Digestion in the Stomach
The stomach, which is located just below the diaphragm,
stores food and begins digestion of proteins. With accordion like
folds and a very elastic wall, this organ can stretch to
accommodate about 2 L of food and fluid. The stomach secretes
a digestive fluid called gastric juice and mixes it with
the food through a churning action. This mixture of ingested
food and gastric juice is called chyme.
Chemical Digestion in the Stomach
Two components of gastric juice carry out chemical digestion.
One is hydrochloric acid (HCl), which disrupts the
extracellular matrix that binds cells together in meat and
plant material. The concentration of HCl is so high that the
pH of gastric juice is about 2, acidic enough to dissolve iron
nails (and to kill most bacteria). This low pH denatures (unfolds)
proteins in food, increasing exposure of their peptide
bonds. The exposed bonds are attacked by the second component
of gastric juice—a protease, or proteindigesting
enzyme, called pepsin. Unlike most enzymes, pepsin works
best in a very acidic environment. By breaking peptide
bonds, it cleaves proteins into smaller polypeptides. Further
digestion to individual amino acids occurs in the small
intestine.
Why doesn’t gastric juice destroy the stomach cells that
make it? The answer is that the ingredients of gastric juice
are kept inactive until they are released into the lumen (cavity)
of the stomach.
The components of gastric juice are produced by two
types of cells in the gastric glands of the stomach. Parietal
cells use an ATPdriven pump to expel hydrogen ions into
the lumen. At the same time, chloride ions diffuse into the lumen through specific membrane channels of the parietal
cells. It is therefore only within the lumen that hydrogen
and chloride ions combine to form HCl.
Meanwhile, chief cells release pepsin into the lumen
in an inactive form called pepsinogen. HCl
converts pepsinogen to active pepsin by
clipping off a small portion of the
molecule and exposing its active site.
Through these processes, both HCl
and pepsin form in the lumen of the
stomach, not within the cells of
the gastric glands.
After hydrochloric acid converts a small amount of pepsinogen
to pepsin, pepsin itself helps activate the remaining
pepsinogen. Pepsin, like HCl, can clip pepsinogen to expose the enzyme’s active site. This generates more pepsin, which activates more pepsinogen. This series of events is an example of positive feedback.
Why don’t HCl and pepsin eat through the lining of the stomach? For one thing, mucus secreted by cells in gastric glands protects against selfdigestion. In addition, cell division adds a new epithelial layer every three
days, replacing cells before they are fully eroded by digestive juices. Under certain circumstances, however, damaged areas of the stomach lining called gastric ulcers can appear.
It had been thought that they were caused by psychological stress and resulting excess acid secretion. However, Australian researchers Barry Marshall and Robin Warren discovered that infection by the acidtolerant bacterium Helicobacter pylori causes ulcers. They also demonstrated that an antibiotic could cure most gastric ulcers.
Stomach Dynamics
Chemical digestion by gastric juice is facilitated by the churning action of the stomach. This coordinated series of muscle contractions and relaxations mixes the stomach contents about every 20 seconds. As a result of mixing and enzyme action, what begins as a recently swallowed meal becomes the acidic, nutrientrich broth known as chyme.
Most of the time, sphincters close off the stomach at both ends. The sphincter between the esophagus and the stomach normally opens only when a bolus arrives. Occasionally, however, a person experiences acid reflux, a backflow of chyme from the stomach into the lower end of
the esophagus. The resulting irritation of the esophagus is commonly called “heartburn.” Peristaltic contractions typically empty the contents of
the stomach into the small intestine within 2–6 hours after a meal. The sphincter located where the stomach opens to the small intestine helps regulate passage into the small intestine, allowing only one squirt of chyme at a time.
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