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Chapter 21

Chapter 21. Digestion. About this Chapter. Overview of the digestive system & how it is organized How products are moved and the role of digestive secretions How and where food is broken down and absorbed How digestive wastes are concentrated and eliminated

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Chapter 21

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  1. Chapter 21 Digestion

  2. About this Chapter • Overview of the digestive system & how it is organized • How products are moved and the role of digestive secretions • How and where food is broken down and absorbed • How digestive wastes are concentrated and eliminated • How digestion is regulated in the gastrointestinal (GI) tract

  3. Processes of the Digestive System: Overview • Ingestion • Digestion: enzymatic • Motility • Secretion • Absorption • Elimination • (Self protection) Figure 21-1: Processes of the digestive system

  4. Digestive Anatomy: Overview • Oral cavity • Salivary glands • Esophagus • Stomach • Fundus • Body • (rugae) • Antrum • pyloris

  5. Chemical and mechanical digestion in the mouth • Saliva • Lubricates and softens food • Salivary amylase • Breaks down starches into smaller carbs • Mechanical digestion; mastication • Protection • Lysosomes and immunoglobulins kill viruses and bacteria.

  6. Figure 21-17: Integration of secretion in the stomach

  7. Intestinal Phase: ENDOCRINE RESPONSE • The pyloric sphincter sends chyme into the duodenum in spurts. • Acidic chyme in the duodenum stimulates hormone release into the blood of: • Secretin, cholecystokinin, GIP

  8. Digestive Anatomy: Overview Figure 21-2a: ANATOMY SUMMARY: The Digestive System

  9. More Digestive Anatomy • Small intestine • Duodenum • Jejunum • Ileum • Pancreas • Liver • Large intestine • Colon • Rectum • Anus

  10. Digestive Anatomy: Histological Overview • Stomach wall • Mucosa • Gastric glands • Muscularis mucosa • Submucosa • Muscularis Externa • Serosa • Small intestine wall • Vili & microvilli • Peyer's patches

  11. Digestive Anatomy: Histological Overview Figure 21-2c: ANATOMY SUMMARY: The Digestive System

  12. Digestive Anatomy: Histological Overview Figure 21-2e: ANATOMY SUMMARY: The Digestive System

  13. Motility: Smooth Muscle Contractions • Tonic – support • Phasic – move products • Parastalsis – moves  • Segmentation – mixes Figure 21-4: Contractions in the GI tract

  14. Digestive Secretions: (7 L / Day From Tissues into Lumen) • Salivary glands • Pancreas • Water • Enzymes • Mucus • Ions: H+, K+, Na+ • HCO3-, Cl- • Mass Balance (H2O) Figure 21-5: Daily mass balance in the digestive system

  15. Chemistry of Digestion: Carbohydrates • Complex carbohydrate foods • Long polymers • Enzyme hydrolysis • Amylase • Maltase • Sucrase • Lactase

  16. Chemistry of Digestion: Carbohydrates Figure 21-6: Carbohydrate digestion

  17. Chemistry of Digestion: Carbohydrates • Disccharides • Monosccharides • (Absorption)

  18. Chemistry of Digestion: Proteins • Proteins  enzyme hydrolysis  amino acids • Enzymes: endopeptidases & exopeptidases Figure 21-7: Endopeptidases and exopeptidases

  19. Chemistry of Digestion: Fats • Bile emulsification to small fat droplets • Enzymes: lipases, colipases & phospholipases • Triglycerides  monoglycerides & free fatty acids

  20. Chemistry of Digestion: Fats Figure 21-8: Fat digestion

  21. Regulating Digestion: CNS and Enteric Nervous System (ENS) • Long (cephalic) reflexes: CNS, feed forward & emotional reflexes • Short reflexes – ENS ("little brain")  integration, motility & secretion: enzymes & hormone/paracrine GI peptides

  22. Regulating Digestion: CNS and Enteric Nervous System (ENS) Figure 21-9: The enteric nervous system

  23. Phases of Digestion: Overview Figure 21-11: Overview of functions in different regions of the digestive system

  24. Cephalic and Oral Phases of Digestion • Cephalic: anticipation of food • CNS  ANS long reflex • Enteric  cells short reflex • GI motility • GI secretions • Mouth: starts digestion • Grind, mix & liquefy • Saliva: water, enzymes, mucus & lysozyme

  25. Cephalic and Oral Phases of Digestion Figure 21-12: Long and short reflexes in the stomach

  26. Swallowing reflex: Soft Palate & Esophagus • Deglutition • Epiglottis • Upper esophageal sphincter • Lower esophageal sphincter • (heartburn)

  27. Swallowing reflex: Soft Palate & Esophagus Figure 21-13: The swallowing reflex

  28. Gastric Phase: The Stomach • Storage • Digestion • HCl – parietal C. • Lipase – chief C. • Pepsin – chief C. • Protect walls • HCO3- • Mucus

  29. Secretion in the Stomach • Parietal cells – pH-1 • Chief cells – inactive pepsinogen  active pepsin • D cells – somatostatin • Enterochromaffin-like (ECL) cells – histamine • G cells – the hormone gastrin. • Mucuos cells – mucus and bicarbonate

  30. Gastric Phase: The Stomach Figure 21-17: Integration of secretion in the stomach

  31. Gastric Phase: The Stomach Figure 21-15: The mucus-bicarbonate barrier of the gastric mucosa

  32. Regulation of the Stomach Digestive Activities • CNS – cephalic input • ENS – ANS integration w/hormones & paracrines

  33. Regulation of the Stomach Digestive Activities Figure 21-17: Integration of secretion in the stomach

  34. Intestinal Phase: Reflexes Direct Digestive Action • Limit chyme entrance rate & motility • Neutralize HCl, add bile & enzymes

  35. Intestinal Phase: EXOCRINE RESPONSE • Pancreatic bicarbonate secretions • Secretin stimulates bicarbonate release. • Neutralize HCl, add bile & enzymes • Pancreatic exocrine enzyme secretions • CCK stimulates pancreatic release of inactive enzymes • Figure 21-20 • Liver adds bile via gall bladder • CCK stimulates gall bladder contraction • Bile; a non-enzyme • Bile salts • Act as detergents • Bilirubin • cholesterol

  36. Intestinal Phase: Pancreatic Secretions • Enzymes • HCO3- • Insulin • Glucagon Figure 21-18: The intestinal phase of gastric function

  37. Intestinal Phase: Pancreatic Secretions Figure 21-19: The hepatic portal system

  38. Intestinal Phase: Liver-Nutrient Storage & Conversions • Bile secretion • Hepatic portal system directs absorbed nutrients

  39. Intestinal Phase: Liver-Nutrient Storage & Conversions Figure 21-24: Carbohydrate absorption

  40. Intestinal Phase: Carbohydrate Digestion & Absorption • Hydrolysis to simple sugars • Absorption: transport • Na+/ glucose symport • Fructose GLUT • To ECF  capillary Figure 21-23: Bile salts

  41. Let’s remember what “fat” is. . . • Phospholipids • Triglycerides • cholesterol

  42. Intestinal Phase: Fat Digestion & Absorption • Hydrolyzed to small peptides, dipeptides, & amino acids • Membrane transport: H+, Na+ cotransports & transcytosis • To ECF then into capillary

  43. Intestinal Phase: Fat Digestion & Absorption Figure 21-26: Fat digestion and absorption

  44. Intestinal Phase: Large Intestine, H2O Absorption & Defecation • (Small intestine reabsorbs  7.5 L/day of water) • Large Intestine reabsorbs 1.4 L/day • Defecation Reflex: mass movement  rectal distension •  internal sphincter (invol)  external sphincter (vol)

  45. Intestinal Phase: Large Intestine, H2O Absorption & Defecation Figure 21-27: Anatomy of the large intestine

  46. Intestinal Phase: Large Intestine Digestion & Absorption • Bacterial digest significant amounts of complex carbs and proteins through fermentation. • Most remaining water is reabsorbed Figure 21-28: NaCl reabsorption by colonocytes

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