The Digestion and Absorption of Food
As in the esophagus, the stomach produces peristaltic
waves in response to the arriving food. Each wave begins in
the body of the stomach and produces only a ripple as it pro-
ceeds toward the antrum, a contraction too weak to produce
much mixing of the luminal contents with acid and pepsin.
As the wave approaches the larger mass of wall muscle sur-
rounding the antrum, however, it produces a more powerful
contraction, which both mixes the luminal contents and
a ring of smooth muscle and connec-
tive tissue between the antrum and the duodenum (
). The pyloric sphincter muscles contract upon arrival
of a peristaltic wave. As a consequence of the sphincter clos-
ing, only a small amount of chyme is expelled into the duo-
denum with each wave. Most of the antral contents are forced
backward toward the body of the stomach, thereby contribut-
ing to the mixing activity in the antrum. Recall that the lower
esophageal sphincter prevents this retrograde movement of
stomach contents from entering the esophagus.
What is responsible for producing gastric peristaltic waves?
Their rhythm (three per minute) is generated by pacemaker
cells in the longitudinal smooth muscle layer. These smooth
muscle cells undergo spontaneous depolarization-repolariza-
tion cycles (slow waves) known as the
basic electrical rhythm
of the stomach. These slow waves are conducted through gap
junctions along the stomach’s longitudinal muscle layer and
also induce similar slow waves in the overlying circular mus-
cle layer. In the absence of neural or hormonal input, how-
ever, these depolarizations are too small to cause signiﬁ cant
contractions. Excitatory neurotransmitters and hormones act
upon the smooth muscle to further depolarize the membrane,
thereby bringing it closer to threshold. Action potentials may
be generated at the peak of the slow wave cycle if threshold is
) and may thus cause larger contrac-
tions. The number of spikes ﬁ red with each wave determines
the strength of the muscle contraction.
Thus, whereas the frequency of contraction is determined
by the intrinsic basic electrical rhythm and remains essentially
constant, the force of contraction, and therefore the amount
of gastric emptying per contraction, are determined reﬂ exly by
neural and hormonal input to the antral smooth muscle.
Conversion of pepsinogen to pepsin in the lumen of the stomach.
An increase in HCl acidiﬁ es the stomach contents. High acidity (low
pH) cleaves pepsin from pepsinogen. The pepsin thus formed also
catalyzes its own production by acting on additional molecules of
pepsinogen. The parietal cells also secrete intrinsic factor, which is
needed to absorb vitamin B
in the small intestine.
Control of HCL Secretion During a Meal
Parasympathetic nerves to enteric
Gastric contents (gastric phase)
Long and short neural reﬂ exes and direct stimulation
of gastrin secretion
Intestinal contents (intestinal phase)
Long and short neural reﬂ exes; secretin,
other duodenal hormones