1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 18 The Digestive System 18-1

2. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Overview  Inside gastrointestinal (GI) tract, food is broken down by hydrolysis into molecular monomers  Absorption of monomers occurs in small intestine 18-3

4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig 18.1 18-4

5. Functions of GI Tract 18-5

6. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Motility 18-6

7. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Includes release of exocrine & endocrine products into GI tract  Exocrine secretions include: HCl, H 2 0, HC0 3 -, 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 Secretion 18-7

8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Is passage of digested end products into blood or lymph Absorption 18-8

9. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Includes temporary storage & subsequent elimination of indigestible components of food Storage and Elimination 18-9

10. Structure of Digestive System 18-10

11. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestive System continued  Organs include oral cavity, pharynx, esophagus, stomach, & small & large intestine  Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, & pancreas Fig 18.2 18-12

13. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Layers of GI Tract  Are called tunics  The 4 tunics are mucosa, submucosa, muscularis, & serosa Fig 18.3 18-13

14. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Mucosa Fig 18.3 18-14

15. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Submucosa Fig 18.3 18-15

16. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Muscularis 18-16

17. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Regulation of GI Tract 18-18

19. From Mouth to Stomach 18-19

20. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Mastication (chewing) mixes food with saliva which contains salivary amylase  An enzyme that catalyzes partial digestion of starch From Mouth to Stomach 18-20

21. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Deglutition (swallowing) begins as voluntary activity  Oral phase is voluntary & forms a food bolus  Pharyngeal & esophageal phases 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 From Mouth to Stomach continued 18-21

22. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 From Mouth to Stomach continued 18-22

23. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Peristalsis propels food thru GI tract  = wave-like muscular contractions  After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux From Mouth to Stomach continued Fig 18.4 18-23

24. Stomach 18-24

25. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Stomach 18-25

26. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Stomach continued Fig 18.5 18-27

28. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach continued  Gastric mucosa has gastric pits in its folds  Cells that line folds deeper in the mucosa, are exocrine gastric glands Fig 18.7 18-28

29. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Gastric glands contain cells that secrete different products that form gastric juice  Goblet cells secrete mucus  Parietal cells secrete HCl & intrinsic factor (necessary for B 12 absorption in intestine)  Chief cells secrete pepsinogen (precursor for pepsin) Stomach continued Fig 18.7 18-29

30. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Enterochromaffin- like cells secrete histamine & serotonin  G cells secrete gastrin  D cells secrete somatostatin Stomach continued Fig 18.7 18-30

31. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HCl in Stomach  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 H 2 CO 3  Cl- comes from blood side of cell in exchange for HC0 3 - Fig 18.8 18-31

32. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 HCl in Stomach continued 18-32

33. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 HCl in Stomach continued 18-33

34. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Include:  Impermeability of parietal & chief cells to HCl  A layer of alkaline mucus containing HC0 3 -  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 Protective Mechanisms of Stomach 18-36

37. Small Intestine 18-37

38. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Small Intestine (SI) Fig 18.10 18-38

39. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Absorption of digested food occurs in SI  Facilitated by long length & tremendous surface area Small Intestine (SI) continued 18-39

40. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Surface area increased by foldings & projections  Large folds are plicae circulares  Microscopic finger- like projections are villi  Apical hair-like projections are microvilli Small Intestine (SI) continued Fig 18.10 18-40

41. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Attached to microvilli are brush border enzymes that are not secreted into lumen  Enzyme active sites are exposed to chyme Intestinal Enzymes 18-43

44. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intestinal Contractions and Motility  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 Fig 18.14 18-44

45. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Occur automatically via endogenous pacemaker activity  Contractions are driven by graded depolarizations called slow waves Intestinal Contractions & Motility continued Fig 18.15 18-45

46. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Slow waves are produced by non- neuronal interstitial cells of Cajal (ICC)  Conducted to smooth muscle via gap junctions  Slow waves spread from 1 smooth muscle cell to another thru nexuses Intestinal Contractions & Motility continued Fig 18.16 18-46

47. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  When slow waves exceed threshold, trigger APs in smooth muscle by opening V-gated Ca 2+ channels  Influx of Ca 2+ produces depolarization phase of AP & stimulates contraction  Repolarization via K + efflux  Contractions are modified by ANS activity  ACh from Parasymp increases amplitude & duration of slow waves  NE & Epi from Symp decrease activity of intestines Intestinal Contractions & Motility continued 18-47

48. Large Intestine 18-48

49. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Has no digestive function but absorbs H 2 0, 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 Large Intestine (LI) or Colon 18-49

50. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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 Large Intestine (LI) or Colon continued Fig 18.17 18-50

51. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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  LI can also secrete H 2 0 via AT of NaCl into intestinal lumen Fluid & Electrolyte Absorption in LI 18-51

52. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

53. Liver 18-53

54. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Structure of Liver  Liver largest internal organ  Hepatocytes form hepatic plates that are 1–2 cells thick  Plates separated by sinusoids which are fenestrated & permeable even to proteins  Contain phagocytic Kupffer cells Fig 18.20 18-54

55. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Food absorbed in SI is delivered 1st to liver  Capillaries in digestive tract drain into hepatic portal vein which carries blood to liver  Hepatic vein drains liver  Liver also receives blood from hepatic artery Hepatic Portal System 18-55

56. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver Lobules  Are functional units formed by hepatic plates  In middle of each is central vein  At edge of each lobule are branches of hepatic portal vein & artery which open into sinusoids Fig 18.20 18-56

57. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver Lobules  Bile is secreted by hepatocytes in bile canaliculi  Empty into bile ducts which flow into hepatic ducts that carry bile away from liver Fig 18.21 18-57

58. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Enterohepatic Circulation  Is recirculation of compounds between liver & intestine  Many compounds are released in bile, reabsorbed in SI, & returned to liver to be recycled  Liver excretes drug metabolites into bile to pass out in feces Fig 18.22 18-58

59. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 18-59

60. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Amounts to 250–1500 ml/day  Bile pigment (bilirubin) is produced in spleen, bone marrow, & liver  Is a derivative of heme groups (minus iron) from Hb  Carried in blood attached to albumin  Free bilirubin combines with glucuronic acid to form conjugated bilirubin that is secreted into bile  Converted by intestinal bacteria to urobilinogen  30-50% of urobilogen is absorbed by intestine & enters hepatic vein  Thus enters enterohepatic circulation to be recycled or filtered by kidneys & excreted in urine Bile Production and Secretion 18-60

61. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolism of Heme and Bilirubin Fig 18.23 18-61

62. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bile Acids  Are formed in major breakdown pathway for cholesterol  Are mostly cholic & chenodeoxycholic acids  Form bile salts by combining with glycine or taurine  Bile salts aggregate as micelles  95% of bile acids are absorbed by ileum Fig 18.25 18-62

63. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Detoxification of Blood  Liver can remove hormones, drugs, & other biologically active molecules from blood by:  Excretion into bile  Phagocytosis by Kupffer cells  Chemical alteration of molecules  E.g. ammonia is produced by deamination of amino acids in liver  Liver converts it to urea which is excreted in urine 18-63

64. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Liver conjugates steroid hormones & xenobiotics with groups that make them anionic  Which can be transported into bile or urine by multispecific organic anion transport carriers & excreted  Cytochrome P450 enzymes are involved in hepatic metabolism of steroids & drugs Detoxification of Blood continued 18-64

65. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secretion of Glucose, Triglycerides & Ketones  Liver helps regulate blood glucose by removing it from blood or releasing it to blood  Removes it via glycogenesis & lipogenesis  Or produces it via glycogenolysis & gluconeogenesis  Can convert free fatty acids into ketone bodies (ketogenesis) that can be used for energy during fasting 18-65

66. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Production of Plasma Proteins  Albumin & most of plasma globulins are produced by liver  Albumin makes up 70% of total plasma protein & contributes most to colloid osmotic pressure of blood  Globulins transport cholesterol & hormones, inhibit trypsin, & are involved in blood clotting  Constitute many of the clotting factors 18-66

67. Gall Bladder & Pancreas 18-67

68. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Is a sac-like organ attached to inferior surface of liver  Stores & concentrates bile continuously produced by liver  When SI is empty, sphincter of Oddi in common bile duct closes & bile is forced into gallbladder  Expands as it fills with bile  When food is in SI, sphincter of Oddi opens, gall bladder contracts, & bile is ejected thru cystic duct into common bile duct then to duodenum Gallbladder Fig 18.26 18-68

69. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreas  Is located behind stomach  Has both endocrine & exocrine functions  Endocrine function performed by islets of Langerhans  Secretes insulin & glucagon  Exocrine secretions include bicarbonate solution & digestive enzymes  These pass in pancreatic duct to SI  Exocrine secretory units are acini 18-69

70. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig 18.26 18-70

71. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig 18.28 18-71

72. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Contains water, bicarbonate, & digestive enzymes  Digestive enzymes include amylase for starch, trypsin for proteins, and lipase for fats  Brush border enzymes are also required for complete digestion Pancreatic Juice 18-72

73. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreatic Juice  Most pancreatic enzymes are produced in inactive form (zymogens)  Trypsin is activated by brush border enzyme, enterokinase  Trypsin activates other zymogens Fig. 18.29 Fig 18.30 18-73

74. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 18-74

75. Control & Phases of Digestion 18-75

76. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Neural & endocrine mechanisms modify activity of GI system  Vagus nerve is heavily involved in regulating & coordinating digestive activities  GI tract is both an endocrine gland & target for action of hormones  Hormones include secretin, gastrin, CCK, & GIP Neural and Endocrine Regulation 18-76

77. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Gastric motility & secretion occur automatically  Waves of contraction are initiated spontaneously by pacesetter cells & secretion occurs in absence of hormonal & neural input  ANS & hormonal effects are superimposed on automatic activity  Extrinsic control of gastric function is divided into cephalic, gastric, & intestinal phases Regulation of Gastric Function 18-77

78. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cephalic Phase  Refers to control by brain of vagus activity  Stimulated by sight, smell, & taste of food  Activation of vagus:  Stimulates chief cells to secrete pepsinogen  Directly stimulates G cells to secrete gastrin  Directly stimulates ECL cells to secrete histamine  Indirectly stimulates parietal cells to secrete HCl  Continues into 1 st 30 min of a meal 18-78

79. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Arrival of food in stomach stimulates gastric phase  Gastric secretion stimulated by distension of stomach & chemical nature of chyme Gastric Phase 18-79

80. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Short polypeptides & amino acids stimulate G cells to secrete gastrin & chief cells to secrete pepsinogen  Gastrin stimulates ECL cells to secrete histamine which stimulates parietal cell secretin of HCl  This is a positive feedback mechanism: As more HCl & pepsinogen are secreted, more polypeptides & amino acids are released Gastric Phase continued Fig 18.31 18-80

81. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Secretion of HCl is also regulated by a negative feedback mechanism:  HCl secretion decreases if pH < 2.5; at pH 1 gastrin secretion stops  D cells stimulate secretion of somatostatin which inhibits gastrin secretion Gastric Phase continued Fig 18.31 18-81

82. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Begins with inhibition of gastric activity when chyme enters SI  Arrival of chyme in SI is detected by sensory neurons of vagus  This causes a neural reflex that inhibits gastric motility & secretion  Fat in chyme stimulates SI to secrete enterogasterones--hormones that inhibit gastric motility & secretion  Enterogasterones include somatostatin, CCK, & GLP-1 Intestinal Phase 18-82

83. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Submucosal & myenteric plexuses contain as many neurons as spinal cord  Includes preganglionic Parasymp axons, ganglion cell bodies, postganglionic Symp axons; & afferent intrinsic & extrinsic sensory neurons; interneurons, & glia  Peristalsis is controlled by enteric NS Enteric Nervous System 18-83

84. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  For peristalsis:  ACh & substance P stimulate smooth muscle contraction above bolus  NO, VIP, & ATP stimulate smooth muscle relaxation below bolus Insert fig. 18.31 Enteric Nervous System continued Fig 18.32 18-84

85. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Paracrine Regulators of Intestine  ECL cells release serotonin & motilin in response to pressure & chemical stimuli in SI  Serotonin stimulates intrinsic afferents which activate motor neurons in intrinsic NS  Motilin stimulates contraction in duodenum & stomach antrum  Guanylin, from ileum & colon, stimulates production of cGMP which inhibits absorption of Na + & causes secretion of Cl - & H 2 0 18-85

86. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intestinal Reflexes  Can be mediated by enteric NS & paracrines; & regulated by ANS & hormones  Gastroileal reflex refers to increased motility of ileum & movement of chyme thru ileocecal sphincter in response to increased gastric activity  Ileogastric reflex decreases gastric motility in response to distension of ileum  Intestino-intestinal reflex causes relaxation of rest of intestine when any part is overdistended 18-86

87. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Secretion of pancreatic juice & bile is stimulated by secretin & bile  Secretin is secreted in response to duodenal pH < 4.5  Stimulates release of HC0 3 - into SI by pancreas & into bile by liver  CCK is secreted in response to fat & protein content of chyme in duodenum  Stimulates production of pancreatic enzymes  Enhances secretin  Stimulates contraction of sphincter of Oddi Secretion of Pancreatic Juice 18-87

88. Digestion & Absorption of Food Types 18-88

89. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestion & Absorption of Carbohydrates  Most carbohydrates are ingested as starch-- a polymer of glucose  Salivary amylase begins starch digestion  Pancreatic amylase converts starch to oligosaccharides  Oligosaccharides hydrolyzed by SI brush border enzymes Fig 18.33 18-89

90. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Begins in stomach when pepsin digests proteins to form polypeptides  In SI, endopeptidases (trypsin, chymotrypsin, elastase) cleave peptide bonds in interior of polypeptides  SI exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides Digestion & Absorption of Protein 18-90

91. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Protein digestion in SI results in free amino acids, dipeptides, & tripeptides  Which are transported into SI cells where di- & tripeptides are broken down to amino acids  Which are secreted into blood Digestion & Absorption of Protein continued Fig 18.34 18-91

92. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Occurs in SI  Arrival of lipids in duodenum causes secretion of bile  Fat is emulsified by bile salt micelles  Forms tiny droplets of fat dissolved in bile salt micelles  Greatly increases surface area for fat digestion Digestion & Absorption of Lipids 18-92

93. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Pancreatic lipase hydrolyzes triglycerides to free fatty acids & monglycerides  Phospholipase A breaks down phospholipids into fatty acids & lysolecithin Digestion & Absorption of Lipids continued Fig 18.35 18-93

94. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Products of fat digestion dissolve in micelles forming mixed micelles which move to brush border Digestion & Absorption of Lipids continued Fig 18.36 18-94

95. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Free fatty acids, monoglycerides, & lysolecithin leave micelles & enter epithelial cells  Inside epithelial cells, they are resynthesized into triglycerides & phospholipids Digestion & Absorption of Lipids continued Fig 18.37 18-95

96. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Triglycerides & phospholipids combine with a protein to form small particles called chylomicrons  Which are secreted into central lacteals of SI villi Digestion & Absorption of Lipids continued Fig 18.37 18-96

97. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  In blood, endothelial lipoprotein lipase hydrolyzes triglycerides to free fatty acids & glycerol for use in cells  Cholesterol-containing remnants are taken up by liver Transport of Lipids 18-97

98. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Cholesterol & triglycerides from liver form VLDLs which are secreted & take triglycerides to cells  Once triglycerides are removed, VLDLs become LDLs  LDLs transport cholesterol to organs & blood vessels  HDLs transport excess cholesterol back to liver  High ratio of HDL-cholesterol to total cholesterol is believed to confer protection against atherosclerosis Transport of Lipids continued 18-98