Nutrition 3

• Monogastric G.I. Tract

  • Mouth
  • Oesophagus
  • Stomach
  • Small Intestine
  • Large Intestine
  • Rectum
  • Anus
  • Caecum
• Salivary amylase

  Digests small amount of starch
• Gastric juice

  Produced at 400-800ml per meal, contains H+ ions so that pH < 2.0, kills ingested bacteria
• Stomach

  Muscular bag whose contractions break food down into smaller pieces, increasing SA for digestion. Holds food, releases into duodenum at constant rate.
• Pancreatic fluid

  Drains into the 1st part of the small intestine, the duodenum. Contains sodium bicarbonate (pH to 8.0) and digestive enzymes: amylase, lipase, peptidase.
• Bile

  Produced in liver, stored in gall bladder, enters duodenum via bile duct. Emulsifies ingested fat, aids digestion.
• Small intestine

  Large number of villi and microvilli dramatically increase surface area for reabsorption of sugars, peptides, fatty acids etc.
• Caecum

  Small pouch situated between last part of small intestine (ileum) and first part of large intestine (ascending colon). Repository for gut bacteria and important for fermentation of cellulose. Limited importance in humans.
• Large Intestine

  Important for the reabsorption of water. Large bacterial population 10^14 (c.f. 10^13 cells in human body), that make up 50% of faeces.
• Dental Adaptations

  • Carnivore (e.g cat) - pointed incisors + canines kill prey and rip/cut away flesh, jagged molars crush and shred food
  • Herbivore (e.g horse) - teeth with broad, ridged surfaces to grind tough plant material, modified incisors + canines for biting vegetation (or are absent)
  • Omnivore (e.g human) - eats both vegetation and meat, unspecialized teeth (32) including blade-like incisors (biting), pointed canine (tearing), premolars (grinding) and molars (crushing)
• Carnivore

  200kg lion - 40kg meat in one meal. Large expandable stomachs.
• Herbivore

  Longer alimentary canals (relative to size). Cell walls of vegetation more difficult to digest. Longer tracts = more time for digestion and greater surface area for absorption.
• Mutualistic symbiosis

  Mutually beneficial interaction between two species. Vertebrates do not produce enzymes that hydrolyse cellulose, but herbivores digest plants = much of energy in cellulose walls. Solution: House population of mutualistic bacteria and protists in fermentation chambers in digestive tracts - digest cellulose into sugars etc. that can be absorbed by animal.
• Examples of mutualism

  • Hoatzin (S.American bird) - Bacteria in muscular crop (oesophageal pouch)
  • Koala - Bacteria in caecum ferment finely shredded eucalyptus leaves
  • Termites - Mainly wood diet (cellulose)
  • Giant Tubeworms - entirely dependent on bacteria (no mouth or digestive system)
• Herbivores

  50% of organic carbon tied up in cellulose. Vertebrates lack cellulase enzyme, but microbes possess them - so herbivores utilise fermentation by microbial gut populations for energy.
• Ruminant Stomachs (e.g. cow)

  • Mouth
  • Rumen
  • Reticulum
  • Omasum
  • Abomasum
  • Small Intestine
  • Caecum
  • Colon
• Rumination


  Cows eat forage rapidly then latter "regurgitate" a bolus ("chewing the cud") during process of rumination. Rumination reduces particle size and exposes fibrous carbohydrates for bacterial fermentation.
• Saliva

  ~180 litres per day (chewing 6-8 hours). Rich in buffers (Sodium bicarbonate + phosphates) that neutralize acids produced by rumen fermentation, maintaining neutral pH that favours bacterial growth.
• Rumen

  Primary site for microbial fermentation of ingested feed. Retention of long forage particles in a fibrous mat stimulate rumination. Microbial fermentation produces volatile fatty acids (VFAs), which are then absorbed and represent primary source of energy for cow. Fermentation allows microbe growth (protein-rich). CO2 and methane produced (500-1000 litres per day); eructated ('belched').
• Reticulorumen

  Represents one functional space.
• Cellulose

  Linear polymer of glucose linked by glycosidic bonds, embedded in matrix of hemicellulose + phenolic polymers (i.e. tough to break down!)
• VFAs

  70% acetate, oxidised in tissues to generate ATP. 20% proprionate, generates glucose in liver. 10% butyrate, oxidised in tissues for energy. Major source of ruminant energy supply (~70%).
• MCT1

  Membrane transporter that allows reabsorption of VFAs across ruminal epithelial cells.
• UT-B

  Urea transporters in bovine blood that allow synthesis of protein from urea.
• Omasum

  Absorption of water, VFAs and minerals. Large particles are trapped in the numerous folds (also called leaves) of mucosal wall, contributing to selective retention of large particles in rumen.
• Abomasum

  "True stomach", similar to that in monogastrics. Secretion of HCl (to kill bacteria) and digestive enzymes. Digestion of carbohydrates and proteins that escaped rumen fermentation. Digestion of microbial protein produced by rumen (1 to 2.5 kg per day).
• Small Intestine

  Secretion of digestive enzymes. Receives digestive secretions from pancreas and liver. Site of enzymatic digestion of proteins, carbohydrates and lipids. Site of absorption of water, minerals, amino acids, glucose and fatty acids.
• Caecum

  Fermentation of unabsorbed digestion products by a bacterial population. Absorption of water. Formation of faeces.
• Equine G.I. Tract

  • Mouth
  • Stomach
  • Small Intestine
  • Caecum
  • Large intestine
• Equine G.I. physiology

  Non-ruminant / Monogastric - digest carbohydrates, protein and fat by enzymatic action (e.g. Humans). Ruminants - use bacteria in the stomachs to digest fibre by fermentation and use enzymatic digestion in the small intestines (e.g. Cattle). Horses are neither, differing in type and function of digestive system and falling between a ruminant and a non ruminant - sometimes called hindgut fermentors.
• Horse G.I. Tract

  Stomach - Small, ~8% of the G.I. tract. Primary product of digestion is lactic acid. Small Intestine - ~30% of the capacity of the G.I. Tract. Carbohydrates, protein, fats and some vitamins and minerals are absorbed here. 50-60% of crude protein digestion, virtually all soluble carbohydrate digestion (fibre excluded).
• Dual system

  Allows the horse to: Digest simple carbohydrate sources (e.g. Starch) from grain in the fore gut. Digest fibrous sources (e.g. oats, hay and pasture) in the hind gut. But cannot use large quantity of microbial protein.
• Caecum and colon

  Greatest capacity of any segment of the horse's digestive system at ~40-50%. Bacterial digestion of fibre occurs, large quantities of VFAs are produced through fermentation and are subsequently absorbed. Water is mainly absorbed here.
• Rabbit G.I. Tract

  i.e. Cecal digestor. Cecotrophy = ingestion of cecotrophs. Cecotrophs are smaller, occurs in bundles stuck together with mucus. Rabbits take cecotrophs directly from anus and ingest them, but don't ingest faeces. By ingesting cecotrophs, rabbits are using cecal bacteria as a valuable source of protein.