658 Chapter 18
2. The exocrine acini of the pancreas produce pancreatic
juice, which contains various digestive enzymes and
bicarbonate.18.6 Regulation of the Digestive System 645
A. The regulation of gastric function occurs in three phases.
- In the cephalic phase, the activity of higher brain
centers, acting via the vagus nerves, stimulates gastric
juice secretion. - In the gastric phase, the secretion of HCl and pepsin is
controlled by the gastric contents and by the hormone
gastrin, secreted by the gastric mucosa. - In the intestinal phase, the activity of the stomach is
inhibited by neural reflexes and hormonal secretion
from the duodenum.
B. Intestinal function is regulated, at least in part, by short,
local reflexes coordinated by the enteric nervous system. - The enteric nervous system contains interneurons,
intrinsic sensory neurons, and autonomic motor neurons. - Peristalis is coordinated by the enteric nervous system,
which produces smooth muscle contraction above the
bolus and relaxation below the bolus of chyme. - Short reflexes include the gastroileal reflex, ileogastric
reflex, and intestino-intestinal reflexes.
C. The secretion of the hormones secretin and cholecystokinin
(CCK) regulates pancreatic juice and bile secretion. - The secretion of secretin is stimulated by the arrival of
acidic chyme into the duodenum. - CCK secretion is stimulated by the presence of fat in the
chyme arriving in the duodenum. - Contraction of the gallbladder occurs in response to a neural
reflex and to the secretion of CCK by the duodenum.
D. Gastrointestinal hormones may be needed for the mainte-
nance of the GI tract and accessory digestive organs.
18.7 Digestion and Absorption of Food 650
A. The digestion of starch begins in the mouth through the
action of salivary amylase.
1. Pancreatic amylase digests starch into disaccharides and
short-chain oligosaccharides.
2. Complete digestion into monosaccharides is
accomplished by brush border enzymes.
B. Protein digestion begins in the stomach through the action of
pepsin.
1. Pancreatic juice contains the protein-digesting enzymes
trypsin and chymotrypsin, among others.
2. The brush border contains digestive enzymes that help
to complete the digestion of proteins into amino acids.
3. Amino acids, like monosaccharides, are absorbed and
secreted into capillary blood entering the portal vein.
C. Lipids are digested in the small intestine after being emulsi-
fied by bile salts.
1. Free fatty acids and monoglycerides enter particles
called micelles, formed in large part by bile salts, and
they are absorbed in this form or as free molecules.
2. Once inside the mucosal epithelial cells, these subunits
are used to resynthesize triglycerides.
3. Triglycerides in the epithelial cells, together with
proteins, form chylomicrons, which are secreted into the
central lacteals of the villi.
4. Chylomicrons are transported by lymph to the thoracic
duct and from there enter the blood.Test Your Knowledge
- Which of these statements about intrinsic factor is true?
a. It is secreted by the stomach.
b. It is a polypeptide.
c. It promotes absorption of vitamin B 12 in the intestine.
d. It helps prevent pernicious anemia.
e. All of these are true.- Intestinal enzymes such as lactase are
a. secreted by the intestine into the chyme.
b. produced by the intestinal crypts (of Lieberkühn).
c. produced by the pancreas.
d. attached to the cell membrane of microvilli in the
epithelial cells of the mucosa.- Which of these statements about gastric secretion of HCl is
false?
a. HCl is secreted by parietal cells.
b. HCl hydrolyzes peptide bonds.
REVIEW ACTIVITIES
c. HCl is needed for the conversion of pepsinogen to pepsin.
d. HCl is needed for maximum activity of pepsin.- Most digestion occurs in
a. the mouth.
b. the stomach.
c. the small intestine.
d. the large intestine. - Which of these statements about trypsin is true?
a. Trypsin is derived from trypsinogen by the digestive
action of pepsin.
b. Active trypsin is secreted into the pancreatic acini.
c. Trypsin is produced in the crypts (of Lieberkühn).
d. Trypsinogen is converted to trypsin by the brush border
enzyme enterokinase.