Download Gastrointestinal Physiology – Part 2

Gastrointestinal Physiology –
Part 2
11/04
Digestive secretions: saliva
• Functions of saliva in non-ruminants:
– Lubricates food to facilitate swallowing
– Contains antibodies and lysozyme
– Some amylase in saliva of swine and rats, but
much less than in humans
• Functions of saliva in ruminants:
– Maintains fluid consistency of rumen
– Slightly alkaline; helps neutralize acids formed
by fermentation
– May help prevent frothing in rumen
Gastric pits in
gastric
mucosa
• Gastric pits
are openings
to ducts into
which gastric
glands empty
their
secretions
Exocrine gland
• Any gland that
directs its
secretions through
a duct into a
compartment that
is contiguous with
the exterior of the
body
Cardiac
Pyloric
Fundic
Glandular
regions of
simple
stomach
Glandular regions
of equine, porcine
and ruminant
stomachs
• Cardiac, fundic
and pyloric gland
regions are
glandular
• Esophageal
regions (“E”) are
non-glandular
epithelium
Cardiac
Pyloric
Fundic
Cardiac
gland
region
(cardiac
mucosa)
Cardiac
Pyloric
Fundic
Fundic
gland
region
• Mucous
neck cells
• Parietal
cells
• Chief cells
Fundic gland
region
• Mucous neck
cells secrete
mucus
• Parietal cells
secrete:
– HCl
– Intrinsic factor
(not in cats);
necessary for
absorption of
Vitamin B12
Fundic gland
region
• Chief cells
secrete
pepsinogen
– HCl converts
pepsinogen
into pepsin
– Pepsin is a
proteolytic
enzyme
(breaks down
proteins into
peptides)
Cardiac
Pyloric
Fundic
Pyloric
gland
region
• Mucous
secreting
cells
• G cells
Pyloric gland
region
• G cells are
endocrine gland
cells – secrete the
endocrine hormone,
gastrin
• Endocrine cells
secrete hormones
into the bloodstream
– travel to a distant
part of the body
where they produce
an effect on another
cell type
Gastrin
may
interact
with
either
receptor
Substances that
stimulate HCl
secretion by
parietal cell
• Gastrin
• Acetylcholine
• Histamine
– Histamine secreted
by ECL cells in
gastric mucosa
Rennin
• Enzyme secreted by abomasal mucosa of
young ruminant
– Acts to coagulate milk protein, which
facilitates its digestion
Hormone secretions of small
intestine: cholecystokinin
Site of
secretion
Stimuli for
secretion
Primarily
duodenal
mucosa
•Inhibits gastric
Chyme with a:
emptying
•High amino
•↑ secretion of
acid
pancreatic enzymes
concentration
and HCO3•High fatty acid
•Stimulates gall
concentration
bladder contractions
•Low pH
Actions
Hormone secretions of the small
intestine: secretin
Site of secretion Stimuli for
secretion
Actions
Duodenal
mucosa
•↓ HCl
production in
stomach
•↑ pancreatic
HCO3- secretion
•↑ biliary HCO3secretion
Chyme with a:
•Low pH
•High fatty acid
concentration
Exocrine pancreatic secretions
• Liquid rich in
HCO3• Pancreatic
digestive
enzymes
• Both are
secreted into
duodenum
Pancreatic digestive enzymes
• Pancreatic
proteolytic
enzymes
(pancreatic
proteases) are
secreted as
proenzymes
(zymogens)
Activation of
pancreatic
proteases in
small intestine
• CCK stimulates
duodenal
mucosal cells to
produce
enteropeptidase
Proelastase
Elastase
Absorption of
amino acids
from small
intestine
Hormone secretions of small intestine: CCK
Site of
secretion
Primarily
duodenal
mucosa
Stimuli for
secretion
Chyme with a:
•High amino
acid
concentration
•High fatty acid
concentration
•Low pH
Actions
•Inhibits gastric
emptying
•↑ secretion of
pancreatic enzymes
and HCO3•Stimulates release of
enteropeptidase
•Stimulates gall
bladder contractions
Carbohydrate
digestion
• Pancreatic
amylase breaks
down starches
into smaller
polysaccharides
• Intestinal
mucosal cells
release maltase,
sucrase and
lactase
Absorption of
monosaccharides
from small
intestine
• Most adult animals
are unable to produce
lactase
Action of
pancreatic
lipase
Digestive secretions of liver
• Bile salts
synthesized by
hepatocytes from
cholesterol
Bile salts
emulsify
dietary fats
• Emulsification
provides
greater
surface area
for action of
lipases
Bile salts
• Help absorption of
lipids – fatty acids,
monoglycerides,
cholesterol, fatsoluble vitamins –
from small intestine
CCK: most
important
stimulus for
gall bladder
contraction
and release
of bile into
small
intestine
Hormone secretions of small intestine: CCK
Site of
secretion
Primarily
duodenal
mucosa
Stimuli for
secretion
Chyme with a:
•High amino
acid
concentration
•High fatty acid
concentration
•Low pH
Actions
•Inhibits gastric
emptying
•↑ secretion of
pancreatic enzymes
and HCO3•Stimulates release of
enteropeptidase
•Stimulates gall
bladder contractions
Hormone secretions of the small
intestine: secretin
Site of secretion Stimuli for
secretion
Actions
Duodenal
mucosa
•↓ HCl
production in
stomach
•↑ pancreatic
HCO3- secretion
•↑ biliary HCO3secretion
Chyme with a:
•Low pH
•High fatty acid
concentration
Carbohydrate digestion
Absorption of monosaccharides
Action of lipase on triglycerides