872
UNIT 4
Maintenance of the Body
23
from excessive acidity and match the small intestine’s process-
ing abilities to the amount of chyme entering it.
If pushed beyond its capacity to accept chyme, the small in-
testine’s refusal is quick and recognizable—nausea and vom-
iting. Tis phenomenon, called the
dumping syndrome
, is an
unpleasant experience in many who have had their stomach
volume reduced in order to lose weight (see p. 943).
In addition, the factors we just named trigger the release
of several intestinal hormones, collectively called
enterogas-
trones
. Tey include
secretin
(se-kre
9
tin),
cholecystokinin
(CCK)
(ko
0
le-sis
0
to-ki
9
nin), and
vasoactive intestinal peptide
(VIP)
. All of these hormones inhibit gastric secretion when the
stomach is very active and also play other roles (see ±able 23.1).
intestinal mucosal cells to release
intestinal (enteric) gastrin
,
a hormone that encourages the gastric glands to continue their
secretory activity. Tis stimulatory effect is brief because as
the intestine distends with chyme containing large amounts of
H
1
, fats, partially digested proteins, and various irritating sub-
stances, the inhibitory component is triggered in the form of the
enterogastric reflex
.
Te enterogastric reflex is actually a trio of reflexes that (1)
inhibit the vagal nuclei in the medulla, (2) inhibit local re-
flexes, and (3) activate sympathetic fibers that cause the pyloric
sphincter to tighten and prevent further food entry into the
small intestine. As a result, gastric secretory activity declines.
Tese “brakes” on gastric activity protect the small intestine
Stomach lumen
Chief cell
Parietal cell
Interstitial
fluid
CO
2
Carbonic
anhydrase
Alkaline
tide
CO
2
+
H
2
O
H
2
CO
3
HCO
3
-
-
Cl
-
antiporter
Gastric gland
HCO
3
-
H
+
Cl
H
Cl
-
Cl
-
K
+
K
+
H
+
HCI
HCO
3
-
Blood
capillary
H
+
-K
+
ATPase
Figure 23.18
Mechanism of HCl secretion by parietal cells.
H
1
and HCO
3
2
(bicarbonate
ions) are generated from the dissociation of carbonic acid (H
2
CO
3
) within the parietal cell. As
H
1
-K
1
ATPase pumps H
1
into the lumen, K
1
enters the cell. Meanwhile, the HCO
3
2
-Cl
2
antiporter
transports HCO
3
2
into the interstitial space in exchange for chloride ions (Cl
2
), establishing the
alkaline tide. Cl
2
and K
1
then diffuse into the lumen through membrane channels.
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