672
UNIT FIVE
phases of gastric secretion, parasympathetic impulses stim-
ulate the pancreas to release digestive enzymes. A peptide
hormone,
secretin,
stimulates the pancreas to secrete abun-
dant fl uid when acidic chyme enters the duodenum. Secretin
is released into the blood from the duodenal mucous mem-
brane in response to the acid in chyme. The pancreatic juice
secreted at this time contains few, if any, digestive enzymes
but has a high concentration of bicarbonate ions. These ions
neutralize the acid in chyme
(f
g. 17.24)
.
Proteins and fats in chyme in the duodenum also stimulate
the release of
cholecystokinin
from the intestinal wall. As in the
case of secretin, cholecystokinin reaches the pancreas in the
bloodstream. Pancreatic juice secreted in response to chole-
cystokinin has a high concentration of digestive enzymes.
PRACTICE
32
Where is the pancreas located?
33
List the enzymes in pancreatic juice.
34
What are the functions of the enzymes in pancreatic juice?
35
What regulates secretion of pancreatic juice?
Painful
acute pancreatitis
results from blockage in the release of pancre-
atic juice. Trypsin, activated as pancreatic juice builds up, digests parts
of the pancreas. Alcoholism, gallstones, certain infections, traumatic
injuries, or the side eF
ects of some drugs can cause pancreatitis.
Pancreatic juice contains two types of
nucleases,
which
are enzymes that break down nucleic acid molecules into
nucleotides. A high concentration of bicarbonate ions makes
the juice alkaline, which provides a favorable environment
for the actions of the digestive enzymes and helps neutral-
ize acidic chyme as it arrives from the stomach. The alkaline
environment in the small intestine also blocks the action of
pepsin, which might otherwise damage the duodenal wall.
Regulation of Pancreatic Secretion
The nervous and endocrine systems regulate release of pan-
creatic juice, much as they regulate gastric and small intesti-
nal secretions. For example, during the cephalic and gastric
Right hepatic duct
Left hepatic duct
Cystic duct
Common hepatic duct
Bile duct
Bile duct
Pancreatic
duct
Gallbladder
Minor duodenal
papilla
Major duodenal
papilla
Sphincter muscles
Head of pancreas
Duodenum
Major duodenal papilla
Intestinal lumen
Pancreatic duct
Tail of pancreas
Hepatopancreatic
ampulla
Hepatopancreatic
sphincter
Pyloric sphincter
FIGURE 17.23
The pancreas is closely associated with
the duodenum.
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