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• 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

Type of % of total Gland saliva Secreted

Parotid Serous

Submandibular Mucous / Serous

Sublingual Mucous 10% / Serous

Buccal Mucous <1%

90%

• 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 villus-

like structures inside of canaliculi

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

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 % 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

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