The Digestive System

1. The Digestive System • General anatomy and digestive processes • Mouth through esophagus • Stomach • Liver, gallbladder and pancreas • Small intestine • Chemical digestion and absorption • Large intestine 25-

2. Digestive Functions • Ingestion • intake of food • Digestion • breakdown of molecules • Absorption • uptake nutrients into blood/lymph • Defecation • elimination of undigested material 25-

3. Stages of Digestion • Mechanical digestion • physical breakdown of food into smaller particles • teeth and churning action of stomach and intestines • Chemical digestion • series of hydrolysis reactions that break macromolecules into their monomers • enzymes from saliva, stomach, pancreas and intestines • results • polysaccharides into monosaccharides • proteins into amino acids • fats into glycerol and fatty acids 25-

4. Digestive Processes • Motility • muscular contractions that break up food, mix it with enzymes and move it along • Secretion • digestive enzymes and hormones • Membrane transport • absorption of nutrients 25-

5. Subdivisions of Digestive System • Digestive tract (GI tract) • 30 foot long tube extending from mouth to anus • Accessory organs • teeth, tongue, liver, gallbladder, pancreas, salivary glands 25-

6. Tissue Layers of GI Tract • Mucosa • epithelium • lamina propria • muscularis mucosae • Submucosa • Muscularis externa • inner circular layer • outer longitudinal layer • Adventitia or Serosa • areolar tissue or mesothelium 25-

7. Tissue Layers of GI Tract 25-

8. Enteric Nervous Control • Able to function independently of CNS • Composed of two nerve networks • submucosal plexus • controls glandular secretion of mucosa • contractions of muscularis mucosae • myenteric plexus • controls peristalsis • contractions of muscularis externa 25-

9. Relationship to Peritoneum • Only duodenum, pancreas and parts of large intestine are retroperitoneal • Dorsal mesentery suspends GI tract and forms serosa (visceral peritoneum) of stomach and intestines • Ventral mesentery forms lesser and greater omentum • lacy layer of connective tissue that contains lymph nodes, lymphatic vessels, blood vessels 25-

10. Lesser and Greater Omentum • Lesser - attaches stomach to liver • Greater - covers small intestines like an apron 25-

11. Mesentery and Mesocolon • Mesentery of small intestines holds many blood vessels • Mesocolon anchors colon to posterior body wall 25-

12. Regulation of Digestive Tract • Neural control • short myenteric reflexes (swallowing) • long vagovagal reflexes (parasympathetic stimulation of digestive motility and secretion) • Hormones • messengers diffuse into bloodstream, distant targets • Paracrine secretions • messengers diffuse to nearby target cells 25-

13. Mouth or Oral Cavity 25-

14. Features of Oral Cavity • Cheeks and lips • keep food between teeth for chewing; essential for speech and suckling in infants • vestibule - space between teeth and cheeks • lips: cutaneous area versus red area (vermilion) • Tongue is sensitive, muscular manipulator of food • papillae and taste buds on dorsal surface • lingual glands secrete saliva, tonsils in root • Hard and soft palate • allow breathing and chewing at same time • palatoglossal and palatopharyngeal arches 25-

15. Dentition • Baby teeth (20) by 2 years; Adult (32) between 6 and 25 • Occlusal surfaces and cusp numbers differ 25-

16. Permanent and Deciduous Teeth 25-

17. Tooth Structure • Periodontal ligament is modified periosteum • anchors into alveolus • Cementum and dentin are living tissue • Enamel is noncellular secretion formed during development • Root canal leads into pulp cavity • nerves and blood vessels • Gingiva or gums 25-

18. Mastication or Chewing • Breaks food into smaller pieces to be swallowed • ↑ surface area exposed to digestive enzymes • Contact of food with sensory receptors triggers chewing reflex • tongue, buccinator and orbicularis oris manipulate food • masseter and temporalis elevate the teeth to crush food • medial and lateral pterygoids swing teeth in side-to-side grinding action of molars 25-

19. Saliva • Functions of saliva • moisten, begin starch and fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus • Hypotonic solution of 99.5% water and solutes • salivary amylase, begins starch digestion • lingual lipase, digests fat activated by stomach acid • mucus, aids in swallowing • lysozyme, enzyme kills bacteria • immunoglobulin A, inhibits bacterial growth • electrolytes = Na+, K+, Cl-, phosphate and bicarbonate • pH of 6.8 to 7.0 25-

20. Salivary Glands • Small intrinsic glands found under mucous membrane of mouth, lips, cheeks and tongue - secrete at constant rate • 3 pairs extrinsic glands connected to oral cavity by ducts • parotid, submandibular and sublingual 25-

21. Histology of Salivary Glands • Compound tubuloacinar glands • Mucous cells secrete mucus • Serous cells secrete thin fluid rich in amylase • Mixed acinus has both 25-

22. Salivation • Total of 1 to 1.5 L of saliva per day • Cells filter water from blood and add other substances • Food stimulates receptors that signal salivatory nuclei in medulla and pons • parasympathetic stimulation → salivary glands produce thin saliva, rich in enzymes • sympathetic stimulation → produce less abundant, thicker saliva, with more mucus • Higher brain centers stimulate salivatory nuclei so sight, smell and thought of food cause salivation 25-

23. Pharynx • Skeletal muscle • deep layer – longitudinal orientation • superficial layer – circular orientation • superior, middle and inferior pharyngeal constrictors 25-

24. Esophagus • Straight muscular tube 25-30 cm long • nonkeratinized stratified squamous epithelium • esophageal glands in submucosa • skeletal muscle in upper part and smooth in bottom • Extends from pharynx to cardiac stomach passing through esophageal hiatus in diaphragm • inferior pharyngeal constrictor excludes air from it • Lower esophageal sphincter closes orifice to reflux 25-

25. Swallowing 25-

26. Swallowing (Deglutition) • Series of muscular contractions coordinated by center in medulla and pons • motor signals from cranial nerves V, VII, IX and XII • Buccal phase • tongue collects food and pushes it back into oropharynx • Pharyngeal-esophageal phase • soft palate rises and blocks nasopharynx • infrahyoid muscles lift larynx; epiglottis folded back • pharyngeal constrictors push bolus down esophagus • liquids in 2 seconds -- food bolus may take 8 seconds • lower esophageal sphincter relaxes 25-

27. X-ray: Swallowing in Esophagus 25-

28. Stomach • Mechanically breaks up food, liquifies food and begins chemical digestion of protein and fat • resulting soupy mixture is called chyme • Does not absorb significant amount of nutrients • absorbs aspirin and some lipid-soluble drugs 25-

29. Gross Anatomy of Stomach • Muscular sac (internal volume from 50ml to 4L) • J - shaped organ with lesser and greater curvatures • regional differences • cardiac region just inside cardiac orifice • fundus - domed portion superior to esophageal opening • body - main portion of organ • pyloric region - narrow inferior end • antrum and pyloric canal • Pylorus - opening to duodenum • thick ring of smooth muscle forms a sphincter 25-

30. Innervation and Circulation • Innervation by • parasympathetic fibers from vagus • sympathetic fibers from celiac plexus • All blood from stomach enters hepatic portal circulation and is filtered through liver before returning to heart 25-

31. Gross Anatomy of Stomach • Notice: bulge of fundus, narrowing of pyloric region, thickness of pyloric sphincter and greater and lesser curvatures 25-

32. Gross Anatomy of Stomach 25-

33. Unique Features of Stomach Wall • Mucosa • simple columnar glandular epithelium • lamina propria is filled with tubular glands (gastric pits) • Muscularis externa has 3 layers • outer longitudinal, middle circular and inner oblique layers 25-

34. Gastric Gland 25-

35. Cells of Gastric Glands • Mucous cells secrete mucus • Regenerative cells • divide rapidly to produce new cells that migrate to surface • Parietal cells • secrete HCl acid and intrinsic factor • Chief cells • secrete pepsinogen • chymosin and lipase in infancy • Enteroendocrine cells • secrete hormones and paracrine messengers 25-

36. Opening of Gastric Pit 25-

37. Gastric Secretions • 2 to 3 L of gastric juice/day (H2O, HCl and pepsin) • Parietal cells contain carbonic anhydrase (CAH) CAH • CO2 + H2O → H2CO3→ HCO3- + H+ • H+ is pumped into stomach lumen by H+K+ATPase • antiporter uses ATP to pump H+ out and K+ in • HCO3- exchanged for Cl-(chloride shift) • Cl- pumped out to join H+ forming HCl • ↑ HCO3- in blood causes alkaline tide (blood pH ↑) 25-

38. Functions of Hydrochloric Acid • Activates pepsin and lingual lipase • Breaks up connective tissues and plant cell walls • liquefies food to form chyme • Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) • absorbed and used for hemoglobin synthesis • Destroys ingested bacteria and pathogens 25-

39. Gastric Enzymes and Intrinsic Factor • Intrinsic factor • essential for B12 absorption by small intestine • RBC production (lack causes pernicious anemia) • Pepsin - protein digestion • secreted as pepsinogen (inactive) • HCl converts it to pepsin (active) • Gastric lipase and chymosin • lipase digests butterfat of milk in infant • chymosin curdles milk by coagulating proteins 25-

40. Production and Action of Pepsin 25-

41. Chemical Messengers • Many produced by enteroendocrine cells • hormones enter blood → distant cells • paracrine secretions → neighboring cells • Gut-brain peptides • signaling molecules produced in digestive tract and CNS 25-

42. Gastric Motility • Swallowing center signals stomach to relax • Food stretches stomach activating a receptive-relaxation response • resists stretching briefly, but relaxes to hold more food • Rhythm of peristalsis controlled by pacemaker cells in longitudinal muscle layer • gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice • stronger contraction at pyloric region; ejects 3 ml • typical meal emptied from stomach in 4 hours 25-

43. Vomiting • Induced by • excessive stretching of stomach, psychological stimuli or chemical irritants (bacterial toxins) • Emetic center in medulla causes • retching • lower esophageal sphincter to relax • stomach and duodenum to contract spasmodically • vomiting • when abdominal contraction forces upper esophageal sphincter to open 25-

44. Healthy Mucosa and Peptic Ulcer 25-

45. Regulation of Gastric Secretion 25-

46. Regulation of Gastric Function (Phases 1-2) • Cephalic phase • sight, smell, taste or thought of food; vagus nerve stimulates gastric secretion and motility • Gastric phase • activated by presence of food or semidigested protein • by stretch or ↑ in pH • secretion stimulated by • ACh (from parasympathetic fibers), histamine (from gastric enteroendocrine cells) and gastrin (from pyloric G cells) • receptors on parietal and chief cells 25-

47. Regulation of Gastric Function (Phase 3) • Intestinal phase - duodenum regulates gastric activity through hormones and nervous reflexes • at first gastric activity increases (if duodenum is stretched or amino acids in chyme cause gastrin release) • enterogastric reflex - duodenum inhibits stomach • caused by acid and semi-digested fats in duodenum • chyme stimulates duodenal cells to release secretin, cholecystokinin (CCK) and gastric inhibitory peptide • all 3 suppress gastric secretion and motility 25-

48. Positive Feedback Control- Gastric Secretion 25-

49. Liver, Gallbladder and Pancreas • All release important secretions into small intestine to continue digestion 25-

50. Gross Anatomy of Liver • 3 lb. organ located inferior to the diaphragm • 4 lobes - right, left, quadrate and caudate • falciform ligament separates left and right • round ligament, remnant of umbilical vein • Gallbladder adheres to ventral surface between right and quadrate lobes 25-