) to enter
Mucosa
(stratified squamous
epithelium)
Submucosa
(areolar connective
tissue)
Lumen
Muscularis externa
Adventitia
(fibrous
connective tissue)
Simple columnar
epithelium of stomach
Esophagus-stomach
junction
(a)
(b)
• Circular layer
• Longitudinal layer
Figure 23.12
Microscopic structure of the esophagus.
(a)
Cross-sectional view of the
esophagus taken from the region close to the stomach junction (10
3
). The muscularis is
composed of smooth muscle.
(b)
Longitudinal section through the esophagus-stomach junction
(130
3
). Notice the abrupt transition from the stratified squamous epithelium of the esophagus
(top) to the simple columnar epithelium of the stomach (bottom).
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