The Digestive System
in food digestion. Te hormones include cholecystokinin and
Bile salts themselves are the major stimulus for enhanced bile
. 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.
, 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
, 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
Scattered amid the acini are the lightly staining
. 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
by the pancreas, and as HCO
is secreted into the pancreatic
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
(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,
), an intestinal brush border protease,
. ²rypsin, in turn, activates
more trypsinogen and two other pancreatic proteases (
) to their active forms,
Other pancreatic enzymes—
are secreted in active form, but require that ions or bile be present
in the intestinal lumen for optimal activity.
Check Your Understanding
What do zymogen granules contain?
What is the functional difference between pancreatic acini
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”
Activation of pancreatic proteases in the small
Pancreatic proteases are secreted in an inactive form and
are activated in the duodenum.