The Digestion and Absorption of Food
549
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
closes
the
pyloric sphincter,
a ring of smooth muscle and connec-
tive tissue between the antrum and the duodenum (
Figure
15–22
). 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
reached (
Figure 15–23
) 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.
Figure 15–21
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
12
in the small intestine.
Pepsinogen
Stomach lumen
HCl
Pepsin
Protein
Peptides
Intrinsic
factor
Parietal cell
Chief cell
Stomach wall
Table 15–5
Control of HCL Secretion During a Meal
Stimuli
Pathways
Result
Cephalic phase
Sight
Smell
Taste
Chewing
Parasympathetic nerves to enteric
nervous system
HCl secretion
Gastric contents (gastric phase)
Distension
Peptides
H
+
concentration
Long and short neural refl exes and direct stimulation
of gastrin secretion
HCl secretion
Intestinal contents (intestinal phase)
Distension
H
+
concentration
Osmolarity
Nutrient concentrations
Long and short neural refl exes; secretin,
CCK, and
other duodenal hormones
HCl secretion
previous page 577 Vander's Human Physiology The Mechanisms of Body Function read online next page 579 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off