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Secretory Functions of the Alimentary Tract

Secretory Functions of the Alimentary Tract. Prof. Dr. Bayram Yılmaz Yeditepe University Faculty of Medicine Department of Physiology. The Major Components o f t he Human Digestive System. Anatomical Types of Glands. Single-cell mucous glands (Goblet cells)

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Secretory Functions of the Alimentary Tract

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  1. Secretory Functions of the Alimentary Tract Prof. Dr. Bayram Yılmaz Yeditepe University Faculty of Medicine Department of Physiology

  2. The Major Components of the Human Digestive System

  3. Anatomical Types of Glands • Single-cell mucous glands (Goblet cells) • Specialized, deep secretory cells; Lieberkühn crypts • Deep tubular glands of the stomach and upper duodenum • Complex glands (salivary glands, pancreas and liver)

  4. Salivary Glands Parotid glands Submandibular (submaxillary) glands Sublingual glands Smaller glands in mucosa of tongue, palate, etc.

  5. Anatomical Types of Glands

  6. Basic Mechanisms of Stimulation of the Alimentary Tract Glands • Effect of contact of food with the epithelium (function of the enteric nervous stimuli) • Tactile stimulation • Chemical irritation • Distention of the gut wall • Autonomic stimulation of secretion • Parasympathetic stimulation • Sympathetic stimulation – dual effect • Regulation of glandular secretion by hormones

  7. Basic Mechanism of Secretion • Secretion of organic substances: • 1) Transport of nutrient material into the cell • 2) Mitochondria form ATP in glandular cells • 3) Synthesis of proteins in ribosomes, organic substances in ER • 4) Formation of secretory vesicles in Golgi apparatus • 5) Secretion of these vesicles by exocytosis

  8. Lubricating and Protective Effects of Mucus • Mucus is a thick secretion, mainly composed of water, electrolytes and glycoproteins • Lubricant and protectant of the gut wall • Buffering effects of glycoproteins • Low resistance for slippage of the food • Protection against chemical damage to the epithelium • Formation of feces

  9. Secretion of Saliva • Salivary glands; characteristics of saliva: • Parotid, submandibular and sublingual glands – and buccal glands • Daily secretion of saliva (800-1500 ml) • 1) Serous secretion that contains a-amylase • 2) Mucus secretion contains mucin • Parotid gland secretes almost entirely serous secretion • Submandibular and sublingual glands secrete both serous and mucous secretion • pH of saliva: between 6 and 7

  10. Secretion of Ions in Saliva • Saliva contains especially large quantities of K and HCO3 ions • Na and Cl concentrations are lower than plasma

  11. Secretion of Ions in Saliva • Two stages of salivary secretion: • The acini secretes the primary secretion that contains ptyalin and/or mucin in a solution of ions not different from that of plasma • As the primary secretion flows through the ducst, Na ions are actively reabsorbed and K ions actively secreted • HCO3 are secreted • Concentration of Na and Cl is about 15 mEq/L • Concentration of K is about 30 mEq/L • During maximal salivation, secretion rate can increase as much as 20 fold

  12. Secretion of Ions in Saliva

  13. Saliva and Oral Hygiene • Mucus secretion of saliva 0.5 ml/min at rest • Flow of saliva helps wash away the bacteria as well as food particles • Thiocyanate ions and proteolytic enzymes (lysozyme) can destroy bacteria • Protein antibodies in saliva against infectious agents

  14. Nervous Regulation of Salivary Secretion • Parasympathic nervous signals, superior and inferior salivary nuclei in the brain stem • Salivation can also be stimulated or inhibited by nervous signals arriving in the salivatory nuclei from higher areas of the central nervous system • Sympathetic stimulation can also increase salivation a slight amount • Blood supply to the salivary glands • Esophageal secretions are entirely mucous, lubrication for swallowing, formation of ulcers...

  15. Nervous Regulation of Salivary Secretion

  16. Neural regulation of secretion

  17. OVERVIEW OF GASTRIC FUNCTION Anatomy of the stomach

  18. Gastric Secretion • Oxyntic (acid-forming) glands • HCl and intrinsic factor (parietal cells) • Pepsinogen (peptic cells) • Mucus (neck cells) • Pyloric gland mainly secretes mucus and also gastrin • Oxyntic glands are located in the body of the stomach, pyloric glands in the antrum

  19. Gastric Secretion

  20. Gastric Secretion • Secretion and activation of pepsinogen (pH 1.8-3.5) • Secretion of intrinsic factor and B12 vitamin • Pyloric glands, secretion of mucus and gastrin • Surface mucous cells – its alkaline nature

  21. HCO3- secretion and mucosa protection

  22. Stimulation of gastric acid secretion • Parietal cells are the only cells that secrete HCl • Acid secretion is controlled by both nervous and hormonal stimuli • Enterochromaffin-like cells (ECL cells) secrete histamine • Factors that stimulate acid secretion: • Gastrin • Acetylcholine • ECL cells • Enteric hormones

  23. Acetylcholine, Gastrin, and Histamine Directly and Indirectly Induce Acid Secretion by Parietal Cells Direct stimulation of parietal cells Indirect stimulation of parietal cells, through histamine released by ECL

  24. Regulation of Pepsinogen Secretion • Stimulation of peptic cells by Acetylcholine • Stimulation of peptic cell secretion in response to acid in the stomach

  25. Phases of Gastric Secretion

  26. Inhibition of gastric secretion by post-stomach intestinal factors • Enterogastric reflexes • Intestinal hormones • Secretin • Gastric inhibitory peptide • VIP and somatostatin • Gastric secretion during the interdigestive period • Mucus secretion • Emotional stimuli and peptic and acidic gastric secretions • Chemical composition of gastrin and other GI hormones

  27. Gastric phase of gastric-acid secretion

  28. Intestinal phase of gastric-acid secretion

  29. Pancreatic Secretion • Pancreatic digestive enzymes are secreted by the pacreatic acini • Large volumes of HCO3 are secreted by the ducts • These secretions empty into the duodenum by papilla of Vater surrounded by Oddi sphincter • Pancreatic juice is secreted in response to the presence of chyme in the upper portions of the small intestine • Insulin secretion – endocrine function

  30. Pancreatic Acinus-duct morphology

  31. Pancreatic Digestive Enzymes • Enzymes for protein digestion: • Trypsin • Chymotrypsin • Carboxypolypeptidase • Pancreatic amylase digests carbohydrates into disaccharides • Main enzymes for fat digestion • Pancreatic lipase • Cholesterol esterase • Phospholipase

  32. Pancreatic Digestive Enzymes • Inactiveforms of proteolyticenzymes • Activationbyenterokinasewhich is secretedbytheintestinalmucosawhenchymecomes in contactwiththemucosa • Secretion of trypsininhibitorpreventsdigestion of thepancreasitself • Trypsininhibitor • Whenpancreasbecomesseverelydamagedor a ductgetsblocked; acutepancreatitisdevelops

  33. Secretion of Bicarbonate Ions

  34. Pancreatic HCO3- secretion and NaCl-rich fluid Duct cells

  35. Regulation of Pancreatic Secretion • Acetylcholine • Cholecystokinin • Secretin • Multiple effects of different stimuli • Phases of pancreatic secretion • Cephalic (20 %) • Gastric (10%) and • Intestinal (70%) phases

  36. Secretion of Pancreatic Secretion

  37. Regulation of Pancreatic Secretion • Secretin stimulates large quantities of HCO3 – Neutralization of acidic stomach • Cholecystokinin: presence of fatty acids and peptones in the chyme

  38. Secretion of Bile • Fat digestion • Emulsification of large fat particles • Absorption of fatty acids • Excretion of waste products (bilirubin, cholesterol) • Biliary secretion • Bile is secreted by hepatocytes • Second portion of secretion from ducts include water, Na and HCO3 • Role of secretion

  39. Secretion of Bile

  40. Storing and Concentration of Bile • Bile is continually secreted by the liver cells • Stored in gallbladder ; 30-60 ml • 450 ml secretion in 12 h • Water, NaCl and other electrolytes are continually absorbed by gallbladder mucosa, leaving bile salts, cholesterol, lecithin and bilirubin • Composition of bile: • Emptying of the gallbladder • Stimulatory role of cholecystokinin • Acetylcholine

  41. Composition of Bile

  42. Liver secretion and gallbladder emptying

  43. Function of Bile Salts in Fat Digestion and Absorption • Emulsifying or detergent action of bile salts • Formation of micelles and absorption of fatty acids, monoglycerides, cholesterol and other lipids • Enterohepatic circulation of bile salts • Role of secretin in controlling bile secretion

  44. Liver secretion of cholesterol and gallstone formation • Concentration of bile, lecithin, bile salts and cholesterol • Precipitation of cholesterol in the gallbladder may cause formation of cholesterol gallstones • Inflammation

  45. Intestinal Secretions Duodenal Secretions mucus and an aqueous component isoosmolar with plasma. Submucosa: Brunner’s glands Secretions of the Small Intestine mucus Goblet cells aqueous secretion Inhibited by VIP Epithelial cells Secretions of the Colon Small volume rich in mucus mucus Goblet cells aqueous secretion rich in K+ and HCO3H- Epithelial cells Stimulated by mechanical irritation of mucosa and cholinergic pathways. Inhibited by sympathetic nerves

  46. Secretions of Small Intestine • Crypts of Lieberkühn • Intestinal secretions at a rate of 1800 ml/day • Slightly alkaline pH

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