gi tract secretion. secretory function of the gi tract digestion –movement of food materials...
TRANSCRIPT
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• Adhesiveness• Ability to coat the tract• Low resistance against movement• High resistance to digestion• Buffering
• Composition– Serous
• Amylase
– Mucous• Mucin
– Ions• Bicarbonate• Potassium• Low sodium and chloride ion concentrations
Salivary Glands
Type of % of total Gland saliva Secreted
Parotid Serous
Submandibular Mucous / Serous
Sublingual Mucous 10% / Serous
Buccal Mucous <1%
90%
• 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
• Oxyntic glands– Cells
• Mucous neck cells• Parietal cells
– HCL
– Intrinsic factor
• Peptic/chief cells– Pepsinigen
K+K+K+
K+
Na+Na+
H+
Na+ Na+
Cl-Cl-
Cl-
Cl-
H2O
H2O
osmosis
H2O HO- + H+
HCO
3
HCO
3
CO2CO2
CarbonicAnhydrase
Final ResultsHCl - 155 mEq/LKCl - 15 mEq/LNaCl - 3 mEq.L
pH = 0.8
P
P
BLOOD LUMEN
P
P
• Pepsinogen– Activated by HCL– Secretion
• Acetylcholine• Stomach acid
• Inhibition of gastric secretion– Reverse enterogastric reflex– Presence of chemicals within the intestine
• 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 % pancreaticsecretion
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– 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
• Sucrase• Maltase• Lactase• Isomaltase
– Lipase
• Secretion– Regulated by the local stimuli
• Enteric nervous reflex