Chapter 23
The Digestive System
883
23
in food digestion. Te hormones include cholecystokinin and
secretin.
Bile salts themselves are the major stimulus for enhanced bile
secretion
(Figure 23.28)
. AFer a fatty meal, when the entero-
hepatic circulation is returning large amounts of bile salts to the
liver, its output of bile rises dramatically.
Secretin
, released by
intestinal cells exposed to fatty chyme, also stimulates liver cells
to secrete bile.
When no digestion is occurring, the hepatopancreatic
sphincter (guarding the entry of bile and pancreatic juice into
the duodenum) is closed and the released bile backs up the
cystic duct into the gallbladder, where it is stored until needed.
Although the liver makes bile continuously, bile does not usu-
ally enter the small intestine until the gallbladder contracts.
Te major stimulus for gallbladder contraction is
cholecysto-
kinin
, which is released to the blood when acidic, fatty chyme
enters the duodenum (±igure 23.28). Besides causing the gall-
bladder to contract, CCK (1) stimulates secretion of pancreatic
juice and (2) relaxes the hepatopancreatic sphincter so that bile
and pancreatic juice can enter the duodenum.
Both secretin and cholecystokinin prompt the pancreas to
secrete its juice. Secretin, released in response to the presence
of HCl in the intestine, mainly targets the pancreatic duct cells,
containing the inactive digestive enzymes (proenzymes) they
manufacture.
Scattered amid the acini are the lightly staining
pancreatic is-
lets
. Tese mini-endocrine glands release insulin and glucagon,
hormones that play an important role in carbohydrate metabo-
lism (see Chapter 16).
Composition of Pancreatic Juice
Approximately 1200 to 1500 ml of clear pancreatic juice is pro-
duced daily. It consists mainly of water, and contains enzymes
and electrolytes (primarily bicarbonate ions). Te acinar cells
produce the enzyme-rich component of pancreatic juice. Te
epithelial cells lining the smallest pancreatic ducts release the
bicarbonate ions that make it alkaline (about pH 8).
Normally, the amount of HCl produced in the stomach is ex-
actly balanced by the amount of bicarbonate (HCO
3
2
) secreted
by the pancreas, and as HCO
3
2
is secreted into the pancreatic
juice, H
1
enters the blood. Consequently, the pH of venous
blood returning to the heart remains relatively unchanged be-
cause acidic blood draining from the pancreas neutralizes the
alkaline blood draining from the stomach.
Te high pH of pancreatic fluid helps neutralize acid chyme
entering the duodenum and provides the optimal environ-
ment for intestinal and pancreatic enzymes. Like pepsin of the
stomach, pancreatic
proteases
(protein-digesting enzymes) are
produced and released in inactive forms, which are activated
in the duodenum, where they do their work. Tis protects the
pancreas from digesting itself.
±or example, within the duodenum,
enteropeptidase
(for-
merly called
enterokinase
), an intestinal brush border protease,
activates
trypsinogen
to
trypsin
. ²rypsin, in turn, activates
more trypsinogen and two other pancreatic proteases (
pro-
carboxypeptidase
and
chymotrypsinogen
) to their active forms,
carboxypeptidase
(kar-bok
0
se-pep
9
tĭ-dās) and
chymotrypsin
(ky
0
mo-trip
9
sin), respectively
(Figure 23.27)
.
Other pancreatic enzymes—
amylase
,
lipases
, and
nucleases
are secreted in active form, but require that ions or bile be present
in the intestinal lumen for optimal activity.
Check Your Understanding
37.
What do zymogen granules contain?
38.
What is the functional difference between pancreatic acini
and islets?
For answers, see Appendix H.
Regulation of Bile and Pancreatic
Secretion and Entry into
the Small Intestine
Describe how bile and pancreatic juice secretion into the
small intestine are regulated.
Te same factors—hormones and neural stimuli—that regu-
late the secretion of bile and pancreatic juice also regulate their
release to the small intestine. Tere they “do their business”
Stomach
Pancreas
Epithelial
cells
Trypsinogen
(inactive)
Chymotrypsinogen
(inactive)
Procarboxypeptidase
(inactive)
Trypsin
Chymotrypsin
Carboxypeptidase
Membrane-bound
enteropeptidase
Figure 23.27
Activation of pancreatic proteases in the small
intestine.
Pancreatic proteases are secreted in an inactive form and
are activated in the duodenum.
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