was discussed as an endocrine gland in chap-
ter 13 (p. 509). It also has an exocrine function—secretion of
a digestive ﬂ
Structure of the Pancreas
The pancreas is closely associated with the small intestine
and is posterior to the parietal peritoneum. It extends hori-
zontally across the posterior abdominal wall, with its head
in the C-shaped curve of the duodenum (portion of the small
intestine) and its tail against the spleen (
f g. 17.23
ence plate 19).
The cells that produce pancreatic juice, called
atic acinar cells,
make up the bulk of the pancreas. These
cells form clusters called
(acinus, singular) around tiny
tubes into which they release their secretions. The smaller
tubes unite to form larger ones, which, in turn, give rise to
extending the length of the pancreas and
transporting pancreatic juice to the small intestine. The pan-
creatic duct usually connects with the duodenum at the same
place where the bile duct from the liver and gallbladder joins
the duodenum, although other connections may be present
gs. 13.34 and 17.23).
The pancreatic and bile ducts join at a short, dilated tube
(ampulla of Vater). A
band of smooth muscle, called the
(sphincter of Oddi), surrounds this ampulla.
Pancreatic juice contains enzymes that digest carbohydrates,
fats, proteins, and nucleic acids. The carbohydrate-digesting
splits molecules of starch or
glycogen into disaccharides. The fat-digesting enzyme,
breaks triglyceride molecules into fatty acids
and monoglycerides. (A monoglyceride molecule consists of
one fatty acid bound to glycerol.)
The protein-splitting (proteolytic) enzymes are
Each of these
enzymes splits the bonds between particular combinations
of amino acids in proteins. No single enzyme can split all
possible amino acid combinations, so several enzymes are
necessary to completely digest protein molecules.
The proteolytic enzymes are stored in tiny cellular struc-
These enzymes, like gastric
pepsin, are secreted in inactive forms and must be activated
by other enzymes after they reach the small intestine. ±or
example, the pancreatic cells release inactive
activated to trypsin when it contacts the enzyme
which the mucosa of the small intestine secretes.
Chymotrypsin and carboxypeptidase are activated, in turn,
by trypsin. This mechanism prevents enzymatic digestion of
proteins in the secreting cells and the pancreatic ducts.
diaphragm so it presses downward over the stomach, and
contracting the abdominal wall muscles to increase pres-
sure inside the abdominal cavity. As a result, the stomach is
squeezed from all sides, forcing its contents upward and out
through the esophagus, pharynx, and mouth.
Drugs (emetics), toxins in contaminated foods, and rapid
changes in body motion stimulate activity in the vomiting
center. With changes in motion, sensory impulses from the
labyrinths of the inner ears reach the vomiting center and
can produce motion sickness. The vomiting center can also
be activated by stimulation of higher brain centers through
sights, sounds, odors, tastes, emotions, or mechanical stimu-
lation of the back of the pharynx.
emanates from activity in the vomiting center or
in nerve centers near it. During nausea, stomach movements
usually are diminished or absent, and duodenal contents
may move back into the stomach.
How is chyme produced?
What factors inF
uence how quickly chyme leaves the stomach?
Describe the enterogastric reF
Describe the vomiting reF
Which factors may stimulate the vomiting reF
in stomach wall
fills with chyme
The enterogastric reF
ex partially regulates the rate
at which chyme leaves the stomach.