Nutrition and the Digestive System 385
Common Disease, Disorder, or ConditionHiatal HerniaA hiatal hernia is caused by a widening of the esophageal hiatus (the opening where the
esophagus penetrates through the diaphragm muscle to connect with the stomach).
This results in a portion of the stomach protruding upward through the diaphragm. This
condition occurs most often in adults, with 40% of the popula-tion affected. The major
symptom of this condition is gastroesophageal reflux or acid reflux, the backflow of the
acidic contents of the stomach into the esophagus. This condition is also called
heartburn and is quite uncomfortable. A hiatal hernia can also compress blood vessels
of the mucosa of the stomach lining resulting in ul-cers or gastritis, which can be quite
painful.The Esophagus
The esophagus (eh-SOFF-ah-gus) is a collapsible, mus-
cular tube that is situated behind the trachea or wind-pipe.
It is about 10 inches (23 to 25 cm) long and begins at the
end of the laryngopharynx. It passes through the
mediastinum (mee-dee-ass-TYE-num) (the space be-
tween the lungs), pierces the diaphragm (DYE-ah-fram)
through an opening called the esophageal hiatus, and
ends at the superior portion of the stomach. The func-tion
of the esophagus is to secrete mucus and transport food to
the stomach. It does not produce any digestive enzymes and
it does not absorb food. Food is pushed through the
esophagus by smooth muscle contractions, called
peristalsis, repeated in wavelike motions that push the food
toward the stomach (Figure 16-8). Movement of solid or
semisolid foods from the mouth to the stomach takes 4 to 8
seconds; liquids pass in about 1 second. Just above the
diaphragm muscle, the esophagus is slightly narrowed by
the lower esophageal or gastroesophageal
sphincter (gas-troh-eh-soff-ah-JEE-al SFINGK-ter).
This sphincter connects the esophagus with the stomach
and controls the passage of food into the stomach.
The Stomach
The stomach is an enlargement of the gastrointestinal tract.
It lies in the upper part of the abdominal cavity just under
the diaphragm muscle. It has the shape of the let-ter J.
When it is empty, it is about the size of a large sau-sage.
However, it can be stretched to accommodate large
amounts of food.
The stomach is divided into four parts (Figure 16-9A):^
(1) the cardia surrounds the gastroesophageal sphincter;
(2) the fundus is the rounded portion above and to the left
of the cardia; (3) below the fundus is the large central
portion of the stomach known as the body; and (4) the
pylorus or antrum is the narrow inferior region that con-
nects with the duodenum of the small intestine via the
pyloric sphincter.
When there is no food in the stomach, the mucosa lies
in large folds called rugae (ROO-ghee), which are visible
with the unaided eye. As the stomach fills, the ru-gae
smooth out and disappear like an accordion when it is
extended with air. The mucosa of the stomach con-tains
many pits or gastric glands that have three kinds of
secreting cells: (1) the zymogenic (zye-moh-JEN-ik) or
chief cells secrete the principal gastric enzyme pep-
sinogen (pep-SIN-oh-gen); (2) the parietal (pah-RYE-
eh-tal) cells secrete hydrochloric acid, which activates the
pepsinogen to become pepsin, the enzyme that begins to
break down proteins; and (3) the mucous cells, which
secrete mucus that protects the stomach from being di-
gested. The secretions of these gastric glands collectively
are referred to as gastric juice (see Figure 16-9B). The
muscularis coat of the stomach has uniquely three, not just
two, layers of smooth muscle: an inner oblique, a middle
circular, and an outer longitudinal. These three layers allow
the stomach to contract in a variety of ways to break up
food into small pieces, churn it, and mix it with the gastric
juice. When the stomach is empty and this activity occurs,
we experience the stomach growling.
The main chemical activity of the stomach is to be-gin
the digestion of proteins by the enzyme pepsin. The protein
components of the stomach cells themselves are protected
from being digested by the mucus secreted by the mucous
cells. The stomach then empties all its