670
UNIT FIVE
Stomach
Food
entering
Pyloric sphincter
contracted
Pyloric sphincter
relaxed
Chyme
Duodenum
(a)
(b)
(c)
FIGURE 17.21
Stomach movements. (
a
) As the stomach f
lls, its muscular wall stretches, but the pyloric sphincter remains closed. (
b
) Mixing
movements combine Food and gastric juice, creating chyme. (
c
) Peristaltic waves move the chyme toward the pyloric sphincter, which relaxes and
admits some chyme into the duodenum.
A
t the barbecue, Perry W. consumed two
burgers, three hot dogs, beans in a spicy
sauce, loads oF chips, several beers, and
ice cream. Later, a Feeling oF Fullness became
abdominal pain, then heartburn, as his stomach
contents backed up into his esophagus.
Perry Found temporary relieF with an over-
the-counter antacid product, which quickly raised
the pH oF the stomach. These products usually
include a compound containing either sodium,
calcium, magnesium, or aluminum. Another
ingredient in some products is simethicone,
which breaks up gas bubbles in the digestive
tract. IF antacids do not help within a Few min-
utes or they are used For longer than two weeks, a
doctor should be consulted. The problem may be
more serious than overeating.
Avoiding acid indigestion and heartburn is a
more healthFul approach than gorging and then
reaching For medication—or even taking prod-
ucts that lower acid production beFore a large or
spicy meal. Some tips:
Avoid large meals. The more Food, the more
acid the stomach produces.
Eat slowly so that stomach acid secretion is
more gradual.
Do not lie down immediately aFter eating.
Being upright enables gravity to help Food
move along the alimentary canal.
IF prone to indigestion or heartburn, avoid
ca±
eine, which increases stomach acid
secretion.
Cigarettes and alcohol irritate the stomach
lining and relax the sphincter at the junction
between the stomach and the esophagus.
This makes it easier For Food to return to the
esophagus, causing heartburn.
Do not eat acidic Foods, such as citrus Fruits
and tomatoes, unless it is at least three
hours beFore bedtime.
Use a pillow that elevates the head six to
eight inches above the stomach.
17.2
CLINICAL APPLICATION
Oh, My Aching Stomach!
inhibiting peristalsis, and intestinal f
lling slows
(f g. 17.22)
.
IF chyme entering the intestine is Fatty, the intestinal wall
releases the hormone cholecystokinin, which Further inhibits
peristalsis.
Vomiting
results From a complex refl
ex that empties the
stomach in the reverse oF the normal direction. Irritation or
distension in the stomach or intestines can trigger vomit-
ing. Sensory impulses travel From the site oF stimulation to
the
vomiting center
in the medulla oblongata, and motor
responses Follow. These include taking a deep breath, rais-
ing the soFt palate and thus closing the nasal cavity, clos-
ing the opening to the trachea (glottis), relaxing the circular
muscle f
bers at the base oF the esophagus, contracting the
they are well mixed with gastric juice. ±atty Foods may
remain in the stomach three to six hours; Foods high in pro-
teins move through more quickly; carbohydrates usually
pass through more rapidly than either Fats or proteins.
As chyme Fills the duodenum, internal pressure on
the organ increases, stretching the intestinal wall. These
actions stimulate sensory receptors in the wall, triggering an
enterogastric refl
ex
(en-ter-o-gas
trik re
fl eks). The name oF
this refl ex, like those oF other digestive refl exes, describes
the origin and termination oF refl ex impulses. That is, the
enterogastric refl ex begins in the small intestine (
entero
) and
ends in the stomach (
gastro
). As a result oF the enterogastric
refl ex, Fewer parasympathetic impulses arrive at the stomach,
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