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Chapter 15
c. Small peptides consisting of two to three amino acids can
be actively absorbed into epithelial cells and then broken
down to amino acids, which are released into the blood.
III. The digestion and absorption of fat by the small intestine
requires mechanisms that solubilize the fat and its digestion
products.
a. Large fat globules leaving the stomach are emulsifi ed in the
small intestine by bile salts and phospholipids secreted by
the liver.
b. Lipase from the pancreas digests fat at the surface of the
emulsion droplets, forming fatty acids and monoglycerides.
c. These water-insoluble products of lipase action, when
combined with bile salts, form micelles, which are in
equilibrium with the free molecules.
d. Free fatty acids and monoglycerides diffuse across the
luminal membranes of epithelial cells, where they are
enzymatically recombined to form triglycerides, which are
released as chylomicrons from the blood side of the cell by
exocytosis.
e. The released chylomicrons enter lacteals in the intestinal
villi and pass, by way of the lymphatic system and the
thoracic duct, to the venous blood returning to the heart.
IV. Fat-soluble vitamins are absorbed by the same pathway used
for fat absorption. Most water-soluble vitamins are absorbed in
the small intestine by diffusion or mediated transport. Vitamin
B
12
is absorbed in the ileum by endocytosis after combining
with intrinsic factor secreted into the lumen by parietal cells in
the stomach.
V. Water is absorbed from the small intestine by osmosis
following the active absorption of solutes, primarily sodium
chloride.
How are Gastrointestinal Processes Regulated?
I. Most gastrointestinal refl exes are initiated by luminal stimuli:
(a) distension, (b) osmolarity, (c) acidity, and (d) digestion
products.
a. Neural refl exes are mediated by short refl exes in the enteric
nervous system and by long refl exes involving afferent and
efferent neurons to and from the CNS.
b. Endocrine cells scattered throughout the epithelium of
the stomach secrete gastrin, and cells in the small intestine
secrete secretin, CCK, and GIP. Table 15–4 lists the
properties of these hormones.
c. The three phases of gastrointestinal regulation—cephalic,
gastric, and intestinal—are named for the location of the
stimulus that initiates the response.
II. Chewing breaks up food into particles suitable for swallowing,
but it is not essential for the eventual digestion and absorption
of food.
III. Salivary secretion is stimulated by food in the mouth acting
refl exly via chemoreceptors and pressure receptors and by
sensory stimuli (e.g., sight or smell of food). Both sympathetic
and parasympathetic stimulation increase salivary secretion.
IV. Food moved into the pharynx by the tongue initiates
swallowing, which is coordinated by the swallowing center in
the brainstem medulla oblongata.
a. Food is prevented from entering the trachea by inhibition of
respiration and by closure of the glottis.
b. The upper esophageal sphincter relaxes as food is moved
into the esophagus, and then the sphincter closes.
c. Food is moved through the esophagus toward the stomach
by peristaltic waves. The lower esophageal sphincter remains
open throughout swallowing.
d. If food does not reach the stomach with the fi rst
peristaltic wave, distension of the esophagus initiates
secondary peristalsis.
V. Table 15–5 summarizes the factors controlling acid secretion
by parietal cells in the stomach.
VI. Pepsinogen, secreted by the gastric chief cells in response
to most of the same refl exes that control acid secretion, is
converted to the active proteolytic enzyme pepsin in the
stomach’s lumen, primarily by acid.
VII. Peristaltic waves sweeping over the stomach become stronger
in the antrum, where most mixing occurs. With each wave,
only a small portion of the stomach’s contents are expelled
into the small intestine through the pyloric sphincter.
a. Cycles of membrane depolarization, the basic electrical
rhythm generated by gastric smooth muscle, determine
gastric peristaltic wave frequency. Contraction strength
can be altered by neural and hormonal changes in
membrane potential, which is imposed on the basic
electrical rhythm.
b. Distension of the stomach increases the force of
contractions and the rate of emptying. Distension of the
small intestine, and fat, acid, or hypertonic solutions in
the intestinal lumen inhibit gastric contractions.
VIII. The exocrine portion of the pancreas secretes digestive
enzymes and bicarbonate ions, all of which reach the
duodenum through the pancreatic duct.
a. The bicarbonate ions neutralize acid entering the small
intestine from the stomach.
b. Most of the proteolytic enzymes, including trypsin, are
secreted by the pancreas in inactive forms. Trypsin is
activated by enterokinase located on the membranes of the
small intestine cells; trypsin then activates other inactive
pancreatic enzymes.
c. The hormone secretin, released from the small intestine
in response to increased luminal acidity, stimulates
pancreatic bicarbonate secretion. The small intestine
releases CCK in response to the products of fat and
protein digestion. CCK then stimulates pancreatic enzyme
secretion.
d. Parasympathetic stimulation increases pancreatic
secretion.
IX. The liver secretes bile, the major ingredients of which are bile
salts, cholesterol, lecithin, bicarbonate ions, bile pigments,
and trace metals.
a. Bile salts undergo continuous enterohepatic recirculation
during a meal. The liver synthesizes new bile salts to
replace those lost in the feces.
b. The greater the bile salt concentration in the hepatic
portal blood, the greater the rate of bile secretion.
c. Bilirubin, the major bile pigment, is a breakdown product
of hemoglobin and is absorbed from the blood by the liver
and secreted into the bile.
d. Secretin stimulates bicarbonate secretion by the cells
lining the bile ducts in the liver.
e. Bile is concentrated in the gallbladder by the absorption
of NaCl and water.
f. Following a meal, the release of CCK from the small
intestine causes the gallbladder to contract and the
sphincter of Oddi to relax, thereby injecting concentrated
bile into the intestine.
X. In the small intestine, the digestion of polysaccharides and
proteins increases the osmolarity of the luminal contents,
producing water fl ow into the lumen.
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