Reticulum

1. Reticulum • Honeycomb lining • No secretions • Formation of food bolus • Regurgitation initiated here • Collects hardware (nails, wire)

2. Rumen • Digestion and fermentation vat • 40-50 gallons • No secretions • Contains anaerobic microbes (25-50 billion bacteria/mL fluid) • Also protozoa, fungi • Produce VFA, protein • Papillae lining • Increase surface area • Absorption of VFA • Passive diffusion

3. Omasum • Laminae/manyply lining • Muscular folds • No secretions • Reduces particle size • Absorption of water • ~60% removed • Absorption of VFAs • ~2/3 of VFAs entering or 10% of total produced • Prevents buffering of the abomasum

4. Abomasum • True gastric stomach - four gallons in a cow • Three regions (cardiac, fundic, and pyloric) • Digestive secretions • Proteolytic enzymes and HCl • pH decreases from 6 to 2.5 • Denatures proteins • Kills bacteria and pathogens • Dissolves minerals • Gastric digestion

5. Small Intestine • Composed of 3 segments (proximal to distal) • Duodenum • Releases bile and pancreatic secretions • Active site of digestion • Jejunum • Active site of nutrient absorption • Ileum • Active site of nutrient absorption • Most water, vitamins & minerals • Some bacterial presence • Fermentation The pH of the small intestine increases towards 7.0 as food moves from the duodenum to the ileum

6. Intestinal Epithelial Cell Brush border

7. Specialized Cells Lining Villi Nutrients Mucus • Absorptive epithelial cell • Contain brush border on lumen/apical side • Brush border: • Enzymes • Nutrient transport molecules • Goblet cell • Secretes mucus

8. Specialized Cells Lining Villi Anti-microbial compounds • Endocrine cell • Secrete hormones into bloodstream or local cells • Paneth cell • Secretory granules with anti-microbial properties CCK, Secretin, etc.

9. Small Intestine – Absorptive Surface • Villi • Enterocyte • Brush border • Cell migration from crypts to tips of villus • 2-3 days

10. Small Intestine - Structure • Lumen • Mucosa • Villi • Crypts • Lacteal • Enterocyte • Brush border

11. Intestinal Wall Villi Mucosa

12. Enhanced Surface Area for Increased Nutrient Absorption Intestinal villi

13. Increased Surface Area in Small Intestine for Absorption

14. Nutrient Absorption in the Small Intestine • Principal site of absorption of amino acids, vitamins, minerals and lipids • Glucose and other sugars in monogastrics • Generally, most absorption occurs in the proximal (upper) part of the small intestine but some absorption occurs in all segments • Duodenum, jejunum and ileum • Digestion and absorption within SI is rapid • Within 30 minutes of entering SI

15. Nutrient Absorption • Variety of mechanisms • Diffusion • Facilitated diffusion • Active transport • Pinocytosis or endocytosis • Dependent upon • Solubility of the nutrient (fat vs. water) • Concentration or electrical gradient • Size of the molecule to be absorbed

16. Diffusion • Water and small lipid molecules pass freely through membrane • Move down concentration gradient to equalize concentrations

17. Facilitated Diffusion • Carrier loads particle on outside of cell • Carrier releases particle on inside of cell • Reverse Allows equalization of concentrations across membrane

18. Active Transport • Carrier loads particle on outside of cell • Carrier releases particle on inside of cell • Carrier returns to outside to pick up another particle

19. Active Transport • Unidirectional movement • Transports nutrients against concentration gradient

20. Pinocytosis or Endocytosis • Substance contacts cell membrane • Membrane wraps around or engulfs substance into sac • Sac formed separates from the membrane and moves into cell

21. Transporters

22. Secretions Entering SI Secreted from within SI • Intestinal mucus • Brush border enzymes • Pancreatic juices • Produced & stored in pancreas • Bile • Produced in liver • Stored in gallbladder • Horse has no gallbladder • Direct bile secretion into duodenum • Cannot store bile—continuous intake of food Enters from ducts into SI

23. Intestinal Mucus • Secreted by glands in wall of duodenum • Brunner’s glands • Acts as lubricant and buffer to protect duodenal wall

24. Primary Enzymes for Carbohydrates

25. Primary Enzymes for Proteins

26. Primary Enzymes for Lipids

27. Bile • Green, viscous liquid • Alkaline ph (neutralize acidic chyme) • Secreted by liver via bile duct to duodenum • Stored in gall bladder (except in horses) • Functions to emulsify fats • Composition • Bile salts (glycocholic and taurocholic acids) • Bile pigments (bilirubin and biliverdin) • Cholesterol • 95% reabsorbed and returned to liver • NOT AN ENZYME

28. Nutrient Digestion - Lipids Large Lipid Droplet Action of bile salts Lipid emulsion Small Bile salts & pancreatic lipase and colipase Water soluble micelles

29. Clear, watery juice Enters duodenum via pancreatic duct Aids in fat, starch, and protein digestion Contains HCO3- Trypsinogen Chymotrypsinogen Procarboxypeptidase Amylase Lipase Nuclease Pancreatic Juice Pro-enzymes

30. Importance of Pancreas for Digestion • Produces enzymes responsible for • 50% of carbohydrate digestion • 50% of protein digestion • 90% of lipid digestion • Produces sodium bicarbonate for neutralization of chyme in duodenum

31. Activation of Pancreatic Enzymes • Enterokinase • Secreted from crypts in duodenum • Trypsinogen trypsin • Trypsin then converts: • Trypsinogen trypsin • Chymotrypsinogen chymotrypsin • Procarboxypeptidase carboxypeptidase

32. Stomach Pepsinogen Chymosin (rennin) Pancreas Trypsinogen Chymotrypsinogen Procarboxypeptidase Amylase Lipase Nuclease Brush Border (SI) Sucrase Maltase Lactase Aminopeptidase Dipeptidase Enterokinase Overview of Digestive Enzymes

33. Ruminant Small Intestine • Similar in structure and function to monogastric • Differences are subtle but important • Limited ability to digest starches and sugars • Little to none presented except in exceptional circumstances (high-grain feeding)

34. Small Intestine Digesta pHFunctions Duodenum 2.7 - 4 Enzymes pH change Flow rate regulation Jejunum 4 – 7 Enzymes Absorption Ileum 7 - 8 Absorption Limited fermentation • Rate of pH increase through small intestine is slower than monogastrics • Better for peptic activity • May limit pancreatic protease and amylolytic activity

35. Pancreatic Secretions • Secretion pH is 7.2-7.8 • Enzymes • Amylase • Lipase • Proteases • Trypsinogen converted to trypsin • Chymotrypsinogen converted to chymotrypsin • Procarboxypeptidase converted to carboxypeptidase • Nucleases

36. Activity of Pancreatic Enzymes • Concentration of enzymes in pancreatic juice comparable to monogastrics • Activity is lower and may be affected by: • Less juice secreted/kg BW • Low digesta pH • High rate of passage • Limited activity particularly a problem for intestinal digestion of starch escaping ruminal digestion • For ruminants fed high grain diets, less than 50% of starch reaching small intestine is digested

37. Bile • Secreted with pancreatic juice in the common bile duct of sheep • Secreted in the bile duct of cattle

38. Large Intestine • Composed of three segments • Cecum • Colon • Rectum • Function • Fermentative digestion • No enzyme secretion • Relies on microbes or secretions washed out of the SI • Absorption of remaining water, volatile fatty acids (VFAs) from microbial fermentation and minerals • Digesta storage Degree of development is species dependent

39. Monogastric Cecum • Located at junction of small and large intestine • Function similar to rumen in ruminants • Microbial activity and digestion of feeds • Contains a microbial population similar to the rumen • Cellulolytic & hemicelluloytic bacteria • Since cecum is located AFTER major site of nutrient absorption (small intestine), then microbial cell proteins are not available to the animal • Fecal loss

40. Monogastric Large Intestine • Function: • Absorption of liquid • Mass movements move fecal matter to anus • Usually only a few times a day • Associated with defecation

41. Bacteria • Cellulolytic – digest cellulose (forages) • Amylolytic – digest starches and sugars (concentrates or grains) • Other types: • Proteolytic • Clostridium • Organic acid utilizers • Methanogens • Produce CO2, H2, formate, CH4

42. Ruminant Large Intestine • Fermentative digestion • Bacteria similar to rumen, but no protozoa • Digestion in colon may account for as much as: • 27% of cellulose digestion • 40% of hemicellulose digestion • 10% of starch digestion • Only important in conditions that increase the amount of fermentative carbohydrate entering the large intestine • Increased rate of passage of forages • High grain diets • May account for as much as 17% of total VFA absorption • VFAs are efficiently absorbed, but primarily used as energy source for large intestinal mucosa cells

43. Ruminant Large Intestine • Absorption of ammonia-N • May account for as much as 30 to 40% of the net transport of N into body fluid • Absorbed N may be used for: • Synthesis of nonessential amino acids • Recycling of N to the rumen • Important on low protein diets • Regulated by: • Increased by increasing N concentration of diet • Decreased by increasing the amount of carbohydrate fermented in the large intestine • Mineral absorption • Water absorption • 90% of water entering the LI is absorbed

44. Rectum • Muscular area of large intestine used for storage of feces and ultimately for defecation • Feces includes sloughed cells, undigested food and microbial matter

45. Digestive Adaptations to Varying Feed Sources • Gastric capacity and structure • Capacity is greatest in pregastric fermentors • Stomach acts as reservoir • Small stomach in carnivores is related to high nutrient density of the diet • Distribution and composition of epithelial lining varies between species and dietary adaptations

46. Digestive Adaptations to Varying Feed Sources • Intestinal length and functions • Small intestine • Less variable among species than stomach and hind gut, but generally shorter in carnivores than in herbivores • Large intestine • Importance of hind gut fermentation dictates variation in structure and size • Some hind gut fermentation occurs in most species

47. Adaptations of the Digestive Tracts Stomach Small Intestine Cecum Large Intestine Rule Size = Function

48. Adaptations of Digestive Enzymes • Young animals produce little sucrase, maltase, amylase • Ruminants produce no sucrase • Adult pigs lack lactase • Activity changes with age • Lactase • Sucrase, maltase

49. Utilizing Cellulose • Advantages • Ultra-abundant in the environment • Easily obtained – no need to “hunt” plants • Plant cell walls & fiber high in energy • Disadvantages • Indigestible by mammalian digestive enzymes • Cellulase is found only in bacteria & some protozoans

50. Fermentative Digestion • All mammals have some fermentative capacity that allows for utilization of ingested fiber • The comparative importance of fermentation is related to the fraction of total digesta contained in fermentative compartments of the gastrointestinal (GI) tract