884
UNIT 4
Maintenance of the Body
23
three- to six-hour journey through the small intestine, and it
is here that most of the water and virtually all nutrients are ab-
sorbed. Like the stomach, the small intestine plays no part in
ingestion or defecation.
Requirements for Optimal Intestinal
Digestive Activity
Although a primary function of the small intestine is to break
down chyme chemically, intestinal juice provides little of what
is needed to perform this function. Most of the substances re-
quired for digestion—bile, digestive enzymes (except for the
brush border enzymes), and bicarbonate ions (to provide the
proper pH for enzymatic catalysis)—are
imported
from the liver
and pancreas. Hence, anything that impairs liver or pancreatic
function or delivery of their juices to the small intestine severely
hinders our ability to digest food and absorb nutrients. Te
other primary function of the small intestine—absorption—is
efficiently accomplished by its absorptive cells with their rich
crop of apical microvilli.
Optimal digestive activity in the small intestine also depends
on a slow, measured delivery of chyme from the stomach. Why
is this so? Entering chyme is usually hypertonic. For this reason,
if large amounts of chyme were rushed into the small intestine,
resulting in a watery
bicarbonate-rich
pancreatic juice. CCK, re-
leased in response to proteins and fats in chyme, stimulates the
acini to release
enzyme-rich
pancreatic juice, and potentiates
the effect of secretin. ±able 23.1 summarizes CCK and the other
digestive hormones and paracrines (p. 868).
Parasympathetic impulses delivered by vagus nerve fibers are
a minor stimulus for gallbladder contraction. Vagal stimulation
also prompts the release of pancreatic juice, mainly during the
cephalic and gastric phases of gastric secretion.
Check Your Understanding
39.
What is the makeup of the fluid in the pancreatic duct? In
the cystic duct? In the bile duct?
40.
What stimulates CCK release and what are its effects on the
digestive process?
For answers, see Appendix H.
Digestive Processes in the Small Intestine
By the time food reaches the small intestine it is unrecognizable,
but still far from being digested. Carbohydrates and proteins are
partially degraded, but fat digestion has only begun. Te pro-
cess of digestion accelerates during the chyme’s tortuous
1
2
3
4
5
6
Chyme entering
duodenum causes duodenal
enteroendocrine cells to
release cholecystokinin
(CCK) and secretin.
CCK (red dots) and
secretin (yellow dots) enter
the bloodstream.
CCK induces secretion of
enzyme-rich pancreatic juice.
Secretin causes secretion of
HCO
3
-
-rich pancreatic juice.
Bile salts and, to a lesser
extent, secretin transported
via bloodstream stimulate
liver to produce bile more
rapidly.
CCK (via bloodstream)
causes gallbladder to contract
and hepatopancreatic
sphincter to relax. Bile enters
duodenum.
During cephalic and
gastric phases, vagal nerve
stimulates gallbladder to
contract weakly.
CCK secretion
Secretin secretion
Figure 23.28
Mechanisms promoting
secretion and release of bile and
pancreatic juice.
When digestion is not
occurring, bile is stored and concentrated
in the gallbladder. Acidic fatty chyme
entering the small intestine initiates
several mechanisms that accelerate the
output of pancreatic juice and bile, and
cause the gallbladder to contract and the
hepatopancreatic sphincter to relax. This
allows bile and pancreatic juice to enter the
small intestine. The single most important
stimulus for bile secretion is an increased level
of bile salts in the enterohepatic circulation.
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