1. Digestive System
Chapter 24

2. Digestive System Basic processes of the digestive system: 1. Ingestion
2. Secretion
3. Mixing and Propulsion
4. Digestion
5. Absorption
6. Defecation

3. Digestive System Two groups of organs: Gastrointestinal tract (GI)
mouth, pharynx, esophagus, stomach, small
intestine, and large intestine.
Accessory organs
teeth, tongue, gallbladder, salivary glands,
liver, and pancreas

4. Digestive System

5. Histology Mucosa - secretion of mucous, absorption, protection
1. Surface epithelium
2. Lamina propria
3. Muscularis mucosae
Submucosa - blood and lymphatic vessels
Muscularis
Serosa - visceral peritoneum

6. Neural Innervation

7. Peritoneum Greater omentum Falciform Ligament Lesser omentum Mesentery
Largest serous membrane
of the body
visceral layer covers
organs
parietal layer lines the
walls of body cavity
Peritoneal cavity
space between layers
contains serous fluid
Greater omentum
Falciform Ligament
Lesser omentum
Mesentery
Mesocolon

8. Peritoneum

9. Mouth
Palate
Uvula
Vestibule
Tongue
Gums
Teeth

10. Salivary Glands
salivation

11. Tongue

12. Teeth – Structure and Composition

13. Teeth

14. Digestion in the Mouth Mechanical digestion (mastication or chewing)
breaks into pieces
mixes with saliva so it forms a bolus
Chemical digestion
amylase
begins starch digestion at pH of 6.5 or 7.0 found in mouth
when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis ceases
lingual lipase
secreted by glands in tongue
begins breakdown of triglycerides into fatty acids and glycerol

15. Digestive Activities of the Mouth

16. Pharynx

17. Esophagus

18. Swallowing

19. Digestive Activities of the Pharynx and Esophagus

20. Stomach
Regions:
Cardia
Fundus
Body
Pylorus

21. Histology of the Stomach

22. Histology of the Stomach

23. Histology of the Stomach

24. Secretion of HCl

25. Mechanical and Chemical Digestion in Stomach
Mechanical digestion
Mixing waves – gentle, rippling peristaltic movements – creates chyme
Chemical digestion
Digestion by salivary amylase continues until inactivated by acidic gastric juice
Acidic gastric juice activates lingual lipase
Digest triglycerides into fatty acids and diglycerides
Parietal cells secrete H+ and Cl- separately but net effect is HCl
Kills many microbes, denatures proteins
Pepsin secreted by chief cells digest proteins
Secreted as pepsinogen
Gastric lipase splits triglycerides into fatty acids and monoglycerides
Small amount of nutrient absorption
Some water, ions, short chain fatty acids, certain drugs (aspirin) and alcohol

26. Digestive Activities of the Stomach

27. Pancreas

28. Pancreatic juice 1200 - 1500ml daily Mostly water
Sodium bicarbonate – buffers acidic stomach chyme
Enzymes
Pancreatic amylase
Proteolytic enzymes – trypsin (secreted as trypsinogen), chymotrypsin (chymotrypsinogen), carboxypeptidase (procarboxypeptidase), elastase (proelastase)
Pancreatic lipase
Ribonuclease and deoxyribonuclease

29. Regulation of Pancreatic Secretions
Secretin
acidity in intestine causes increased sodium bicarbonate release
GIP
fatty acids & sugar causes increased insulin release
CCK
fats and proteins cause increased digestive enzyme release

30. Liver and Gall bladder

31. Role and Composition of Bile
Hepatocytes secrete mL of bile daily
Mostly water, bile salts, cholesterol, lecithin, bile pigments and several ions
Partially excretory product/ partially digestive secretion
Bilirubin – principal bile pigment
Derived from heme of recycled RBCs
Breakdown product stercobilin gives feces brown color
Bile salts play role in emulsification
Also aid in absorption of lipids following digestion

32. Bile Secretion and Regulation

33. Bile Secretion and Regulation

34. Small intestine

35. Histology of Small Intestine

36. Cells of Small Intestine

37. Brush Border

38. Segmentation Rhythmic local contractions. Mixes food with
digestive juices

39. Digestion of Carbohydrates
Mouth: salivary amylase
Esophagus & stomach: nothing happens
Duodenum: pancreatic amylase
Brush border enzymes (maltase, sucrase & lactase) act on disaccharides
produces monosaccharides: fructose, glucose & galactose
lactose intolerance (no enzyme; bacteria ferment sugar)--gas & diarrhea

40. Digestion of Proteins Stomach Pancreas
HCl denatures or unfolds proteins
pepsin turns proteins into peptides
Pancreas
digestive enzymes: split peptide bonds between different amino acids
brush border enzymes: aminopeptidase or dipeptidase: split off amino acid at amino end of molecule or split dipeptide

41. Digestion of Lipids Mouth: lingual lipase Small intestine
emulsification by bile
pancreatic lipase---splits into fatty acids & monoglyceride
no enzymes in brush border

42. Digestion of Nucleic Acids
Pancreatic juice contains 2 nucleases
ribonuclease which digests RNA
deoxyribonuclease which digests DNA
Nucleotides produced are further digested by brush border enzymes (nucleosidease and phosphatase)
Absorbed by active transport

43. Summary of Digestive Enzymes

44. Absorption in Small Intestine

45. Absorption of Monosaccharides
Absorption into epithelial cell
glucose & galactose----sodium symporter(active transport)
fructose-----facilitated diffusion
Movement out of epithelial cell into bloodstream
by facilitated diffusion

46. Absorption of Lipids
Small fatty acids enter cells & then blood by simple diffusion
Larger lipids exist only within micelles (bile salts coating)
Lipids enter cells by simple diffusion leaving bile salts behind in gut
Bile salts reabsorbed into blood & reformed into bile in the liver
Fat-soluble vitamins are enter cells since were within micelles

47. Absorption of Lipids
Inside epithelial cells fats are rebuilt and coated with protein to form chylomicrons
Chylomicrons leave intestinal cells by exocytosis into a lacteal
travel in lymphatic system to reach veins near the heart
removed from the blood by the liver and fat tissue

48. Absorption of Amino Acids
Absorption into epithelial cell
active transport with Na+ or H+ ions (symporters)
Movement out of epithelial cell into blood
diffusion

49. Absorption of Electrolytes
Sources of electrolytes
GI secretions & ingested foods and liquids
Enter epithelial cells by diffusion & secondary active transport
sodium & potassium move = Na+/K+ pumps (active transport)
chloride, iodide and nitrate = passively follow
iron, magnesium & phosphate ions = active transport
Intestinal Ca2+ absorption requires vitamin D & parathyroid hormone

50. Absorption of Vitamins
Fat-soluble vitamins
travel in micelles & are absorbed by simple diffusion
Water-soluble vitamins
absorbed by diffusion
B12 combines with intrinsic factor before it is transported into the cells
receptor mediated endocytosis

51. Absorption of Water 9.3 liters of fluid dumped into GI tract each day
Small intestine reabsorbs 8.3 liters
Large intestine reabsorbs 90% of that last liter
Absorption is by osmosis through cell walls into vascular capillaries inside villi

52. Summary of Digestive Activities
Pancreas
Liver
Gallbladder
Small Intestine

53. Large intestine

54. Histology of Large Intestine

55. Cells of Large Intestine

56. Summary of Digestive Activities of Large Intestine

57. Summary of Organs of Digestive System

58. Gastric Phases 1. Cephalic phase - triggered by the aroma, taste,
sight, or thought of food.
2. Gastric phase - food in the stomach stimulates
local nerves and glands (distension and gastrin).
3. Intestinal phase - excitatory starts with appearance
of digested food in the duodenum; production of a
hormone (intestinal gastrin) which stimulates gastrin
secretion in the stomach; inhibitory phase.

59. Regulation of Gastric emptying
Distention & stomach contents increase secretion of gastrin hormone & vagal nerve impulses
Enterogastric reflex regulates amount released into intestines inhibition of gastric emptying

60. Hormones that Control Digestion