1. Digestion and Absorption

2. Digestive System Reduces particle size
Helps to absorb micronutrients and trace elements
Sets a physical and immunologic barrier

3. Digestion
Breaks down
Carbohydrates (starch and sugar) → single sugar molecules
Proteins → amino acids
Fats → fatty acids, glycerol

4. Enzymes
Proteins that break bonds

5. The Digestive System
Is a long tube from the mouth to the anus

6. Digestive System Mouth to anus Taste and smell
Epithelium lines the lumen
Barrier to invaders
Submucosal layer
Muscularis
Taste and smell

7. Peristalsis
Muscular contractions that move food along throughout GI tract

8. The Mouth Mastication: chewing Saliva
Salivary glands produce about 1.5 liters of saliva daily
Salivary amylase (ptyalin) begins to break down starch (inactivated in the stomach)
Salivary lipase: begins fat digestion, but has minimal effect
Mucus to lubricate the food for easier swallowing
Lysozyme to kill bacteria

9. Tongue Taste receptors: sweet, sour, salty, bitter, savory
Enzymes to help break down fatty acids

10. Swallowing Bolus is the food swallowed at one time
Epiglottis blocks windpipe, prevents choking

11. The Esophagus Long tube Connects pharynx to the stomach
Peristalsis, muscle contraction
Lower esophageal sphincter
Heartburn

12. The Stomach
Lower esophageal sphincter and pyloric sphincter control entry and exit from stomach
Storage capacity of ~4 cups
Secretion of hydrochloric acid, enzymes, including a protease, gastric lipase; mucus, the hormone gastrin, and intrinsic factor (B12)
Stomach secretes about ml of gastric juice daily

13. Stomach
pH of stomach is low, 1 to 4; kills many bacteria and micro-organisms
Achlorhydria, gastrectomy, GI dysfunction or disease ↑ risk of bacterial overgrowth in the intestines
Protein digestion begins with pepsinogen which is converted to active pepsin in acidic stomach environment
Chief cells secrete acid-stable gastric lipase, which has minor effect on shorter-chain triglycerides

14. Stomach
Most of a liquid meal exits in 1 to 2 hours and most of a solid meal in 2 to 3 hours
Formation of chyme
Mucus layer protects stomach from digestion
Very little absorption of nutrients

15. Physiology of the Stomach

16. Stomach Acid Activates digestive enzymes
Partially digests dietary protein
Assists in calcium absorption
Makes dietary minerals soluble for absorption

17. The Small Intestine Most digestion and absorption happens here
About 5-7 meters
Duodenum (.5 meters)
Jejunum (2-3 meters)
Ileum (3-4 meters)

18. The Small Intestine Folded walls with villi projections
Absorptive cells are located on the villi
Increases intestinal surface area 600x
Rapid cell turnover

19. The Small Intestine

20. Nutrient Absorption
Passive diffusion: driven by concentration; fats, water, some minerals
Active absorption: uses energy; glucose and amino acids
Phagocytosis and pinocytosis: absorptive cells engulf compounds, generally larger molecules, as in immune substances in breast milk

21. Nutrient Absorption

22. Site of Absorption

23. Absorption Digestion → small particles End products of digestion:
Carbohydrates → monosaccharides
Proteins → amino acids
Fats → glycerol, fatty acids

24. Absorption Through small intestine walls Absorbed into
Blood – water soluble nutrients
Lymph – fat soluble nutrients
Blood → liver → general circulation
Liver detoxifies and repackages

25. The Large Intestine ~3 1/2 feet in length No villi or enzymes present
Little digestion occurs
Absorption of water, some minerals, vitamins
Bacteria break down fiber; produce Vitamin K
Formation of feces for elimination

26. Rectum Stool remains Stimulates elimination Muscle contraction
Anal sphincters
Voluntary control
Opens for elimination

27. Accessory Organs
Salivary glands
Pancreas
Gallbladder
Liver

28. Salivary Glands Saliva Works in mouth Moistens Salivary amylase
Digests starch

29. The Pancreas Produces glucagon and insulin (endocrine)
Manufactures digestive enzymes→ small intestine
Secretes pancreatic juices
Bicarbonate needed to neutralize chyme when it enters small intestine

30. The Liver
Produces bile (fat digestion)
Enterohepatic circulation

31. The Gallbladder Stores bile Concentrates it
Releases to small intestine when needed

32. The Digestive System

33. Sites of Secretion, Digestion, and Absorption

34. Digestion in the Stomach
Different nutrients leave the stomach at different rates:
—Carbohydrates first, followed by protein, fat, and fibrous foods

35. Summary of Enzymatic Digestion and Absorption
Secretion
Enzyme
Substrate
Action
Final Product
Saliva
Ptyalin
Starch
Hydrolysis to form dextrins
Gastric juice
Pepsin
Protein
Hydrolysis of peptide bonds
Gastric lipase
Fats
Hydrolysis into free fatty acids

36. Summary of Enzymatic Digestion and Absorption —cont’d
Secretion
Enzyme
Substrate
Action
Final Product
Pancreatic exocrine secretion
Lipase
Fat
Hydrolysis to mono-glycerides
Fatty acids
Cholesterol esterase
Cholesterol
Hydrolysis to esters of cholesterol and fatty acids
Choles-terol
alpha-Amylase
Starch, dextrins
Hydrolysis
Dextrin, maltose

37. Summary of Enzymatic Digestion and Absorption —cont’d
Secretion
Enzyme
Substrate
Action
Final Product
Pancreatic exocrine secretion
Trypsin
Protein
Hydrolysis
Polypeptides
Chymotrypsin
Carboxy-peptidase
Polypep-tides
Amino acids
Ribonuclease
Ribonu-cleic acids
Mono- nucleotides
Elastase
Fibrous protein

38. Summary of Enzymatic Digestion and Absorption —cont’d
Secretion
Enzyme
Substrate
Action
Final Product
Brush border enzymes
Carboxy-peptidase; aminopep-tidase; dipeptidase
Polypep-tides
Hydrolysis
Amino acids
Entero-kinase
Trypsino-gen
Activates to trypsin
Polypep-tidases and peptides
Sucrase
Sucrose
Glucose, fructose

39. Enzymatic Digestion and Absorption —cont’d
Secretion
Enzyme
Substrate
Action
Final Product
Brush border enzymes
Isomaltase
Dextrin
Hydrolysis
Glucose
Maltase
Maltose
Lactase
Lactose
Glucose, galactose
Nucleotidases
Nucleic acid
Nucleotides
Nucleosidases and phosphorylase
Nucleosidases
Purine and pyrimidine bases

40. Features of Digestion and Absorption
92% to 97% of the diet is absorbed
Small intestine—key role in digestion and absorption
Intestines: 7 meters long
—Villi and microvilli/brush border

41. Villi of the Human Intestine

42. Refeeding after Starvation
GI tract compromised after starvation
Feeding results in cellular proliferation, which restores normal gastrointestinal architecture and function after a few days

43. Lipids
The major portion of fat digestion takes place in the small intestine as a result of pancreatic lipase

44. Summary of Fat Absorption

45. Transport Pathways through the Cell Membrane

46. Large Intestine 5 feet long
Bacterial action to produce gases and organic acids
Nutrients formed here—vitamin K, thiamin, vitamin B12, riboflavin

47. The Gradual Breakdown of Large Starch Molecules

48. Process of Digestion of Carbohydrate

49. Sites of Secretion and Absorption into Gastrointestinal Tract

50. Neurotransmitters Alpha-aminobutyric acid: relaxes LES
Norepinephrine: decreases motility, increases contractions of sphincters, inhibits secretions
Acetylcholine: increases motility, relaxes sphincters, stimulates secretions
Neurotensin: inhibits release of gastric emptying and acid secretion
Neuropeptide-Y: stimulates feeding behavior

51. Hormones and GI Function: Gastrin
Released from gastric mucosa and duodenum in presence of peptides and amino acids; stimulates secretion of hydrochloric acid and pepsinogen; increases gastric antral motility, increases LES tone

52. Hormones and GI function: Secretin
Secretin: from duodenal mucosa in presence of acid in small intestine; stimulates pancreas to secrete water and bicarb; also insulin and pancreatic enzymes

53. Hormones and GI function: Cholecystokinin (CCK)
Released from proximal small bowel in presence of peptices, amino acids, fat, HCL, stimulates pancreas to release pancreatic enzymes

54. Hormones and GI function: Gastric Inhibitory Polypeptide (GIP)
Released from small intestine in presence of glucose and fat; inhibits gastrin-stimulated release of gastric acid

55. Hormones and GI Function (Motilin)
Released from stomach and small and large intestine in presence of biliary and pancreatic secretions; promotes gastric emptying and increases GI motility

56. Hormones and GI function: Somatostatin
Released from stomach, pancreas, and upper small intestine in presence of acidity and products of protein and fat digestion; inhibits release of gastrin, motilin, and pancreatic secretions; decreases motility of GI tract

57. Digestion and Absorption
It’s important to know the physiology and function of the GI tract in order to understand the implications of pathophysiology