Transcript GI tract secretion

GI tract secretion
Secretory function of the GI tract
• Digestion
– Movement of food materials
• Linear movement
• Mixing
– Digestion
• Mechanical
• Chemical
– Absorption
• Secretion by the GI tract
– Aids movement
• Mucus
– Provides protection
– Digestion
• Enzymes
• Bile acids
– In response to the presence of foods in the
tract
Anatomical structures
• Single cell mucous gland
– Mucous cells/goblet cells
• Pits
– Submucosal glands
• Crypts of Lieberkun
• Tubular glands
– Oxyntic glands in stomach
• Complex gland
– Salivary
– Liver
– Pancreas
• GI tract secretion
– Epithelial stimulation
• Direct contact with food
– Enteric nerve stimulation
• Tactile
• Chemical
• Distension
– Parasympathetic stimulation
• Increased secretion in the upper GI tract
– Sympathetic stimulation
• Increase in some area
• Reduced parasympathetic-induced secretion
Mechanism of secretion
• Organic materials
– Exocytosis
• Water and electrolytes
– Increased ion uptake
• Inward movement of chlorine
– Movement of sodium ion
• Osmosis
• Release of fluids by opening of channels
– Hydrostatic pressure
Mucus
• Chemical composition
– Different among different regions within the
tract
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Adhesiveness
Ability to coat the tract
Low resistance against movement
High resistance to digestion
Buffering
Saliva
• Composition
– Serous
• Amylase
– Mucous
• Mucin
– Ions
• Bicarbonate
• Potassium
• Low sodium and chloride ion concentrations
Salivary Glands
Gland
Type of
saliva
Parotid
Serous
Submandibular
Mucous
/ Serous
Sublingual
Mucous
/ Serous
Buccal
Mucous
% of total
Secreted
90%
10%
<1%
• Saliva and oral hygiene
– Removal of pathogens and food particles
– Bactericide activity
– Immunity
• Nervous regulation of secretion
– Parasympathetic nervous system
• Tactile and taste stimulation of salivatory nuclei
– Higher CNS
• Regulation of salivatory nuclei
– Reflexes
• Lower GI tract
Esophageal secretion
• Mucus
– Lubrication for swallowing
• Simple mucous cells
– Protection of the lower esophagus
• Compound glands located in the lower portion
Gastric secretion
• Types of secretion
– Mucous cells
• Mucus
– Oxyntic glands
• Hydrochloric acid
• Pepsinogen
• Intrinsic factor
– Pyloric glands
• Mucus
• gastrin
Phase of secretion
20 %
70 %
• Oxyntic glands
– Cells
• Mucous neck cells
• Parietal cells
– HCL
– Intrinsic factor
• Peptic/chief cells
– Pepsinigen
• HCL
– Formed at the villuslike structures inside of
canaliculi
LUMEN
BLOOD
H2 O
CO2
HCO
HCO
3
3
K+
Na+
ClH2 O
P
HO- + H+
CO2
Carbonic
Anhydrase P
K+
K+
Na+
Na+
K+
P
Cl-
Cl
-
osmosis
Na+
P
H+
H2 O
Cl-
Final Results
HCl - 155 mEq/L
KCl - 15 mEq/L
NaCl - 3 mEq.L
pH = 0.8
• Pepsinogen
– Activated by HCL
– Secretion
• Acetylcholine
• Stomach acid
• Inhibition of gastric secretion
– Reverse enterogastric reflex
– Presence of chemicals within the intestine
Pancreatic secretion
• Digestive enzymes
– Pancreatic acini
• Sodium bicarbonate
– Ducts
• Secretion
– Presence of chyme
• Determines the
composition of
pancreatic secretion
– Stimulated by
• Acetylcholine
– Acinar secretion
• Cholecystokinin
– Acinar secretion
• Secretin
– Ductal secretion
• Pancreatic enzymes
– Carbohydrate digestion
• Amylase
– Protein digestion
• Trypsin
• Chymotrypsin
• Carboxylpolypeptidase
– Lipid digestion
• Lipase
• Cholesterol esterase
• Phospholipase
Phase of secretion
20 % pancreatic
secretion
5-10 %
Pancreatic
secretion
Majority of secretion
(secretin)
Bile secretion
• Secretion
– Hepatocytes
• Initial secretion
– Drained into ducts
• Secretion to the SI
• Accumulation in the
gallbladder
• Gallbladder
– Increased concentrations
of bile salts
• Removal of sodium ions
– Absorption of chloride
ions
– Osmosis of water
– Emptying
• In response to fats in
the chyme
– Secretion of
cholecystokinin
• Contraction of
gallbladder wall and
relaxation of the
sphincter of Oddi
• Function
– Fat digestion and absorption
• Emulsification
– Detergent function
• Formation of micelles
– Bile is amphipathic in nature (cholesterol plus salt)
– Hydrophobic core/pocket for lipids
• Bile salts
– Recycled
• Enterohepatic circulation
Secretion of the small intestine
• Compound mucous cells
– Brunner’s gland
• Site where pacreatic
secretion and bile enter
the intestine
• Alkaline mucus for
protection
– Added effects with
pacreatic bicarbonate
ions and bile
• Secretion
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Tactile stimulation
Irritation
Vagus stimulation
Secretin
• Crypts of Liererkuhn
– Goblet cells
• Mucus
– Enterocytes
• Water and electrolytes
• Secretion
– Rapidly reasborbed by
the villi
• Secretion of the watery mucus
– Active secretion of chloride ions
– Active secretion of bicarbonate ions
– Passive/drag movement of sodium ions
– Results of these:
• Osmosis
• Digestive enzymes
– Peptidases
– Enzymes for disaccharide digestion
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Sucrase
Maltase
Lactase
Isomaltase
– Lipase
• Secretion
– Regulated by the local stimuli
• Enteric nervous reflex
Secretion by the large intestine
• No enzyme secretion
– Only mucus
• Bicarbonate ions
– Non-mucous cells
• Secretion
– Tactile stimulation
– Local nervous reflex to the cryts of Leiberkuhn
• Protection of the large intestine
– Acids from bacterial metabolism
• Adhesive for feces formation