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Physiology of GIS (Secretory Function of GI-tract)
The document summarizes the physiology of the gastrointestinal tract secretions. It describes the various glands in the GI tract, including goblet cells, tubular glands, and others. It explains the secretions of these glands, such as mucus, digestive enzymes, acid, and bicarbonate. It also discusses the stimulation and regulation of secretion, including through nervous and hormonal mechanisms. The secretions aid in digestion and protect the lining of the GI tract.
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Physiology of GIS (Secretory Function of GI-tract)
- 1. PHYSIOLOGY OF GIS KeumalaHayati Monday, April 18, 2016
- 2. Secretory Function ofGI-Tract Secretory glands Digestive enzymes Mucus 18/04/2016 2
- 3. Types of GI-tractGlands Goblet Cells Also called : single-cell mucous glands or mucous cell. Located on the surface of epithelium (in most part of GIT) Specialized Secretory Cells Located in the pits (invagination of the epithelium to submucosa) of many surface areas of the GIT. E.g : crypts of Lieberkühn in small intestine. Tubular Glands Abundant in stomach and duodenum E.g : oxyntic gland in stomach (acid- and pepsinogen-secreting gland) Other glands associated with GIT Salivary glands Pancreatic glands Liver 18/04/2016 3
- 4. Daily Secretion ofIntestinal Juices 18/04/2016 4
- 5. Stimulation of GITGlands (basic mechanism) • Contact of foods with epithelium • Contact of foods in particular segments stimulates local and adjacent glands to secret juices • E.g : direct contact of the surface cells by food stimulates mucous cells to secret mucus mucus secretion ↑ • Enteric Nervous Stimuli • Enteric Nervous system activation: • Tactile • Chemical irritation • distention Stimuli • Mucous cells • Deep glands ENS 18/04/2016 5
- 6. Stimulation of GITGlands (basic mechanism) • Autonomic Nervous Stimuli • Parasympathetic stimulation ↑ GIT glandular secretion rate Parasympathetic stimulation Upper portion (innervated by glossopharyngeal and vagus nerves) Salivary, esophageal, gastric glands, pancreas, & Brunner’s glands in duodenum Lower portion (innervated by pelvic parasympathetic nerves) Some glands in 1/3 distal portion of large intestine 18/04/2016 6
- 7. Stimulation of GITGlands (basic mechanism) • Autonomic Nervous Stimuli • Sympathetic stimulation dual effect 18/04/2016 7 Sympathetic stimulation Alone • Slightly increase secretion rate Presence of hormonal and parasympathetic stimulation • Reduce the secretion
- 8. Glandular Cells Secretion 18/04/20168 • Organic substances 1. Diffusion of nutrients from blood vessels. 2. Formation of ATP by mitochondria. 3. ATP + substrates provided by nutrients to RE synthesis of the organic secretory substances. Proteins secreted are formed by ribosomes adherent to the RE. 4. Secretory materials to Golgi complex 5. In Golgi complex, the materials : modified discharged into cytoplasm in the form of secretory vesicles 6. Stored in apical-ends of the cells until stimulated by hormonal or nervous signals.
- 9. Secretion of Saliva Saliva Serous secretion Ptyalin(α-amylase) Mucus Mucin 18/04/2016 9 Almost entirely serous Parotid glands Bothd serous and mucus Sublingual and submandibular glands Only mucus Buccal glands
- 10. Secretion of Saliva •Stage 1 : primary secretion involved acini • Stage 2 : ions secretion involved salivary duct 18/04/2016 10
- 11. Esophageal Secretion • Entirelymucus • provide lubrication for swallowing • Compound mucous glands at the gastric-end of esophagus • Protection from digestion by acidic gastric juices that often reflux from the stomach 18/04/2016 11
- 12. Gastric Secretion Tubular Glandsof Stomach Oxyntic (acid-forming) glands HCl Pepsinogen Mucus Pyloric gland Mucus Gastrin 18/04/2016 12
- 13. Gastric Secretion • Oxynticglands : • mucous neck cells secrete mainly mucus • peptic (chief) cells secrete pepsinogen • parietal (or oxyntic) cells secrete HCl and intrinsic factor. 18/04/2016 13
- 14. Gastric Secretion • BasicMechanism of HCl secretion 18/04/2016 14
- 15. Gastric Secretion • Acetylcholine: • ↑ secretion of pepsinogen by peptic cells • HCl by parietal cells • mucus by mucous cells • Gastrin and histamine strongly stimulate secretion of acid by parietal cells but have little effect on the other cells • HCl + pepsinogen acvtive pepsin • Intrinsic factor (secreted by parietal cells) important for absorption of vit-B12 18/04/2016 15
- 16. Gastric Secretion • Pyloricglands • Contain mostly mucous cells • Secrete : • Mucus (thick, alkaline, viscid mucus) Lubrication and protection from digestion by gastric acids • Gastrin hormone controlling gastric secretion • Response in stimuli associated with ingestion of meal secretion of gastrin by G cells of the antrum of the stomach. • 2 forms : large, G-34 (34 amino acid). Small, G-17 (17 amino acid) • Food containing protein (antral-end of stomach) stimulate gastrin cells in pyloric glands secretion of gastrin to blood transported to ECL (enterochromaffin-like cells) secretion of histamine into deep oxyntic cells secretion of HCl 18/04/2016 16
- 17. Gastric Secretion • Regulationof Pepsinogen secretion • acetylcholine released from the vagus nerves or from the gastric enteric nervous plexus • acid in the stomach. • Acid : elicits additional enteric nervous reflexes that support the original nervous signals to the peptic cells. 18/04/2016 17
- 18. Pancreatic Secretion • PancreaticDigestive Enzymes • Protein digestion : • Trypsin and chymotrypsin split proteins into peptides (but do not cause release of individual amino acids. • Carboxypolypeptidase splits some peptides into individual amino acids. • Carbohydrate digestion : • Pancreatic amylase hydrolyzes starches, glycogen, and most other carbohydrates (except cellulose) to form disaccharides and a few trisaccharides • Fat digestion : • Pancreatic lipase hydrolyzing neutral fat into fatty acids and monoglycerides • Cholesterol esterase causes hydrolysis of cholesterol esters • Phospholipase splits fatty acids from phospholipids 18/04/2016 18
- 19. Pancreatic Secretion • Bicarbonate secretionby pancreatic ductules and ducts • HCl +NaHCO3 NaCl +H2CO3 18/04/2016 19
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- 21. Bile Secretion bythe Liver • Bile acids • Fat digestion : • Emulsify large fat • Fat absorption through intestinal muosal membrane • Excretion of bilirubin • Bile secretion • Initial secretion • Bile flow (stimulated by secretin) 18/04/2016 21
- 22. Bile Secretion bythe Liver 18/04/2016 22
- 23. Bile Secretion bythe Liver • Composition of bile 18/04/2016 23
- 24. Bile Secretion bythe Liver • Bile stored in gallbladder • Factors secreting bile : • Cholecystokinin • Acethylcholine-secreting nerve fibers (vagus nerves) 18/04/2016 24
- 25. Secretion of SmallIntestine • Brunner’s Glands secrete large amount of alkaline mucus • Stimulation : • Tactile • Vagal • GI hormones (secretin) • Inhibited by sympathetic stimulation • Function : protection 18/04/2016 25
- 26. Secretion of SmallIntestine • Crypts of Lieberkühn and the villi are covered by an epithelium composed of two types of cells : • goblet cells, which secrete mucus • Function : lubricates and protects the intestinal surfaces, • enterocytes, which, in the crypts, secrete large quantities of water and electrolytes and, over the surfaces of adjacent villi, reabsorb the water and electrolytes along with the end products of digestion. 18/04/2016 26
- 27. Secretion of SmallIntestine • Secretion of water and electrolytes : • active secretion of chloride ions into the crypts and • active secretion of bicarbonate ions Causes electrical drag of positively charged sodium ions through the membrane and into the secreted fluid Cause osmotic movement of water 18/04/2016 27
- 28. Secretion of SmallIntestine • The enterocytes of the mucosa contain digestive enzymes that digest specific food substances while they are being absorbed through the epithelium; • Peptidase : small peptides amino acids • Sucrase, Maltase, Isomaltase, Lactase • Intestinal lipase : neutral fat glycerol and fatty acids • Local stimulation : tactile or iritative stimuli from chyme in duodenum enteric nervous reflexes secretion of intestinal digestive juices 18/04/2016 28
- 29. Secretion of Mucusby Large Intestine • Crypts of Lieberkühn (do not have villi) secrete only mucus • Regulated by : direct, tactile stimulation of the epithelial cell lining local nervous reflexes to mucous cells • Parasympathetic stimulation (pelvic nerves) ↑ mucus secretion (1/3 distal part) • Function of mucus : • protects the intestinal wall against excoriation • adherent medium for holding fecal matter together • protects the intestinal wall from the bacterial activity inside the feces • the mucus plus the alkalinity of the secretion (a pH of 8.0 caused by large amounts of sodium bicarbonate) provides a barrier to keep acids formed in the feces from attacking the intestinal wall 18/04/2016 29
- 30. Reference • Hall JE,Guyton AC. Textbook of Medical Physiology. 13th ed. Philadelphia: Elsevier; 2016. 18/04/2016 30
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