The Digestion and Absorption of Food
547
activity of the parasympathetic nerves to the stomach’s enteric
nervous system results in the release of ACh from the plexus
neurons, gastrin from the gastrin-releasing cells, and hista-
mine from ECL cells (
Figure 15–20
).
Once food has reached the stomach, the gastric phase
stimuli—distension from the volume of ingested material and
the presence of peptides and amino acids released by the diges-
tion of luminal proteins—produce a further increase in acid
secretion. These stimuli use some of the same neural pathways
used during the cephalic phase. Nerve endings in the mucosa of
the stomach respond to these luminal stimuli and send action
potentials to the enteric nervous system, which in turn, can
relay signals to the gastrin-releasing cells, histamine-releasing
cells, and parietal cells. In addition, peptides and amino acids
can act directly on the gastrin-releasing endocrine cells to pro-
mote gastrin secretion.
The concentration of acid in the gastric lumen is itself
an important determinant of the rate of acid secretion for the
following reason. Hydrogen ions (acid) stimulate the release
of somatostatin from endocrine cells in the gastric wall.
Somatostatin then acts on the parietal cells to inhibit acid
secretion; it also inhibits the release of gastrin and histamine.
The net result is a negative feedback control of acid secretion.
As the contents of the gastric lumen become more acidic, the
stimuli that promote acid secretion decrease.
Carbonic anhydrase
Epithelial cell
Capillary
Stomach
lumen
H
2
O
H
2
CO
3
H
+
H
+
H
+
K
+
K
+
K
+
Cl
CO
2
+
H
2
O
OH
Cl
Cl
Cl
HCO
3
ADP
ATP
HCO
3
Figure 15–18
Secretion of hydrochloric acid by parietal cells. The hydrogen ions secreted into the lumen by primary active transport are derived from the
breakdown of water molecules, leaving hydroxyl ions (OH
) behind. These hydroxyl ions are neutralized by combination with other hydrogen
ions generated by the reaction between carbon dioxide and water, a reaction catalyzed by the enzyme carbonic anhydrase, which is present in
high concentrations in parietal cells. The bicarbonate ions formed by this reaction move out of the parietal cell on the blood side in exchange
for chloride ions.
Figure 15–19
The four inputs to parietal cells that regulate acid secretion by
generating second messengers. These second messengers control
the transfer of the H
+
/K
+
-ATPase pumps in cytoplasmic vesicle
membranes to the plasma membrane.
Parietal cell
H
+
Acid secretion
Somatostatin
Second messengers
+
+
+
Gastrin
Histamine
ACh
H
+
/K
+
–ATPase
Figure 15–18
physiological
inquiry
Why doesn’t the high concentration of hydrogen ions in the stomach lumen destroy the lining of the stomach wall?
Answer can be found at end of chapter.
previous page 575 Vander's Human Physiology The Mechanisms of Body Function read online next page 577 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off