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Chapter 18

Chapter 18. The Digestive System. 18-1. Chapter 18 Outline Functions of GI Tract Structure of Digestive System From Mouth to Stomach Stomach Small Intestine Large Intestine Liver Gall Bladder & Pancreas Control & Phases of Digestion Digestion & Absorption of Food Types. 18-2.

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Chapter 18

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  1. Chapter 18 The Digestive System 18-1

  2. Chapter 18 Outline • Functions of GI Tract • Structure of Digestive System • From Mouth to Stomach • Stomach • Small Intestine • Large Intestine • Liver • Gall Bladder & Pancreas • Control & Phases of Digestion • Digestion & Absorption of Food Types 18-2

  3. Overview • Inside gastrointestinal (GI) tract, food is broken down by hydrolysis into molecular monomers • Absorption of monomers occurs in small intestine 18-3

  4. Fig 18.1 18-4

  5. Functions of GI Tract 18-5

  6. Motility • Is movement of food through GI tract by means of: • Ingestion--taking food into mouth • Mastication--chewing food & mixing it with saliva • Deglutition--swallowing food • Peristalsis--rhythmic wave-like contractions that move food through GI tract 18-6

  7. Secretion • Includes release of exocrine & endocrine products into GI tract • Exocrine secretions include: HCl, H20, HC03-, bile, lipase, pepsin, amylase, trypsin, elastase, & histamine • Endocrine includes hormones secreted into stomach & small intestine to help regulate GI system • E.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, & somatostatin 18-7

  8. Absorption • Is passage of digested end products into blood or lymph 18-8

  9. Storage and Elimination • Includes temporary storage & subsequent elimination of indigestible components of food 18-9

  10. Structure of Digestive System 18-10

  11. Digestive System • Is composed of GI tract (alimentary canal) & accessory digestive organs • GI tract is 30 ft long; extends from mouth to anus 18-11

  12. Digestive System continued Fig 18.2 • Organs include oral cavity, pharynx, esophagus, stomach, & small & largeintestine • Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, & pancreas 18-12

  13. Layers of GI Tract • Are called tunics • The 4 tunics are mucosa, submucosa, muscularis, & serosa Fig 18.3 18-13

  14. Mucosa • Is the absorptive & secretory layer lining lumen of GI tract • In places is highly folded with villi to increase absorptive area • Contains lymph nodules, mucus-secreting goblet cells, & thin layer of muscle Fig 18.3 18-14

  15. Submucosa • Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood & lymphatic vessels • Contains glands & nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae Fig 18.3 18-15

  16. Muscularis • Is responsible for segmental contractions & peristaltic movement through GI tract • Has an inner circular & outer longitudinal layer of smooth muscle • Activity of these layers moves food through tract while pulverizing & mixing it • Myenteric plexus between these layers is major nerve supply to GI tract • Includes fibers & ganglia from both Symp & Parasymp systems 18-16

  17. Serosa • Is outermost layer; serves to bind & protect • Consists of areolar connective tissue covered with layer of simple squamous epithelium Fig 18.3 18-17

  18. Regulation of GI Tract • Parasympathetic effects, arising from vagus & spinal nerves, stimulate motility & secretions of GI tract • Sympathetic activity reduces peristalsis & secretory activity • GI tract contains an intrinsic system that controls its movements--the enteric nervous system • GI motility is influenced by paracrine & hormonal signals 18-18

  19. From Mouth to Stomach 18-19

  20. From Mouth to Stomach • Mastication (chewing) mixes food with saliva which contains salivary amylase • An enzyme that catalyzes partial digestion of starch 18-20

  21. From Mouth to Stomach continued • Deglutition (swallowing) begins as voluntary activity • Oral phase is voluntary & forms a food bolus • Pharyngeal & esophagealphases are involuntary & cannot be stopped • To swallow, larynx is raised so that epiglottis covers entrance to respiratory tract • A swallowing center in medulla orchestrates complex pattern of contractions required for swallowing 18-21

  22. From Mouth to Stomach continued • Esophagus connects pharynx to stomach • Upper third contains skeletal muscle • Middle third contains mixture of skeletal & smooth • Terminal portion contains only smooth • Passes through diaphragm via esophageal hiatus 18-22

  23. From Mouth to Stomach continued • Peristalsis propels food thru GI tract • = wave-like muscular contractions • After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux Fig 18.4 18-23

  24. Stomach 18-24

  25. Stomach • Is most distensible part of GI tract • Empties into the duodenum • Functions in: storage of food; initial digestion of proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine 18-25

  26. Stomach continued • Is enclosed by gastroesophageal sphincter on top & pyloric sphincter on bottom • Is divided into 3 regions: • Fundus • Body • Antrum Fig 18.5 18-26

  27. Stomach continued • Inner surface of stomach is highly folded into rugae • Contractions of stomach churn chyme, mixing it with gastric secretions • Eventually these will propel food into small intestine Fig 18.5 18-27

  28. Stomach continued Fig 18.7 • Gastric mucosa has gastric pits in its folds • Cells that line folds deeper in the mucosa, are exocrine gastric glands 18-28

  29. Stomach continued • Gastric glands contain cells that secrete different products that form gastric juice • Goblet cells secrete mucus • Parietal cells secrete HCl & intrinsic factor (necessary for B12 absorption in intestine) • Chief cells secrete pepsinogen (precursor for pepsin) Fig 18.7 18-29

  30. Stomach continued • Enterochromaffin-like cells secrete histamine & serotonin • G cells secrete gastrin • D cells secrete somatostatin Fig 18.7 18-30

  31. HCl in Stomach Fig 18.8 • Is produced by parietal cells which AT H+ into lumen via an H+/ K+ pump (pH ≈1) • Cl- is secreted by facilitated diffusion • H+ comes from dissociation of H2CO3 • Cl- comes from blood side of cell in exchange for HC03- 18-31

  32. HCl in Stomach continued • Is secreted in response to the hormone gastrin; & ACh from vagus • These are indirect effects since both stimulate release of histamine which causes parietal cells to secrete HCl 18-32

  33. HCl in Stomach continued • Makes gastric juice very acidic which denatures proteins to make them more digestible • Converts pepsinogen into pepsin • Pepsin is more active at low pHs Fig 18.9 18-33

  34. Digestion & Absorption in Stomach • Proteins partially digested by pepsin • Carbohydrate digestion by salivary amylase is soon inactivated by acidity • Alcohol & aspirin are only commonly ingested substances absorbed 18-34

  35. Gastric and Peptic Ulcers • Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl • In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin • Helicobacter pylori infection is associated with ulcers • Antibiotics are useful in treating ulcers • Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation • Why histamine receptor blockers such as Tagamet & Zantac can treat gastritis 18-35

  36. Protective Mechanisms of Stomach • Include: • Impermeability of parietal & chief cells to HCl • A layer of alkaline mucus containing HC03- • Tight junctions between adjacent epithelial cells • Rapid rate of cell division (entire epithelium replaced in 3 days) • Prostaglandins (PGs) inhibit gastric secretions • Which is why PG blockers such as NSAIDs can cause ulcers 18-36

  37. Small Intestine 18-37

  38. Small Intestine (SI) • Is longest part of GI tract; approximately 3m long • Duodenum is 1st 25cm after pyloric sphincter • Jejunum is next 2/5s • Ileum is last 3/5s; empties into large intestine Fig 18.10 18-38

  39. Small Intestine (SI) continued • Absorption of digested food occurs in SI • Facilitated by long length & tremendous surface area 18-39

  40. Small Intestine (SI) continued Fig 18.10 • Surface area increased by foldings & projections • Large folds are plicae circulares • Microscopic finger-like projections are villi • Apical hair-like projections are microvilli 18-40

  41. Small Intestine (SI) continued • Each villus is covered with columnar epithelial cells interspersed with goblet cells • Epithelial cells at tips of villi are exfoliated & replaced by mitosis in crypts of Lieberkuhn • Inside each villus are lymphocytes, capillaries, & central lacteal Fig 18.12 18-41

  42. Small Intestine (SI) continued • A carpet of hair-like microvilli project from apical surface of each epithelial cell • Create a brush border 18-42

  43. Intestinal Enzymes • Attached to microvilli are brush border enzymes that are not secreted into lumen • Enzyme active sites are exposed to chyme 18-43

  44. Intestinal Contractions and Motility Fig 18.14 • 2 major types of contractions occur in SI: • Peristalsis is weak & slow & occurs mostly because pressure at pyloric end is greater than at distal end • Segmentation is major contractile activity of SI • Is contraction of circular smooth muscle to mix chyme 18-44

  45. Large Intestine 18-48

  46. Large Intestine (LI) or Colon • Has no digestive function but absorbs H20, electrolytes, B & K vitamins, & folic acid • Internal surface has no villi or crypts & is not very elaborate • Contains large population of microflora • LI bacteria produce folic acid & vitamin K & ferment indigestible food to produce fatty acids 18-49

  47. Large Intestine (LI) or Colon continued • Extends from ileocecal valve at end of SI to anus • Outer surface bulges to form pouches (haustra) • Chyme from SI enters cecum, then passes to ascending colon, transverse colon, descending colon, sigmoid colon, rectum, & anal canal Fig 18.17 18-50

  48. Fluid & Electrolyte Absorption in LI • SI absorbs most water but LI absorbs 90% of water it receives • Begins with osmotic gradient set up by Na+/K+ pumps • Water follows by osmosis • Salt & water reabsorption stimulated by aldosterone 18-51

  49. Defecation • After electrolytes & water have been absorbed, waste material passes to rectum, creating urge to defecate • Defecation reflex begins with relaxation of external anal sphincter allowing feces to enter anal canal • Longitudinal rectal muscles contract to increase rectal pressure; internal anal sphincter relaxes • Excretion is aided by contractions of abdominal & pelvic muscles which push feces from rectum 18-52

  50. Liver 18-53

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