The Stomach Begins
Digestion in Earnest
The stomach receives multiple boluses of food swal-
lowed from the mouth during a meal. Think about
how many bites you would take to consume that gen-
erous breakfast. Because you eat and swallow faster
than you can digest, the stomach serves as a storage
site for ingested food. The
fu n d u s
and
body
regions
contain folds of mucosa (rugae) that can expand to
accommodate the volume of your meal. The
pylorus
region is the site of most of the gastric digestion.
The lining of the stomach is composed of a thick
mucosa embedded with numerous gastric pits, where
various secretory cells lie (Figure 14.5). The presence
of food inside the stomach stimulates the secretion of
gastric juices, which are composed of mucus, hydro-
chloric acid (HCl), and digestive enzymes. Numerous
peristaltic contractions from the fundus to the pylorus
of the stomach mix the gastric juices with your break-
fast and reduce it to a thick liquid called chyme (KlM).
The hydrochloric acid secreted into the chyme kills bac-
teria and activates the enzyme pepsin, which breaks
down those sausage and egg proteins into peptides (see
Chapter 2). Gastric juice contains another enzyme
called gastric lipase, which breaks down the generous
quantity of lipids included in your morning meal.
Why doesn’t hydrochloric acid burn a hole in
your stomach? There are three reasons: •
Hydrochloric acid (HCl) secretion occurs only
when there is food present in the stomach.
The chyme dilutes the acid.
The thick layer of mucus covering the mucosa
protects the underlying stomach tissues.
In addition to mixing, secreting, and storing, the
stomach can also absorb some substances. For exam-
ple, mucous cells within the gastric pits absorb some
water, ions, short-chain fatty acids, and some drugs
(for example, aspirin, alcohol, ibuprofen) from the
stomach’s lumen.
Finally, the stomach empties its contents into the
small intestine in a process called gastric emptying.
The peristaltic motions of the stomach muscles pro-
pel small bursts of chyme through the partially closed
pyloric sphincter into the duodenum (first section)
of the small intestine. Once a burst of chyme passes
through it, the pyloric sphincter closes, and reflexes
InSight
T h e s t o m a c h
F ig u r e 1 4 . 5
The stomach is a muscular pouch where food is
stored and digestion begins. The mucosa is thicker
in the stomach than in other areas of the digestive
system. Numerous gastric pits compose the stom-
ach lining. Within these gastric pits, various cells
produce secretions to assist the digestion process.
Esophagus
Fundus
Serosa
Muscularis:
Longitudinal
layer
Circular
layer
Oblique
layer
Rugae
— j-----Duodenum (first portion
of the small intestine)
Anterior view of the stomach
may slow the peristaltic movements and the exit of chyme. The
process continues until the stomach is empty (about 1-2 hours
after you finish a meal). Various foods spend different amounts
of time in the stomach. Fat-rich meals spend more time in your
stomach than protein-rich ones, which in turn hang out longer in
this part of the digestive system than carbohydrate-rich meals.
406 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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