Gastrointestinal Tracts (ch. 4) Steve Boyles

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

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

Generic GI tract Cecum Esophagus Stomach Large Intestine Small intestine

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

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

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

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

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

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

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

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

Pig (minimal fermenter)

Horse (hind gut fermenter)

Pig Rabbit

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

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

Chicken (minimal fermenter)

Small intestine

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)

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

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

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

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

Cow (foregut / pregastric fermenter)

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

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

Camelid stomach C3 C2 C1

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.

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