Gastrointestinal System

1. Gastrointestinal System Faisal I. Mohammed, MD, PhD

2. The Gastrointestinal System- Overviewand Mouth • Objectives: • Introduce the GITS • Give anatomical overview • Describe the functions of the GITS

4. Organs of the GITS • Gastrointenstinal (GI) tract or alimentary canal – mouth, most of pharynx, esophagus, stomach, small intestine, and large intestine • Accessory digestive organs – teeth, tongue, salivary glands, liver, gallbladder, and pancreas

5. Organs of the digestive system

6. Functions of the Gastrointestinal system(Gatrointestinal Processes) • Ingestion • Secretion of water, acid, buffers, and enzymes into lumen • Mixing and propulsion • Digestion • Mechanical digestion churns food • Chemical digestion – hydrolysis • Absorption – passing into blood or lymph • Defecation – elimination of feces

7. Digestive Process

8. Layers of the GI tract • Wall of GI tract from lower esophagus to anal canal has same basic 4 layers • Mucosa – inner lining • Epithelium protection, secretion, absorption • Lamina propria – connective tissue with blood and lymphatic vessels and mucosa-associated lymphatic tissue (MALT) • Muscularis mucosae – thin layer of smooth muscle making folds to increase surface area • Submucosa • Connective tissue binding mucosa to muscularis • Contains many blood and lymphatic vessels • Submucosal plexus (Meissner’s)

9. Layers of the GI tract …cont • Muscularis • Voluntary skeletal muscle found in mouth, pharynx, upper 2/3 of esophagus, and anal sphincter • Involuntary smooth muscle elsewhere • Arranged in inner circular fibers and outer longitudinal fibers • Myenteric plexus between muscle layers (Auerbach’s) • Serosa • Outermost covering of organs suspended in abdominopelvic cavity • Also called visceral peritoneum • Esophagus lacks serosa – has adventitia

10. Layers of the gastrointestinal tract

11. Summary of pathways controlling Gastrointestinal activities

12. Neural innervation • Enteric nervous system (ENS) • Intrinsic set of nerves - “brain of gut” • Neurons extending from esophagus to anus • 2 plexuses • Myenteric plexus – GI tract motility • Submucosal plexus – controlling secretions • Autonomic nervous system • Extrinsic set of nerves • Parasympathetic stimulation increases secretion and activity by stimulating ENS • Sympathetic stimulation decreases secretions and activity by inhibiting ENS

13. Organization of the enteric nervous system

14. Regulation of the GI Tract • Extrinsic innervation: • Parasympathetic nervous system: • Vagus and spinal nerves: • Stimulate motility and GI secretions. • Sympathetic nervous system: • Postganglionic sympathetic fibers that pass through submucosal and myenteric plexuses and innervate GI tract: • Reduce peristalsis and secretory activity.

15. Regulation of the GI Tract (continued) • Enteric nervous system: • Sites where parasympathetic fibers synapse with postganglionic neurons that innervate smooth muscle. • Submucosal and myenteric plexuses: • Local regulation of the GI tract. • Paracrine secretion: • Molecules acting locally. • Hormonal secretion: • Secreted by the mucosa.

16. Mouth • Oral or buccal cavity • Formed by cheeks, hard and sot palates, and tongue • Oral cavity proper is a space that extends from gums and teeth to fauces (opening between oral cavity and oropharynx) • Salivary glands release saliva • Ordinarily, just enough is secreted to keep mouth and pharynx moist and clean • When food enters mouth, secretion increases to lubricate, dissolve and begin chemical digestion • 3 pairs of major salivary glands secrete most of the saliva • Parotid, submandibular, and sublingual

17. Structures of the mouth (oral cavity)

18. The three major salivary glands- parotid, sublingual, and submandibular

19. Salivary Glands

20. Saliva • Saliva • Mostly water 99.5% • 0.5% solutes – ions, dissolved gases, urea, uric acid, mucus, immunoglobulin A, lysozyme, and salivary amylase (acts on starch) • Not all salivary glands produce the same saliva • Salivation • Controlled by autonomic nervous system • Parasympathetic stimulation promotes secretion of moderate amount of saliva • Sympathetic stimulation decreases salivation

21. Saliva: Source and Composition • Secreted from serous and mucous cells of salivary glands • A 97-99.5% water, hypo-osmotic, slightly acidic solution containing • Electrolytes – Na+, K+, Cl–, PO42–, HCO3– • Digestive enzyme – salivary amylase • Proteins – mucin, lysozyme, defensins, and IgA • Metabolic wastes – urea and uric acid

22. Salivary Glands • Produce and secrete saliva that: • Cleanses the mouth • Moistens and dissolves food chemicals • Aids in bolus formation • Contains enzymes that break down starch • Three pairs of extrinsic glands – parotid, submandibular, and sublingual • Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

23. Control of Salivation • Intrinsic glands keep the mouth moist • Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to: • Ingested food which stimulates chemoreceptors and pressoreceptors • The thought of food • Strong sympathetic stimulation inhibits salivation and results in dry mouth

24. Tongue • Tongue • Accessory digestive organ • Skeletal muscle covered by mucous membrane • Maneuvers food for chewing, shapes mass, forces food back for swallowing • Lingual glands secrete salivary lipase

25. Digestion in the mouth • Mechanical digestion in the mouth • Chewing or mastication • Food manipulated by tongue, ground by teeth, and mixed with saliva • Forms bolus • Chemical digestion in the mouth • Salivary amylase secreted by salivary glands acts on starches • Only monosaccharides can be absorbed • Continues to act until inactivated by stomach acid • Lingual lipase secreted by lingual glands of tongue acts on triglycerides • Becomes activated in acidic environment of stomach

26. Thank You

27. Gastrointestinal System L2 Faisal I. Mohammed, MD, PhD

28. Saliva and Stomach • Objectives: • Describe salivary secretion composition, function and regulation • Give anatomical overview of the stomach • Describe gastric Movements, Secretion, Absorption and Digestion • Explain Gastric Emptying and its regulation

29. The three major salivary glands- parotid, sublingual, and submandibular

30. Salivary Glands

31. Saliva: Source and Composition • Secreted from serous and mucous cells of salivary glands around 1.5 liters/day • A 97-99.5% water, hypo-osmotic, slightly acidic solution containing • Electrolytes – Na+, K+, Cl–, PO42–, HCO3– Na+ - 0.1 x plasma (15 mEq/L) Cl- - 0.15 x plasma (15 mEq/L) K+ - 7 x plasma (30 mEq/L) HCO-3 - 3 x plasma (70 mEq/L) • Digestive enzyme – salivary amylase • Proteins – mucin, lysozyme, defensins, and IgA • Metabolic wastes – urea and uric acid

32. Salivary Glands • Produce and secrete saliva that: • Cleanses the mouth, prevent dental caries • Moistens and dissolves food chemicals • Facilitates speech • Aids in bolus formation • Contains enzymes- amylase- that break down starch • Three pairs of extrinsic glands – parotid, submandibular and sublingual • Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

33. Control of Salivation • Salivation is exclusively under neural control • Parasympathetic through glossopharyngeal nerve (IX) is stimulatory and sympathetic is inhibitory • Intrinsic glands keep the mouth moist • Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to: • Ingested food which stimulates chemoreceptors and pressoreceptors salivatory center at medulla oblongata • The thought of food – cerebral cortex • Strong sympathetic stimulation inhibits salivation and results in dry mouth

34. Tongue • Tongue • Accessory digestive organ • Skeletal muscle covered by mucous membrane • Maneuvers food for chewing, shapes mass, forces food back for swallowing • Lingual glands secrete salivary lipase

35. Digestion in the mouth • Mechanical digestion in the mouth • Chewing or mastication • Food manipulated by tongue, ground by teeth, and mixed with saliva • Forms bolus • Chemical digestion in the mouth • Salivary amylase secreted by salivary glands acts on starches ( 1-4 glucosidic linkages) • Only monosaccharides can be absorbed • Continues to act until inactivated by stomach acid • Lingual lipase secreted by lingual glands of tongue acts on triglycerides • Becomes activated in acidic environment of stomach

36. Pharynx • Passes from mouth into pharynx • 3 parts • Nasopharynx • Functions only in respiration • Oropharynx • Digestive and respiratory functions • Laryngopharynx • Digestive and respiratory functions

37. Esophagus • Secretes mucous, transports food – no enzymes produced, no absorption • Mucosa – protection against wear and tear • Submucosa • Muscularis divided in thirds • Superior 1/3 skeletal muscle • Middle 1/3 skeletal and smooth muscle • Inferior 1/3 smooth muscle • 2 sphincters – upper esophageal sphincter (UES) regulates movement into esophagus, lower esophageal sphincter (LES) regulates movement into stomach • Adventitia – no serosa – attaches to surroundings

38. Deglutition (Swallowing) • Act of swallowing • Facilitated by secretions of saliva and mucus • Involves mouth, pharynx, and esophagus • 3 stages • Oral phase: Voluntary – bolus passed to oropharynx • Pharyngeal phase– involuntary passage through pharynx into esophagus (reflex action), Fast 1-2 second. controlled by the medulla and lower pons (Deglutition center) • All routes except into the digestive tract are sealed off • Esophageal phase – involuntary passage through esophagus to stomach (depend on gravity ~ 5seconds) • Peristalsis pushes bolus forward

39. Swallowing (mouth, pharynx and esophagus)

40. Swallowing … cont (esophagus)

41. Deglutition (Swallowing) Bolus of food Tongue Uvula Pharynx Bolus Epiglottis Epiglottis Glottis Esophagus Trachea Bolus (c) Upper esophageal sphincter contracted (a) Upper esophageal sphincter contracted (b) Upper esophageal sphincter relaxed Relaxed muscles Relaxed muscles Circular muscles contract, constricting passageway and pushing bolus down Gastroesophageal sphincter open Bolus of food Longitudinal muscles contract, shortening passageway ahead of bolus Gastroesophageal sphincter closed Stomach (d) (e)

42. Oropharyngeal stages of swallowing

43. Stomach • Serves as mixing chamber and holding reservoir • 4 main regions • Cardia, fundus, body, pylorus • Same 4 layers • Mucosa – gastric glands open into gastric pits • 3 types of exocrine gland cells – mucous neck cells (mucus), parietal cells (intrinsic factor and HCl), and chief cells (pepsinogen and gastric lipase) • G cell – endocrine cell – secretes gastrin • Submucosa • Muscularis – additional 3rd inner oblique layer • Serosa – part of visceral peritoneum

44. Stomach

45. Gastric secretions

47. Gastric emptying (movements) Stomach factors are stimulatory Duodenal factors are inhibitory

48. Mechanical and Chemical Digestion • Mechanical digestion • Mixing waves – gentle, rippling peristaltic movements – creates chyme • Chemical digestion • Digestion by salivary amylase continues until inactivated by acidic gastric juice • Acidic gastric juice activates lingual lipase • Digest triglycerides into fatty acids and diglycerides • Parietal cells secrete H+ and Cl- separately but net effect is HCl • Kills many microbes, denatures proteins

49. Chemical Digestion • Chemical digestion (cont.) • Pepsin secreted by chief cells digest proteins • Secreted as pepsinogen activated into pepsin by HCl • Pepsin is an endopeptidase breaks peptide bonds in the inside of the polypeptides • Gastric lipase splits triglycerides into fatty acids and monoglycerides • Small amount of nutrient absorption • Some water, ions, short chain fatty acids, certain lipid soluble drugs (aspirin) and alcohol

50. Thank You