Digestive System

• Digestive tract


  Also known as gastrointestinal or alimentary tract
• Accessory organs


  • Teeth
  • Tongue
  • Salivary glands
  • Liver
  • Pancreas
  • Gallbladder
• Functions of digestive system

  • Ingestion
  • Propulsion
  • Mechanical processing
  • Digestion
  • Secretion
  • Absorption
  • Excretion
  • Immunity
• Ingestion


  Active process of bringing material into the oral cavity
• Propulsion


  Ingested materials are moved through digestive tract by swallowing and peristalsis
• Peristalsis


  1. Circular smooth muscle behind the bolus contracts while that in front relaxes

  2. Longitudinal smooth muscle layer behind the bolus relaxes while that in front contracts

  3. Increase diameter of the lumen in front of the bolus results in propulsion of bolus down the digestive tract (aborally)
• Segmentation


  Nonadjacent sections of digestive tract contract and relax, mixing the contents
• Mechanical processing


  Materials entering the digestive tract are physically reduced in size, beginning in the oral cavity where food is crushed and sheared before being propelled along the digestive tract
• Mechanical processing in ruminants

  • Food materials are moved from the stomach to the mouth for further reduction in particle size
  • Food is also churned along digestive tract by segmental contractions for further mixing with digestive juices
• Digestion


  Ingested nutrients are chemically broken down into particles small enough for absorption
• Secretion


  From epithelial cells and glandular organs, releasing water, mucus, acids, enzymes, buffers and salts into the lumen of digestive tract
• Absorption


  Organic substrates, electrolytes, vitamins and water pass from the lumen into the body
• Excretion


  Site of elimination of waste products, via defecation or egestion
• Immunity


  Physical barrier to prevent entry of pathogens into the body, and innate immune system
• Layers of digestive tract

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa
• Enteric nervous system


  Brain of the gut, composed of submucosal and myenteric plexuses
• Submucosal plexus


  • Includes sensory and motor neurons and postganglionic fibers of both sympathetic and parasympathetic nervous systems, regulating activity of glands and smooth muscle of mucosa
• Myenteric plexus


  • Coordinates frequency and strength of digestive tract motility, controls patterns of peristalsis and segmentation through automatic local reflex arcs
• Enteric nervous system communication with CNS

  Via afferent fibers of sympathetic and parasympathetic branches, with parasympathetic input enhancing digestive functions
• Salivary glands


  Extramural glands associated with the oral cavity, including minor salivary glands with short ducts and major salivary glands requiring ducts
• Salivary gland secretions


  Can be serous, mucous or mixed
• Serous cells


  • Produce watery secretions containing enzymes, ions and small amount of mucin
• Mucous cells


  • Produce viscous, stringy secretion called mucus
• Major salivary glands and secretions

  Parotid gland produces serous secretion, mandibular gland is a mixed gland, sublingual gland secretes mucus
• Saliva


  Consists of 97-99.5% water, thus hypoosmotic, containing sodium, potassium, chloride, bicarbonate and phosphate, slightly acidic with pH 6.75-7
• Functions of saliva


  • Solubilizes food
  • Provides alkaline buffering fluid
  • Removes wastes
  • Lubricates and binds
  • Initiates starch digestion
  • Assists oral hygiene
  • Enables evaporative cooling
• Saliva solubilizes food


  Dissolves food so they can be tasted and digestive reactions can occur
• Saliva provides alkaline buffering fluid


  Bicarbonate and phosphate can neutralize acidic feedstuffs
• Saliva removes wastes


  Urea and uric acid are excreted in the saliva
• Saliva initiates starch digestion

  Amylase in the saliva begins starch digestion
• Saliva lubricates and binds

  Mucus in the saliva helps bind masticated food into a bolus
• Saliva assists oral hygiene

  • Contains lysozyme, IgA, cyanide, defensins, and lactoferrin to protect the mouth
• Saliva enables evaporative cooling

  Particularly important in dogs, cats, and birds
• Saliva secretions in nonruminants

  As secretion increases, sodium, bicarbonate and chloride increase while potassium decreases
• Saliva secretions in ruminants

  As saliva production increases, sodium and phosphate decrease while bicarbonate, chloride and potassium increase
• Salivary glands and salivation

  Salivary glands continuously secrete saliva, with presence of food, sight/smell/sound/thought of food increasing salivation
• Pavlov training - dogs salivate at the sound of a bell, helping initiate digestion as soon as food enters the oral cavity
• Ruminant saliva


  Isotonic, contain high concentrations of bicarbonate and phosphate and high pH, acting to buffer acids produced during fermentation in the rumen
• Tongue


  Maneuvers food mass in the mouth, assists movement of food in the esophagus, and performs prehension in ruminants
• Chewing (mastication)


  A chemical process including grinding, breaking, and mixing of food with saliva