The Digestion and Absorption of Food
545
because it is not primarily gravity but the peristaltic wave that
moves the food to the stomach.
The lower esophageal sphincter opens and remains relaxed
throughout the period of swallowing, allowing the arriving
food to enter the stomach. After the food passes, the sphincter
closes, resealing the junction between the esophagus and the
stomach.
Swallowing is an example of a refl ex in which multiple
responses occur in a sequence determined by the pattern of
synaptic connections between neurons in a brain coordinating
center. Both skeletal and smooth muscles are involved, so the
swallowing center must direct efferent activity in both somatic
nerves (to skeletal muscle) and autonomic nerves (to smooth
muscle). Simultaneously, afferent fi
bers from receptors in the
esophageal wall send information to the swallowing center; this
can alter the efferent activity. For example, if a large food parti-
cle does not reach the stomach during the initial peristaltic wave,
the maintained distension of the esophagus by the particle acti-
vates receptors that initiate refl exes, causing repeated waves of
peristaltic activity (
secondary peristalsis
). This is usually not
accompanied by the initial pharyngeal events of swallowing.
The ability of the lower esophageal sphincter to maintain a
barrier between the stomach and the esophagus when swallow-
ing is not taking place is aided by the fact that the last portion
of the esophagus lies below the diaphragm and is subject to the
same abdominal pressures as the stomach. In other words, if the
pressure in the abdominal cavity increases, for example, during
cycles of respiration or contraction of the abdominal muscles, the
pressures on both the gastric contents and the terminal segment
of the esophagus are raised together. This prevents the forma-
tion of a pressure gradient between the stomach and esophagus
that could force the stomach’s contents into the esophagus.
During pregnancy, the growth of the fetus not only
increases the pressure on the abdominal contents but also
pushes the terminal segment of the esophagus through the
diaphragm into the thoracic cavity. The sphincter is there-
fore no longer assisted by changes in abdominal pressure.
Consequently, during the last half of pregnancy, increased
abdominal pressure tends to force some of the gastric contents
up into the esophagus. The hydrochloric acid from the stom-
ach irritates the esophageal walls, producing pain known as
heartburn
(because the pain appears to be located in the area
of the heart). Heartburn often subsides in the last weeks of
pregnancy prior to delivery, as the uterus descends lower into
the pelvis, decreasing the pressure on the stomach.
Heartburn also occurs in the absence of pregnancy.
Some people have less effi cient lower esophageal sphincters,
resulting in repeated episodes of gastric contents refl
uxing into
the esophagus (
gastroesophageal refl
ux
), heartburn, and in
extreme cases, ulceration, scarring, obstruction, or perforation
of the lower esophagus. Heartburn can also occur after a large
meal, which can raise the pressure in the stomach enough to
force acid into the esophagus. Gastroesophageal refl
ux can also
cause coughing and irritation of the larynx in the absence of
any esophageal symptoms, and it has even been implicated in
the onset of asthmatic symptoms in susceptible individuals.
The lower esophageal sphincter not only undergoes
brief periods of relaxation during a swallow but also in the
absence of a swallow. During these periods of relaxation,
small amounts of the acid contents from the stomach normally
refl ux into the esophagus. The acid in the esophagus triggers
a secondary peristaltic wave and also stimulates increased sali-
vary secretion, which helps to neutralize the acid and clear it
from the esophagus.
Stomach
The epithelial layer lining the stomach invaginates into the
mucosa, forming many tubular glands. Glands in the thin-
walled upper portions of the
body
of the stomach (
Figure
15–16
) secrete mucus, hydrochloric acid, and the enzyme pre-
cursor
pepsinogen.
The uppermost part of the body of the
stomach is called the
fundus.
The lower portion of the stom-
ach, the
antrum,
has a much thicker layer of smooth muscle.
The glands in this region secrete little acid but contain the
endocrine cells that secrete the hormone gastrin.
The cells at the opening of the glands secrete mucus
(
Figure 15–17
). Lining the walls of the glands are
parietal
cells
(also known as oxyntic cells), which secrete acid and intrin-
sic factor, and
chief cells,
which secrete pepsinogen. Thus, each
of the three major exocrine secretions of the stomach—mucus,
acid, and pepsinogen—is secreted by a different cell type. The
gastric glands in the antrum also contain
enteroendocrine
cells,
which secrete gastrin. In addition,
enterochromaffi n-like
(ECL) cells,
which release the paracrine agent histamine, and
endocrine (D) cells that secrete the peptide messenger
soma-
tostatin,
are scattered throughout the tubular glands.
HCl Secretion
The stomach secretes about 2 L of hydrochloric acid per day.
The concentration of hydrogen ions in the lumen of the stom-
ach may reach >150 mM, which is 1 to 3 million times higher
than the concentration in the blood.
Figure 15–16
The two regions of the stomach: body and antrum. The fundus is
the uppermost portion of the body of the stomach.
Esophagus
Body
(secretes
mucus, pepsinogen,
and HCI)
Duodenum
Pyloric
sphincter
Antrum
(secretes
mucus,
pepsinogen,
and gastrin)
Fundus
Lower esophageal
sphincter
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