1. Gastrointestinal Tracts (ch. 4)
Steve Boyles

2. Digestion and Absorption
Digestion: dismantling feed for absorption
Physical, chemical, enzymatic (pancreatic & intestinal)
Absorption: taking compounds into the body
Transport across GI epithelium

3. GI tracts Where digestion and absorption take place
Tube that reaches from the mouth to the anus:
AKA alimentary tract
Esophagus, stomach, intestines, cecum
Inside of tube called the lumen
Other tissues associated with digestion
Liver, pancreas, blood, lymph

4. Generic GI tract Cecum Esophagus Stomach Large Intestine
Small intestine

5. Types of GI tracts Minimal fermenters Foregut (pregastric) fermenters
Hind gut (postgastric) fermenters
* Classified by whether fermentation provides to the animal

6. Fermentation Microbes harvesting energy anaerobically
Potential end products:
Lactic, propionic, butyric, and other acids
Ethanol
* Animals cannot digest fiber alone – need microbes

7. Microbes Microbes like warm, moist places Microbes are not innate
Can use inorganic N to make protein
Can provide vitamins (B, K) in addition to energy from fiber (acids)
Some animals can use microbial protein
Foregut fermenters

9. Minimal fermenters Carnivores, birds, pigs, humans, omnivores
Microbes do little to aid in digestion
Animal does not handle fiber well
Long small intestine, minimal cecum
Can have some fermentation in colon
Too late for appreciable absorption

10. Foregut fermenters Ruminants, camelids (Camels, alpacas, llamas)
Microbes live in large compartment near beginning of GI tract
Microbes ferment (digest) feed first
Animal can handle lots of fiber
Animal digests leftovers and microbial products

11. Hind gut fermenters Horses, rabbits Microbes live in large cecum
Animal can handle lots of fiber
Otherwise similar to minimal fermenters

12. Non ruminant mammals Mouth: Grasping & chewing (physical)
tongue, lips, teeth
Saliva (enzymes, buffers)
Stomach: HCl (chemical) and enzymes
Small intestine: Enzymes and absorption

13. Non ruminant mammals cont’d
Cecum: Microbial breakdown
Most important in hind gut fermenters
Large intestine: Water
Colonic fermentation of undigested feed
Only some absorption – not a major route

14. Pig (minimal fermenter)

15. Horse (hind gut fermenter)

16. Pig Rabbit

17. Birds
Crop: Storage
No stomach
Proventriculus: site of gastric juices (chemical and enzymatic)
Gizzard: grinds feed (physical)
No teeth
Grit

18. Small intestine: digestion (enzymatic) and absorption
Slightly acidic = some chemical breakdown
Ceca and Large intestine: water
Very little fiber digestion in ceca (microbes)
Cloaca: common chamber of urinary, digestive, and reproductive passages

19. Chicken (minimal fermenter)

20. Small intestine

21. Ruminants Mouth: Grasping & chewing (physical) Stomach: 4 compartments
tongue, lips, teeth (bottom only)
Saliva (buffers, N, P, Na)
Stomach: 4 compartments
Rumen (paunch)
Reticulum (honeycomb)
Omasum (manyplies)
Abomasum (true stomach)

22. Rumen: Large fermentation vat
Ruminants cont’d
Rumen: Large fermentation vat
Bacteria, protozoa, fungi - symbiotic
Microbial breakdown of feed
End products of fermentation:
Absorbed from rumen (papillae)
Flow out to omasum
Burped out (eructation) – CO2, CH4

23. Reticulum: Moves feed between rumen, omasum, and regurgitation
Rumination = regurgitation & mastication
Extra chewing = extra breakdown of feed
Pieces too large to enter omasum
Eructation because fermentation makes gases
Cannot belch = bloat
Animal’s left side is first
Honey comb
Hardware disease

24. Omasum: Muscular grinding (Physical) Abomasum: Glandular stomach
Ruminants cont’d
Omasum: Muscular grinding (Physical)
Abomasum: Glandular stomach
Chemical and enzymatic breakdown
Similar to non ruminant mammal stomach
Reticular groove: shunts milk to omasum in nursing animals
Bypass microbial fermentation

26. Ruminants cont’d
Small intestine, cecum, & large intestine similar structure to minimal fermenter mammals

27. Cow (foregut / pregastric fermenter)

28. Psuedoruminant camelids
3 compartment stomach (C1, C2, C3)
More mixing = more eructation = less bloat
Regurgitation for rumination or spit
C1: larger first compartment (83% of digesta
Full of symbiotic bacteria
No papillae, but highly absorptive (VFAs)
Secretes buffers se

29. Psuedoruminant camelids cont’d
Full of symbiotic bacteria
Mixing, motility, and fermentation
Filters material
Secretes buffers
C3: glandular stomach
Gastric secretions only in last 1/5 of C3
Proteolytic and HCl
First 4/5 of C3 produce mucus

30. Camelid stomach C3 C2 C1

34. From the GI tract – Blood + Lymph
Once nutrients cross into the body proper from the GI tract, the nutrients enter lymph (fat) or blood (all other nutrients).
This is how nutrients are transported through the body to different tissues.

35. Other digestive organs
Pancreas:
Enzymes: proteases, amylases, lipases
No fiber degrading enzymes
Liver:
Bile for emulsifying fat