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Understanding the Digestive System: Processes, Anatomy, and Regulation

This chapter provides a comprehensive overview of the digestive system, including its organization, processes, and anatomy. It examines how food is ingested, digested, absorbed, and eliminated. Key topics include the roles of digestive secretions and motility, along with enzymatic digestion of carbohydrates, proteins, and fats. It also explores the regulatory mechanisms of digestion through the CNS and Enteric Nervous System. Anatomical details of the oral cavity, stomach, small intestine, pancreas, liver, and large intestine are discussed, alongside the roles of various enzymes and hormones in digestion.

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Understanding the Digestive System: Processes, Anatomy, and Regulation

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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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