Digestive system - GIT

• Main Components of GIT

  • From mouth to anus
  • Single stomach and pre-gastric fermentation
  • Associated parts (Liver)
• Mouth & Oral Cavity

  • Function: Gather, sort & process food
  • Physical rupture
  • Disintegration: Size reduction, Moistening, softening, Enhanced surface area
• Monogastric jaws, gums & teeth

  • Food cutting, crushing & grinding
  • Different teeth types: Incisors = cut food, Canine = split food, Premolars = grind food, Molars = grind food
• Monogastric tongue

  • Tough & muscular
  • 8 muscles (4+4)
  • Extrinsic = attached to bones & change the position
  • Intrinsic = not attached to bones & change the shape
  • Papillae & taste buds
  • Salivary glands, full of ducts, glands and blood vessels
• Monogastric salivary glands

  • Functions of saliva:
  • Moisturising and Lubrication
  • Protection from injuries
  • Partial starch & lipid digestion
  • Volume = change with animal size, feed type & chewing
• Oesophagus

  • Move semi-processed food from oral cavity to stomach
  • Peristalsis starts here = muscular contractions
  • Horse / pigs = First 2/3 striated muscles, Last 1/3 smooth muscles
  • Dog = all striated
  • Deglutition (swallowing): Reflex in pharynx, Peristalsis in oesophagus
• GIT

  • A long tube
  • 4 basic layers
• Stomach components

  • Oesophageal = no glands
  • Cardiac = Mucous
  • Fundic = Parietal cells, chief cells
  • Pyloric = Mucous
• Gastric Pits & Motility

  • Many folds in the epithelium
  • Glands release secretions into the pits
  • Multiple glands may empty in the same pit
  • Motility helps mixing & hydrolysis of food to chime
  • The presence of chime stimulate emptying of semi digested foods
• Gastric secretions

  • Acid = HCL: Reduces pH to < 3, Denature protein molecules, Kills pathogens, Activates pepsinogen to pepsin
  • Mucous: Protects gastric linings from acid, fluids & enzymes, Lubrication
  • Rennin: Clots milk
  • Lipase – not all species
• Gastric functions

  • First major digestive process
  • Mixing, churning & moistening
  • Acid, enzyme & hormone secretion
  • Mucous formation
  • Neutralises pathogens
  • Short storage and gradual release of food to SI
  • Proteolysis
• Small intestine

  • 3 parts:
  • Duodenum = Bile & pancreatic fluids for active digestion
  • Jejunum = Site for nutrient absorption
  • Ileum = Active site for absorption – water, vitamins & minerals
• Intestinal epithelium & villi

  • Thin layer of cells that covers the lumen surface of GIT
  • Absorbs useful substances & prevent harmful substances
  • Villi contain specialised cells: Absorptive, Goblet = mucous, Endocrine, Paneth = anti-microbial
  • Villi increase SA 10x & microvilli 20x
• Microstructure of small intestine

  • Lumen
  • Mucosa
  • Villi
  • Crypts
  • Lacteal
  • Enterocytes
  • Brush border
• Absorption of Nutrients in Small intestine

  • Main absorption site for most: AA, Vit, Min, Lipids in most animals, Glucose & sugars in mono-gastric animals
  • Mostly in upper parts of small intestine
  • Very rapid = within 30 mins of entry into small intestine
• Major secretions in small intestine

  • Intestinal mucous (boblet cells) via Brunner's glands in Duodenal wall
  • Brush border enzymes
  • Pancreatic juices via ducts
  • Bile via bile ducts
• Bile


  • Green viscous liquid
  • Salts: Glycocholic, Taurocholic acids
  • Pigments: Bilirubin, Biliverdin, Cholesterol
  • Alkaline = neutralise chime
  • Emulsify fats
  • 95% returns to liver
• Pancreas


  • A mixed gland
  • Opens via ducts in Duodenum
  • Digestive juices as exocrine: Neutralise acidic chyme, Salts, e.g. HCO3, Many enzymes
  • Endocrine = hormones: Insulin, Glucagon
• Hepatic Portal System


  • Differs from a system where organs receive arterial blood & remove venous blood
  • In addition to hepatic artery & vein the liver receives blood from small intestine via the hepatic portal (HPV) – dual vascular supply
  • The venous blood contains absorbed nutrients
  • The HPV delivers the nutrients first to liver instead of general venous circulation
• Dogs – Canine


  • Carnivore, limited post-gastric fermentation
  • Single stomach = cannot utilise high fibre diets
  • Cannot digest some components of grains, fruits & veg
  • Strong teeth to tear & rupture
  • No amylase in saliva but antiseptic
  • Rapid deglutition
  • Two loved pancreas similar to feline
• Large intestine


  • Variable in size, shape and function
  • No mammalian enzyme secretions
  • Digestion by microbes
  • 3 parts: Caecum, Colon, Rectum
• caecum:


  Microbes similar to rumen & hind gut

  MCP

  not available to the host = faecal waste
• colon:


  absorption of:

  • water
  • VFA
  • minerals
• rectum:


  Muscular part

  storage & defecation

  Faeces include undigested food & end products