GASTROINTESTINAL
PHYSIOLOGY
Dr. Meg-angela Christi Amores
Functional anatomy
The gastrointestinal wall
(from outer to inner)
1.
Serosa
2.
Longitudinal muscle
3.
Circular muscle
4.
Submucosa
5.
Mucosa
6.
Mucosal muscle
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Functional anatomy
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motor functions of the gut are performed by the
different layers of smooth muscle
In the longitudinal muscle layer, the bundles extend
longitudinally down the intestinal tract;
in the circular muscle layer, they extend around the
gut
each muscle layer functions as a syncytium
Neural control – ENTERIC nervous
system
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a nervous system all its own
lies entirely in the wall of the gut
especially important in controlling gastrointestinal
movements and secretion
two plexuses:
 myenteric
 controls
plexus or Auerbach's plexus
mainly the gastrointestinal movements
 submucosal
 controls
flow
plexus or Meissner's plexus
mainly gastrointestinal secretion and local blood
Assignment
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Describe the differences between the Myenteric
plexus and the submucosal plexus
Hormonal control
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Gastrin
 secreted
by the "G" cells of the antrum of the stomach
 stimulation of gastric acid secretion and stimulation of
growth of the gastric mucosa

Cholecystokinin
 secreted
by "I" cells in the mucosa of the duodenum and
jejunum in response to digestive products of fat, fatty
acids, and monoglycerides
 strongly contracts the gallbladder, expelling bile into
the small intestine
Hormonal Control

Secretin
 secreted
by the "S" cells in the mucosa of the duodenum in
response to acidic gastric juice emptying into the duodenum
 acts to promote pancreatic secretion of bicarbonate
 to neutralize the acid in the small intestine

Gastric inhibitory peptide
 secreted
by the mucosa of the upper small intestine
 decreasing motor activity of the stomach and therefore
slows emptying of gastric contents
Secretory functions
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2 types of secretion
 Digestive
enzymes
 Mucus

are formed only in response to the presence of
food in the alimentary tract
SECRETIONS
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Saliva
Esophageal secretions
Gastric secretions
Pancreatic secretions
Bile (liver)
Secretions of the small intestine
Secretions of the large intestine
Average daily volume of secretions
mL
pH
Saliva
1000
6.0-7.0
Gastric secretion
1500
1.0-3.5
Pancreatic secretion
1000
8.0-8.3
Bile
1000
7.8
Small intestine secretion
1800
7.5-8.0
Brunner's gland secretion
200
8.0-8.9
Large intestinal secretion
200
7.5-8.0
Total
6700
Salivary secretions
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principal glands of salivation are the parotid,
submandibular, and sublingual glands
two major types of protein secretion:
(1) a serous secretion that contains ptyalin (an αamylase), which is an enzyme for digesting starches
 (2) mucus secretion that contains mucin for lubricating
and for surface protective purposes

Saliva
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0.5 milliliter of saliva, is secreted each minute
Becomes very little during SLEEP
secretion plays an exceedingly important role for
maintaining healthy oral tissues
flow of saliva itself helps wash away pathogenic
bacteria
contains several factors that destroy bacteria
contains significant amounts of protein antibodies
that can destroy oral bacteria
Gastric secretions

oxyntic glands (also called gastric glands)
 hydrochloric
acid, pepsinogen, intrinsic factor, and mucus
 body and fundus (proximal 80 per cent of stomach)
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pyloric glands
 mucus
for protection of the pyloric mucosa from the stomach
acid
 Gastrin
 antral portion of the stomach (distal 20%)
Gastric secretion
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parietal cell (also called
oxyntic cell), demonstrating
that it contains large
branching intracellular
canaliculi
where HCl is produced
Gastric secretion
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Pepsinogen
 has
no digestive activity when first secreted
 as soon as it comes in contact with hydrochloric acid, it
is activated to form active pepsin
 for protein digestion in the stomach
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Intrinsic factor
 essential
for absorption of vitamin B12 in the ileum
Gastric Secretion
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Pyloric glands
 almost
no parietal cells
 contain mostly mucous cells
 Secrete mucus
 Secrete gastrin in G cells
 Act
on histamine to stimulate gastric hydrochloric acid
secretion
Pancreatic Secretions
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Secretes DIGESTIVE enzymes, bicarbonates
secreted most abundantly in response to the
presence of chyme in the upper portions of the
small intestine
digesting proteins : trypsin, chymotrypsin, and
carboxypolypeptidase
digesting carbohydrates: pancreatic amylase
fat digestion: pancreatic lipase, cholesterol esterase,
phospholipase
Liver secretions - BILE
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Functions:
 fat
digestion and absorption
 1)
they help to emulsify the large fat particles of the food
into many minute particles, the surface of which can then be
attacked by lipase enzymes secreted in pancreatic juice
 (2) they aid in absorption of the digested fat end products
through the intestinal mucosal membrane
 serves
as a means for excretion of several important
waste products from the blood
Liver secretions - BILE
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Bile is secreted continually by the liver cells, but
most of it is normally stored in the gallbladder
maximum volume that the gallbladder can hold is
only 30 to 60 milliliters
When food begins to be digested in the upper
gastrointestinal tract, the gallbladder begins to
empty
the most potent stimulus for causing the gallbladder
contractions is the hormone cholecystokinin
Small Intestinal Secretions
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Duodenum: Brunner’s Glands – secrete mucus
Small intestine – crypts of Leiberkuhn – enzymes:
 Peptidase
(small peptides to amino acids)
 Sucrase, maltase, isomaltase, lactase (splitting
disaccharides to monosaccharides)
 Intestinal lipase – neutral fats to glycerol and fatty
acids
Large Intestinal Secretions
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Crypts of Leiberkuhn – mucus
protects the intestinal wall against excoriation
provides an adherent medium for holding fecal
matter together
protects the intestinal wall from the great amount of
bacterial activity
mucus plus the alkalinity of the secretion
SO HOW is FOOD DIGESTED?
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Bread: starch
Mouth:
Esophagus:
Stomach:
Pancreas/Liver:
Small intestine:
Large Intestine
Digestion and Absorption
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To be continued......