The Digestive Tract

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The Digestive Tract

• Anatomy, Function, and Secretions

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Functions of Intestinal Tract

• Protection
• Continuous lining of mucosa prevents microbial
  infiltration and toxin absorption
• Transport of ingested feed
• Digestion
• Secretory Functions
• Absorption
• Excretion (undigested feed residues)

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Terminology

• Digestion process of making food absorbable by
  dissolving and breaking it down to simpler
  compounds that occurs in living body chiefly by
  secreted enzymes in the alimentary canal
• Mechanical forces
• Enzymatic/Chemical action
• Fermentation

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Mechanical Forces

• Physical action through chewing and GIT
  contraction
• Decreases particle size
• Increases surface area
• Increases particle exposure to gastric secretions

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Chemical Action

• HCl secreted from stomach
• Denatures (unfolds) proteins
• Activates proenzymes (pepsinogen)

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Enzymatic Action

• Stomach and Small Intestine
• Secretions by host animal to degrade complex food
  components for absorption
• Amylases
• Proteases
• lipases

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Digestion

• Enzymatic versus Fermentative
• No protein loss (energy loss)
• Ability to obtain energy from indigestible
  ingredients
• Microbial cells source of nutrients
• Vitamins, protein

VERSUS
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Terminology

• Absorption transfer of substances from GIT into
  circulating blood or lymph systems

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Types of Gastrointestinal Tracts (GIT)

• Simple Nonruminant
• Dogs, cats
• Nonruminant
• Omnivores with limited fermentation
• Pigs, rats
• Nonruminant herbivore
• Horse, rabbit
• Pregastric fermentors (ruminants)

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Digestive Organs of the GIT

• Mouth
• Stomach
• Small Intestine
• Large Intestine

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Mouth

• Brings food into the body
• Mix food with saliva
• Dogs/cats swallow large boluses of food
• Little to no chewing
• Teeth structure

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Tooth Structure

• Cat versus Dog
• Similarities same of incisor and canine teeth
• Differences dog contains more premolars and cat
  has enlarged canines
• Dog Increased capacity to chew and crush food
• Indicator of more omnivorous diet
• Cat Designed for gripping and tearing

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Esophagus

• Passage way to stomach
• Secrete mucus to increase lubrication
• Cardiac sphincter at end of esophagus relaxes in
  response to peristaltic movements of esophagus

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Stomach

• 4 regions
• 1. Esophageal stratified squamous epithelia
  with no secretory functions
• 2. Cardiac mucous secretion
• 3. Gastric or Fundic parietal cells secrete
  HCL and chief cells secrete pepsinogen
• 4. Pyloric mucous and some pepsinogen

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Functions of Canine/Feline Stomach

• Storage
• Reduction of particle size
• Mixing of food, saliva, water, gastric juices
• Initiation of chemical digestion

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Stomach

• Retention time
• Dog/Cat 2-8 hours
• Cattle 16-48 hours
• WHY?
• Enzymatic digestion in SI versus fermentation by
  bacteria

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Stomach

• At this point the digestive processes have been
  mostly mechanical
• Material leaving stomach is called CHYME
• Chyme is semisolid mass made up of food particle
  mixed with gastric secretions

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Small Intestine

• Major role chemical/enzymatic digestion
• Less diversity between species
• Major difference is length

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Small Intestine

• 3 sections
• Duodenum first section, shortest section (2-5)
• Active site of digestion
• Jejunum middle section
• Active in nutrient absorption
• Ileum last section
• Active in nutrient absorption

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Small Intestine

• Walls of SI lined with villi small, finger-like
  projections that increase surface area
• Increase absorptive surface

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Functions of Small Intestine

• Regulation of digesta flow
• Peristalsis
• Primary site of chemical and enzymatic digestion
• Primary site of nutrient absorption

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Large Intestine

• 3 sections
• Cecum first section
• Very small in cats
• Relatively larger in dogs
• Colon middle section
• Largest part of large intestine
• Rectum last section

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Functions of Large Intestine

• CHIEF FUNCTION water and electrolyte absorption
• No villi and thus lower capacity for absorption
• No mechanism for transport of other nutrients out
  of LI
• Fermentation

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Fermentation

• Digestion of nutrients by intestinal bacteria
• Significant fermentation under circumstances when
  substrate (food for bacteria) is available
• High fiber, high indigestible protein/CHO
  containing diets
• Results in the production of Short Chain Fatty
  Acids (SCFA), water, carbon dioxide, heat
  increment

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Short Chain Fatty Acids

• Acetate, Propionate, Butyrate, Lactate
• Recent research indicates that SCFA can be
  absorbed from LI
• Improves intestinal health
• Fuels growth of colonocytes

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Secretions of GIT

• Saliva
• Characteristics
• Moisten and lubricate food
• Small amounts secreted continuously (Gustatory
  response salivate at sight/smell of food)
• No salivary enzymes
• Evaporative cooling (esp. dogs)

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Secretions of GIT

• Hydrochloric Acid
• Produced by parietal cells
• Functions
• Initiates unfolding (breakdown) of protein
• Activates pepsinogen to pepsin
• Maintains acidic pH of stomach (pH 2-4)

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Control of HCl release

• 1. Cephalic Phase sight, smell of food
  increases HCl release
• 2. Gastric Phase food in stomach (distension)
  increases HCl release
• 3. Intestinal Phase food in intestine
  decreases HCl release

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Pepsinogen

• Primary enzyme
• Secreted by chief cells
• Converted to active form by acid presence
• Functions
• Preconditions proteins for digestion
• Large proteins peptides
• Proteolytic

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Pancreatic Secretions

• Bicarbonate increase pH in SI
• Enzymes luminal digestion of protein, fat,
  CHO
• Both stimulated by presence of acidic chyme in
  duodenum (at pyloric sphincter)

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Pancreatic Enzymes

• Proteolytic enzymes (2 key enzymes)
• Trypsinogen activated in SI by enterokinase
  enzyme
• Cleaves terminal protein bonds
• Chymotrypsinogen activated by trypsin
• Cleaves in middle of proteins

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Pancreatic Enzymes

• Pancreatic amylase
• Cleaves bonds of carbohydrates
• Optimal pH 6.5 -7
• Thus function of bicarbonate!
• Production driven by level of starch (CHO) in the
  diet

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Pancreatic Enzymes

• Pancreatic lipase
• Cleaves triglycerides
• Requires bile presence for activity

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Secretions of GIT

• Bile
• Produced in liver, stored and secreted by gall
  bladder
• NOT an enzyme
• Secreted into duodenum like pancreatic secretions
• Contains electrolytes and buffers

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Functions of Bile

• 1. Emulsifies lipids (increases surface area for
  enzymatic action potential)
• 2. Increases solubility of fatty acids

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Enzymes associated with SI mucosal cells

• Present on cell surface or inside cell
• epithelial digestion
• Disaccharidases (maltase, sucrase, lactase)
• Dipeptidases (cleaves peptides to AA)
• Enterokinase (activates trypsinogen)

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General Review of Nutrient Digestion/Absorption

• Water
• Carbohydrates
• Proteins
• Lipids
• Minerals
• Vitamins

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Water

• Absorbed Jejunum, Ileum, Large Intestine
• Necessary for nutrient metabolism and excretion!

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Carbohydrates

• Ingested as Starch or cellulose
  (polysaccharides), Sucrose (disaccharide)
• Digested by Amylase enzymes or bacterial
  cellulase (fermentation)
• Absorbed as Glucose, Fructose, Galactose
  (monosaccharides) or Short-Chain Fatty Acids

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Short Chain Fatty Acids

• Discuss further when talk in depth about CHO
• Fatty Acids produced by bacterial digestion of
  fiber (cellulose, hemicellulose,
  oligosaccharides)
• Absorbed in the LI

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Contribution of hindgut fermentation to energy
supply of different animals
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Lipids

• Ingested as Triglycerides
• Digested by Lipase enzymes with action of bile
  as emulsifier
• Absorbed as Monoglycerides, Fatty Acids

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Protein

• Ingested as Polypeptides, Peptides, AA
• Digested by Protease enzymes (pepsin, trypsin,
  chymostrypsin, etc.)
• Absorbed as Amino Acids

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Minerals

• Ingested as the organic or inorganic form
• Converted to ionic form
• Absorbed in Small Intestine

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Vitamins

• Absorbed directly from diet in upper small
  intestine (jejunum)
• Water soluble (rapidly absorbed)
• Fat soluble (requires presence of fat)