gi tract

• Digestion


  The digestive tract breaks down complex materials to their constituent parts so that nutrients can be absorbed and utilized
• Digestion


  Physical and enzymatic means the body uses to render a feedstuff ready for absorption
• End products of digestion

  • Carbohydrates: simple sugars
  • Proteins: amino acids
  • Lipids: fatty acids and glycerol
• Journey of Food Through the GI Tract

  1. Digestion

  2. Accomplished in three ways: physical/mechanical action, chemical action, enzymatic action
• Digestive Systems

  • The type of digestive systems dictates what the animal can successfully use as feed
  • The more complicated the feed, the more complex the tract
• Digestive Systems - Anatomic Classification


  • Monogastric
  • Simple monogastric
  • Hindgut fermenters
  • Ruminant: true, 4 compartments, fermentation occurs in the rumen
  • Pseudoruminants: 3 compartments, fermentation occurs in the cecum
• Digestive System - Diet Consumed

  • Carnivores: flesh eating animals
  • Omnivores: eat animal and vegetable matter
  • Herbivores: eat vegetable-based feeds
• Digestive System - Diet Consumed (by animal type)

  • Monogastric, omnivore: dogs, poultry, swine
  • Monogastric, carnivores: cats, birds of prey
  • Monogastric, herbivores: horses, rabbits
  • Ruminants, herbivores: cattle, sheep, goats
• Steps of Digestion - Prehension

  Means that an animal uses to bring food to their mouth: upper limbs, head, beak, claws, lips, mouth, tongue, teeth
• Step of Digestion - Mastication

  1. Vertical and lateral action of jaw and teeth to crush food

  2. Reduce particle size for swallowing

  3. Carnivores chew only to the extent needed

  4. Herbivores need thorough mastication

  5. Monogastric: food goes directly into the glandular stomach

  6. Ruminant: first stop is the rumen
• Step of Digestion - Mastication (Ruminants)


  1. Ruminants form a bolus and swallow it without much chewing, later regurgitate feed and thoroughly chew it

  2. A typical dairy cow chews 40,000 - 50,000 times a day
• Step of Digestion - Salivation


  1. Secretion and mixing of saliva with food

  2. Main purpose: lubricate food for chewing and swallowing

  3. All animals depend on this salivary function

  4. Water and mucin (protein found in saliva) are responsible for this function

  5. Especially important for herbivores
• Steps of Digestion - Salivary Glands

  • Parotid
  • Mandibular (submaxillary)
  • Palatine
  • Buccal
  • Sublingual
• Steps of Digestion - Salivation (other Functions)

  • Solvent medium
  • Taste (why taste is important in animals)
  • Washing of mouth
  • Cleanse the mouth
  • Enzyme secretion (lysozyme kills bacteria)
  • Buffer (bicarbonates)
• Step of Digestion - Salivation (Ruminants)

  • Bicarbonates found in saliva help maintain pH balance in the rumen
  • Acids produced by microorganisms damage the rumen wall
  • Rumen microbiota
  • Nutrients for rumen microorganisms
  • Urea in saliva is used to make proteins
  • P and Na
  • Antifoam properties prevent ruminal bloat
• Steps of Digestion - Swallowing or Deglutition

  1. Passing of substances from the mouth to the first stomach compartment through the esophagus

  2. Involuntary (under neural control)

  3. Caused by the presence of food in the back of the mouth

  4. Tongue forms a bolus and moves it (voluntarily) to the back of the mouth

  5. Movement of the food to the stomach - peristalsis
• Steps of Digestion - Digestion

  1. Ruminant: rumino-reticulum compartment, true stomach, fermentation

  2. Monogastric: true stomach, glandular stomach: chemical and enzymatic digestion, food storage

  3. Chicken: proventriculus

  4. Cow: abomasum
• Steps of Digestion - Digestion (Stomach)

  • Strong muscle contractions help mix the food with chemicals and enzymes
  • Hydrochloric acid (HCl): denaturing of proteins, provides acidic pH needed for gastric enzymes, keeps stomach sterile
  • Enzymatic digestion: pepsin, gastricin, rennin, gastric lipase
• Steps of Digestion - Digestion (Small intestine)

  1. Duodenum (1 ft long): bile and pancreatic secretions, main site of food breakdown

  2. Jejunum (longest): main site of absorption

  3. Ileum: form a connection with large intestine
• Steps of Digestion - Absorption (Small Intestine)

  • Small intestine is lined with mucous membranes
  • Absorption: defense mechanism against noxious substances and bacteria
  • Folds and villi increase surface area
  • Water-soluble nutrients transported to bloodstream and liver, lipids and fat-soluble vitamins transported by lymphatic system
• Steps of Digestion - Absorption (Large intestine)

  • Water, electrolytes, vitamins, minerals, and VFA absorbed
  • No villi, restricted absorption, no digestive enzymes secreted
• Steps of Digestion - Absorption (Large intestine)


  • Size varies depending on species
  • Monogastric herbivores - largest, monogastric carnivores - smallest
  • Cecum in monogastric herbivores: similar function to rumen, large fermentation capacity, B-complex vitamins, VFA
• Steps of Digestion - Absorption (Large intestine - Cecotrophy)


  1. Microbial protein digestion: free amino acids, no whole proteins

  2. Behavioral adaptation (cecotrophy)

  3. Two types of feces: cecotropes (contents of cecal fermentation) and fecal matter
• Steps of Digestion - Defecation

  1. Discharge of excrement from the body via the rectum or cloaca

  2. Initiated by the defecation reflex, stimulated by pressure of feces in rectum

  3. Fecal material: undigested feed, residues of digestive enzymes, sloughed cells, and bacteria
• Urination


  • Contents: urea in mammals, uric acid in birds and other species
  • Kidney major organ involved in keeping the body properly hydrated and removing various wastes from the body
• Digestion in the Pig

  • Only farm mammal in which amylase is secreted in the saliva, breaks down starches in feed (little nutritional importance)
  • Stomach (2 gals): gastric juices similar to all monogastric animals, lack rennin, protein digestion completed in the intestine, most fat digestion occurs in the small intestine, ~24h to empty a full stomach
• Digestion in the Pig

  • Small intestine (60 ft): chemical and physical means, pancreatic juice (enzymes, sodium carbonate and sodium bicarbonate, insulin), bile, digestion of fats, absorption of fats and vitamins
• Digestion in the Avian


  • Mouth: lack teeth, salivary amylase but no digestion
  • Crop: food storage and moistening reservoir
  • Proventriculus: equivalent to glandular stomach in mammals, gastric juice production (HCl), pH 4, no digestion except carnivorous birds
• Digestion in the Avian


  • Ventriculus (gizzard): specialized grinding organ, very muscular, contains grit (free-ranging), no enzymes secreted
  • Small intestine (digestion and absorption): duodenum (first part) longer in herbivore birds, no specifically separated jejunum or ileum, same enzymes as in mammals but no lactase
• Digestion in the Avian


  • Ceca (junction at small and large intestine): size influenced by diet, large when fed large amounts of fiber, water reabsorption, bacterial action, fiber digestion, water-soluble vitamin synthesis
  • Large intestine: water absorption
  • Cloaca: common office (excretion, copulation, egg laying)
• Digestion in the Horse

  • Non-ruminant herbivore: capable of digesting fiber, active cecal bacteria population
  • Prehension: teeth, flexible upper lip, and tongue, no enzymes in saliva but important lubricant, secretion is abundant (10 gals a day), one-way peristaltic movements: almost impossible to regurgitate or eructate
• Digestion in the Horse

  • Stomach: smaller capacity, reduced muscular activity, susceptible to stomach disorders (colic, rupture)
  • Small intestine: similar to other monogastrics, no gallbladder, bile secreted directly and continually into the duodenum
• Digestion in the Horse

  • Large intestine: cecum, large colon, small colon, rectum, cecum and large colon contain bacteria similar to ruminant, only 2/3 as effective at fiber digestion as ruminants, production of VFA, water-soluble vitamins, microbial protein
• Coprophagy


  The act of eating feces