The Digestive System

1. The Digestive System Chapter 19

2. Gastrointestinal (GI) Tract • Tube that includes: mouth, Pharynx, Esophagus, Stomach, Small intestine, Large intestine • Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas

3. Figure 19.1

4. Overview- Operations • Ingestion: eating • Secretion: release of water, enzymes & buffers • Mixing & propulsion: movement along GI tract • Digestion: mechanical and chemical breakdown of foods • Absorption: getting it into the body • Defecation: dumping waste products = defecation

5. Wall Layers- Everywhere • 4 layers • Mucosa- epithelium, connective layer, glands, muscularis mucosae • Submucosa- connective tissue, blood vessels, lymphatic vessels, enteric nervous system • Muscularis-circular layer, longitudinal layer • In mouth, pharynx & upper esophagus –skeletal muscle • Also in external anal sphincter • Serosa or Visceral peritoneum

6. Figure 19.2

7. Figure 19.3a

8. Figure 19.3b

9. Mouth • Formed by cheeks, hard & soft palate & tongue • Soft palate at back includes a “hangy down” part = uvula • During swallowing uvula prevents entry into nasal cavity • Tongue- muscular accessory organ • maneuvers food for chewing • Adjusts shape for speech & swallowing • Lingual tonsils at base of tongue

10. Salivary Glands • 3 pairs of salivary glands • Ducts empty into oral cavity • Parotid- • inferior & anterior to ears • Submandibular- • in floor of mouth, medial & inferior to mandible • Sublingual • Beneath tongue and superior to submandibular • Saliva contains 99.5% water, salivary amylase, mucus and other solutes • Dissolves food & starts digestion of starches

11. Figure 19.4

12. Teeth • Accessory organs in bony sockets of mandible & maxilla • 3 external regions: • Crown- above gums • Root- 1 or more parts embedded in socket • Neck – between crown and root near gum line • 3 layers of material • Enamel- covers crown • Dentin- majority of interior of tooth • Pulp cavity - nerve, blood vessel & lymphatics

13. Figure 19.5

14. Digestion in the Mouth • Mechanical breakdown- chewing • Mixed with saliva by tongue • Salivary amylase chemically breaks down polysaccharides (starch) • maltose and larger fragments • Continues in the stomach until acidified • Rounds up food into a soft bolus for swallowing

15. Pharynx & Esophagus • On swallowing: • Bolus of food  oropharynx • Laryngopharynx esophagus • Muscular contractions in pharynx help • Upper esophageal sphincter (UES) • Skeletal muscle –controls entry to esophagus • Lower esophageal sphincter (LES) • Smooth muscle- regulates entry to stomach

16. Figure 19.6a,b

17. Swallowing • Voluntary: bolus forced into oropharynx • Triggers oropharyngeal stage • Involuntary & breathing interrupted • Soft palate move up-close nasopharynx • Epiglottis seals off larynx • Bolus moves into esophagus through UES • Esophageal stage peristalsis moves it toward stomach

18. Figure 19.6c

19. Stomach • J- shaped enlargement of tract • Serves as mixing chamber and holding reservoir • Very elastic & muscular • 4 regions • Cardia- surrounds upper opening • Fundus- superior & to left of cardia • Body – large central portion • Pylorus- lower part leading to pyloric sphincter & duodenum

20. Figure 19.7

21. Stomach Wall • Mucosa: • Folds called rugae • Epithelium- simple columnar mucous • Form gastric glands lining gastric pits • Secretory cells: mucous neck cells • Chief cells inactive enzyme pepsinoge • Parietal cells HCl & intrinsic factor • Collectively = gastric juice • Muscularis- 3 Layers: longitudinal, circular & oblique

22. Figure 19.8

23. Figure 19.9

24. Digestion & Absorption • Food entry stretch & rise in pH • Nerve impulses  secretion & mixing waves Food mixed with juice Chyme • Small amount pushed through pyloric sphincter • = gastric emptying- Carb. foods fastest, lipids next & proteins slowest • Entry in duodenum feedback inhibition of stomach activity • Pepsin digests protein peptides • Little absorption- water, ions & some drugs

25. Pancreas • Behind stomach- • Produces pancreatic juice in acinar cells • to duodenum via pancreatic duct • NaHCO3 solution (pH 7.1-8.2)– 1000ml/day • Neutralize stomach acid and dilutes chyme • Panceas- digestive enzymes • Proteases: chymotrypsinogen, trypsinogen, et. al. • Activated by entreokinase from intestine • Starch digesting- pancreatic amylase • Pancreatic lipase • Nucleotidases – RNAase & DNAase

26. Liver & Gall Bladder • Largest organ after the skin • On right below diaphragm • Functional unit is lobule- • Hepatocytes around central vein • Open capillaries = sinusoids • Bile  canaliculi ducts hepatic duct • Gall bladder =Pear-shaped organ on front (stores bile) • cystic duct common bile duct

27. Bile • Bicarbonate, bile salts & waste. – 1000 ml/day • Important for emulsifying fats • Increases surface area for digestion • Pigment is bilirubin- from broken-down heme during RBC recycling • Digested to strecobilin- brown color • Bile salts reabsorbed at end of small intestine- ileum • recycle to liver in portal circulation

28. Figure 19.10

29. Figure 19.11a

30. Figure 19.11b

31. Liver Function • Maintains blood glucose • Stores as glycogen • Uses absorbed sugars & Converts amino acids glucose • Lipid metabolism • Produces cholesterol & triglycerides, makes bile • Makes lipoproteins for lipid transport • Excretion of bilirubin • Processes drugs and other chemicals • Store fat soluble vitamins • Make active vitamin D

32. Small Intestine • 3 parts: duodenum, jejunum, ileum • Where most of the digestion occurs • Essentially all of the nutrient absorption • Ends in ileocecal sphincter

33. Figure 19.12a

34. Figure 19.12b

35. Wall Structure • Same 4 layers • Epithelial- simple columnar • Absorptive cells with microvilli • Goblet cells- secrete mucus • Intestinal glands- intestinal juice & hormones • Secretin, cholecystokinin (CCK), Glucose-dependent-insulinotrophic peptide (GIP) • Lymphatic tissue- defense

36. Wall Structure (Cont.) • Duodenal glands- alkaline mucus • Helps neutralize stomach acid • Circular folds- increase surface area • Villi- finger like projections of mucosa • Increase surface area for absorption • Include lacteals for lipid absorption

37. Figure 19.13

38. Motility & Secretions • Secretions: alkaline, some enzymes • Peptidases-breaks small peptides • Disaccharidases attached to wall • Water and salt to balance osmolality • ~2000 ml/day • Segmentation activity- for mixing • Peristalsis for movement after most absorption completed- slow waves

39. Digestion & Absorption • Chyme enters with partially digested carbohydrates & proteins • Bile + pancreatic juice + intestinal juice completes the job • Absorption is of monosaccharides; amino acids; phosphate sugar & bases of DNA & RNA; fatty acids & monoglycerides

40. Carbohydrate Digestion • Amylases: • Starch & dextrin  maltose • Disaccharidases at surface: • Maltose: maltose  glucose • Sucrase: sucrose  glucose & fructose • Lactase: lactose glucose & galactose

41. Protein & Fat Digestion • Trypsin, chymotrypsin, elastase, carboxypeptidase & pepsin • Proteins small peptides • Peptidases at surface: • Peptides amino acids & di- & tri- peptides • Lipase: • glycerides fatty acids & monoglycerides

42. Absorption • By diffusion, facilitated diffusion, osmosis & active transport • Carbohydrates  monosaccharides • Via portal system to liver • Proteins (jejunum & ileum) amino acids • Via portal system to liver • Lipids reformed to triglycerides • Packaged in chlyomicrons with protein • Via lacteals  lymphatics

43. Absorption (Cont.) • Water & salt • Primarily osmotic movement along with other nutrients • Vitamins: • Fat soluble absorbed with fat • Water soluble with simple diffusion • B12 combines with intrinsic factor & absorbed by active transport in ileum

44. Figure 19.14a

45. Figure 19.14b

46. Large Intestine • Cecum, colon, rectum, anal canal • Ileocecal canal large intestine • Below is cecum with appendix • Colon- ascending, transverse, descending & sigmoid •  rectum anal canal • Standard 4 layers with mucus secretion • Few folds , little specialization for absorption • Muscularis: circular + bands of longitudinal muscle

47. Figure 19.15a

48. Figure 19.15b

49. Figure 19.16

50. Digestion & Absorption • Slow emptying of ileum • Slow peristalsis • Mass peristalsis with food in stomach • Moves from middle of colon  rectum • Bacterial digestion • Produce some B-vitamins & Vit. K • Produce gases= flatus • Colon absorbs salt & water