671
CHAPTER SEVENTEEN
Digestive System
17.7
PANCREAS
The
pancreas
was discussed as an endocrine gland in chap-
ter 13 (p. 509). It also has an exocrine function—secretion of
a digestive fl
uid called
pancreatic juice
(pan
kre-at
ik jo
o
s).
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
and refer-
ence plate 19).
The cells that produce pancreatic juice, called
pancre-
atic acinar cells,
make up the bulk of the pancreas. These
cells form clusters called
acini
(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
a
pancreatic duct
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
(see F
gs. 13.34 and 17.23).
The pancreatic and bile ducts join at a short, dilated tube
called the
hepatopancreatic ampulla
(ampulla of Vater). A
band of smooth muscle, called the
hepatopancreatic sphinc-
ter
(sphincter of Oddi), surrounds this ampulla.
Pancreatic Juice
Pancreatic juice contains enzymes that digest carbohydrates,
fats, proteins, and nucleic acids. The carbohydrate-digesting
enzyme,
pancreatic amylase,
splits molecules of starch or
glycogen into disaccharides. The fat-digesting enzyme,
pan-
creatic lipase,
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
trypsin,
chymotrypsin,
and
carboxypeptidase.
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-
tures called
zymogen granules.
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
trypsinogen,
activated to trypsin when it contacts the enzyme
enteroki-
nase,
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.
Nausea
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.
PRACTICE
27
How is chyme produced?
28
What factors inF
uence how quickly chyme leaves the stomach?
29
Describe the enterogastric reF
ex.
30
Describe the vomiting reF
ex.
31
Which factors may stimulate the vomiting reF
ex?
Vagus
nerve
Nerve impulses
inhibit peristalsis
in stomach wall
Duodenum
fills with chyme
Sensory stretch
receptors are
stimulated
Sensory nerve
impulses travel
to central
nervous system
From CNS
To CNS
4
1
2
3
FIGURE 17.22
The enterogastric reF
ex partially regulates the rate
at which chyme leaves the stomach.
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