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.

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

8. 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

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

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

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

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

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

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

15. 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

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

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

18. Chemistry of Digestion: Carbohydrates
Disccharides
Monosccharides
(Absorption)

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

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

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

22. 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

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

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

25. 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

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

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

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

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

30. 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

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

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

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

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

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

36. 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

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

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

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

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

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

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

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

48. 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)

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

50. 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

51. Intestinal Phase: Large Intestine Digestion & Absorption
Bacterial fermentation: Vit. K , lactate & buterate
Water and electrolyte secretion &/or absorption
Figure 21-28: NaCl reabsorption by colonocytes

52. Digestive Health: Protection & Problems
Immune defense: M-cells, Peyer's patches, lymphocytes
Irritable bowel disease – chronic inflammation
Diarrhea: leads to dehydration (4 million deaths/yr)
Osmotic-solutes prevent H2O reabsorption
Secretory- bacterial toxins ("flush out' pathogens)
Vomiting (emesis) can lead to alkalosis
Ulcers- H. pylori
"heart-burn"  acid reflux disease

53. Summary
Processes of digestion: ingestion, digestion, absorption, secretion, motility, reabsorption & defecation
Anatomy of digestion: mouth, esophagus, stomach, small & large intestines, rectum, anus (pancreas & liver)
Enzyme hydrolysis of carbohydrates, proteins, lipids and nucleic acids provide the nutrients, absorbed & conducted to liver for storage
Regulation from CNS (long reflex) and ENS (short reflex) integrate hormones & paracrines to coordinate digestion