Page 954 - Fundamentals of anatomy physiology
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Chapter 24 The Digestive System 941
Figure 24–22 Major Hormones of the Duodenum.
Major Hormones of the Duodenum 24
Gastrin
Gastrin is secreted by G cells in the duodenum when they are exposed to
large quantities of incompletely digested proteins. The functions of gastrin
include promoting increased stomach motility and stimulating the production
of gastric acids and enzymes. (Gastrin is also produced by the stomach.)
Secretin
Secretin is released when chyme arrives in the duodenum. Secretin’s primary
effect is an increase in the secretion of bile (by the liver) and buffers (by the
pancreas), which in turn act to increase the pH of the chyme. Among its
secondary effects, secretin reduces gastric motility and secretory rates.
Gastric Inhibitory Peptide (GIP)
Gastric inhibitory peptide is secreted when fats and carbohydrates—
especially glucose—enter the small intestine. The inhibition of gastric activity
is accompanied by the stimulation of insulin release at the pancreatic islets.
GIP has several secondary effects, including stimulating duodenal gland
activity, stimulating lipid synthesis in adipose tissue, and increasing glucose
use by skeletal muscles.
Cholecystokinin (CCK)
Cholecystokinin is secreted when chyme arrives in the duodenum, especially
when the chyme contains lipids and partially digested proteins. In the
pancreas, CCK accelerates the production and secretion of all types of
digestive enzymes. It also causes a relaxation of the hepatopancreatic
sphincter and contraction of the gallbladder, resulting in the ejection of bile
and pancreatic juice into the duodenum. Thus, the net effects of CCK are to
increase the secretion of pancreatic enzymes and to push pancreatic
secretions and bile into the duodenum. The presence of CCK in high
concentrations has two additional effects: It inhibits gastric activity, and it
appears to have CNS effects that reduce the sensation of hunger.
Vasoactive Intestinal Peptide (VIP)
Vasoactive intestinal peptide stimulates the secretion of intestinal glands,
dilates regional capillaries, and inhibits acid production in the stomach. By
dilating capillaries in active areas of the intestinal tract, VIP provides an
efficient mechanism for removing absorbed nutrients.
Enterocrinin
Enterocrinin is released when chyme enters the duodenum. It stimulates
mucin production by the submucosal glands.
Intestinal Absorption Checkpoint
On average, it takes about five hours for materials to pass from 2 1. Name the three regions of the small intestine from
the duodenum to the end of the ileum. The first of the materi- proximal to distal.
als to enter the duodenum after you eat breakfast may leave the
small intestine at lunchtime. Along the way, the organ’s absorp- 2 2. How is the small intestine adapted for nutrient
tive effectiveness is enhanced by the fact that so much of the mu- absorption?
cosa is movable. The microvilli can be moved by their support-
ing microfilaments, the individual villi by smooth muscle cells, 23. Does a high-fat meal raise or lower the level of
groups of villi by the muscularis mucosae, and the circular folds cholecystokinin in the blood?
by the muscularis mucosae and the muscularis externa. These
movements stir and mix the intestinal contents, changing the en- 2 4. How would the pH of the intestinal contents be
vironment around each epithelial cell from moment to moment. affected if the small intestine did not produce secretin?
25. The digestion of which nutrient would be most
impaired by damage to the exocrine pancreas?
See the blue Answers tab at the back of the book.
