1 / 96

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.

ena
Télécharger la présentation

Chapter 18

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related