GASTROINTESTINAL SECRETIONS-1•About 9 liters of fluid pass through the GI system eachday, and only about 2 liters are ingested, the restrepresent secretions from the system itself.•About half of the total secreted amount (3.5, liters) issecreted from the exocrine glands, the salivaryglands, the pancreas and the liver,•The other half is secreted by the epithelial cells of thedigestive tract itself.•Nearly all this fluid is absorbed
GASTROINTESTINAL SECRETIONS-1•To put this in perspective a 70 Kilogram man has about42 liters of fluid, so the secretions represent about a sixthof the bodys volume.•The circulation contains about 3.5 liters, so thesesecretions represent twice the bodies circulating volume!•Failures of absorption of the intestinal secretions canthus lead to rapid dehydration and electrolyte imbalance.
GASTROINTESTINAL SECRETIONS-1 Types of gastrointestinal glands1. Single cell mucous glands and goblet cells.2. Pit glands: Invaginations of the epithelia into the submucosa. In the small intestine these are called Crypts of Lieberkuhn.3. Deep tubular glands. These are found in the stomach - the gastric glands, and the upper duodenum - Brunners glands.4. Complex glands, the salivary glands, the pancreas, and the liver.
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 1- Electrolytes and FluidsA large portion of the 7 liters is composed of water and ions. Theionic composition varies from region to region.•The acini of the salivary glands secrete a sodium and chloride richsecretion, this is then turned to a potassium, bicarbonate rich secretion as ittravels down the lumen and ducts of the glands•The Oxyntic cells of the stomach secret Hydrochloric acid•The mucous cells of the stomach secrete a mucous rich in bicarbonates•The pancreatic ducts and ductules secrete a solution rich in bicarbonate•The Crypts of Liberkuhn of the small intestine secrete a solution almostindistinguishable from interstitial fluid.
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions Mechanism of Electrolytes and Fluids Secretion•In its resting state the membrane resting potential isabout -30-40 mV•Neural stimulation causes an influx of -ve chloride ionsdecreasing resting potential by 10-20 mV•Sodium ions follow down the electrical gradient.•Cell contents become hyper osmotic.•Water follows.•Intracellular pressure increases•Increased pressure opens pores on apical side ofglandular cell flushing water and electrolytes
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretionsMechanism of Electrolytes and Fluids Secretion
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 2- Digestive Enzymes LOCATION ENZYME NAME ACTION Alpha amylase Splitting of 1-4 linkages in starchBuccal Cavity Lingual Lipase Triglyceride digestion especially in bacterial wall (immune function) Pepsin Digestion of tertiary proteins into simpler forms Stomach Gastric Lipase Splitting of fatty acids from triglycerides
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 2- Digestive Enzymes LOCATION ENZYME NAME ACTION Pancreatic Amylase Digestion of starch remnants into disaccharides and glucose Pancreatic Lipase Digestion of dietary triglyceridesExocrine Pancreas Phospholipase Digestion of any phospholipids Trypsin Long peptide digestion Chemotrypsin Splitting of chemical bonds between polypeptides
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 2- Digestive Enzymes LOCATION ENZYME NAME ACTION Enterokinase Activation of intestinal proteolytic enzymes Endopeptidases Splitting of bonds inside aIntestinal Epithelium polypeptide Exopeptidases Splitting of terminal bonds Aminopeptidase Splitting of amine end from a polypeptide Carboxypeptidase Splitting of carboxyl end from a polypeptide Sucrase Splitting of sucrose into two glucose molecules Maltase Splitting of maltose Lactase Splitting of lactose
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions Mechanism of formation and secretion of Digestive Enzymes•Digestive enzymes are secreted by glandular cells whichwill store the enzyme in secretory vesicles until they areready to be released.•These cells are characterised by a robust roughendoplastic reticulum and numerous mitochondria.•Passage of materials from the ribosomes, through theendoplasmic reticulum and Golgi body to the secretoryvesicles takes about 20 minutes.
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretionsMechanism of formation and secretion of Digestive Enzymes
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 3- Mucous Secretions•Mucous is a viscous secretion used for protection andlubrication.•It consists mainly of Glycoproteins.•In the mouth about 70% of the mucous is secreted bythe minor salivary glands.•Mucous cells in the stomach.•Goblet cells in the small intestine. Up to 25% of theintestinal epithelial cells are goblet cells.
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 3- Mucous SecretionsMucous has the following properties:1. Adherent properties, it sticks well to surfaces2. Enough body to prevent contact of most food particles with tissue3. Lubricates well - has a low resistance to slippage4. Strongly resistant to digestive enzymes5. Neutralizing properties. As well as a buffer like effect, mucous can also contain large quantities of bicarbonate.
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 4- Hormonal Secretions HORMONE SECRETING TARGET TISSUE RELEASING RESULTING CELL STIMULI EFFECTS G-cells in the gastric 1. Histamine- •Chemical stimuli as Increases acid antrum secreting cells peptides in chyme secretion in GASTRIN 2. Gastric acid •Neural stimuli stomach secreting cells through Cr-X and ENS Endocrine cells of -Gall bladder Fat in the duodenal 1. gallbladder small intestine -Pancreas chyme contractionCHOLECYSTOKININ -gastric muscle 2. inhibits gastric (CCK) emptying; 3. stimulates secretion of Pancreatic Enzymes
GASTROINTESTINAL SECRETIONS-1 Components of gastrointestinal secretions 4- Hormonal SecretionsHORMONE SECRETING TARGET TISSUE RELEASING RESULTING CELL STIMULI EFFECTS Endocrine cells of 1. Exocrine •Chemical stimuli as 1. Increase small intestine Pancreas peptides in chyme bicarbonate SECRETIN 2. Gastric acid •Neural stimuli secretion secreting cells through Cr-X and 2. Inhibit acid 3. Pyloric ENS secretion Musculature 3. Stimulatory for 4. LES sphincters Endocrine cells of -Gastric antral Fasting Evokes hungar small intestine musculature pangs MOTILIN -Duodenal musculature
GASTROINTESTINAL SECRETIONS-1 ORAL CAVITY SECRETION “SALIVA”•The Salivary glands consist of the parotid, submandibular, andsublingual glands as well as numerous smaller buccal glandssecreting both serous and mucoid secretions.•The parotid secretions are mainly serous, the buccal glandsmucus, and the sublingual and submandibular are a mixture of thetwo.•The acini secrete proteins and a fluid similar in consistency tointerstitial fluid, and the ducts exchange the sodium for potassiumand Bicarbonate for chlorine leaving saliva that is rich in Potassiumand bicarbonate ions.•The glands secrete between 800-1500 ml per day
GASTROINTESTINAL SECRETIONS-1 ORAL CAVITY SECRETION “SALIVA”•The sodium ions are actively reabsorbed, and thepotassium ions are actively secreted at the luminal sideof the cell with an excess of sodium reabsorbtion causinga -70mV gradient.•This causes passive reabsorbtion of chlorine ions.•Bicarbonate ions are both passively exchanged , andactively secreted in exchange for chlorine.