Module 3

• Digestive system


  Supplies fuel (energy) & building blocks (cell growth/repair) by: 1) Taking in food; 2) Breaking it down into nutrient molecules; 3) Absorbing nutrient molecules into the bloodstream; 4) Riding the body of indigestible remains
• Organization of digestive system

  • Gastrointestinal tract: oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, large intestine
  • Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, pancreas
• Gastrointestinal tract


  • Mechanical processing, moistening, mixing with salivary secretions
  • Muscular propulsion of materials into the esophagus
  • Transport of materials to the stomach
  • Chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions
  • Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions
• Accessory organs


  • Produce saliva, bile, digestive enzymes that contribute to breakdown of foodstuffs
  • Oral canal works with: teeth, tongue, salivary glands
  • Small intestine works with: gallbladder, liver, pancreas
• Digestive activity


  Provoked by a range of chemical & mechanical stimuli (e.g. pH of contents, stretch receptors, presence of substances)
• Chemical stimuli


  Glands activated/inhibited to secrete digestive juices into lumen or hormones into blood
• Mechanical stimuli


  Lumen contents mixed along length of tract by stimulating smooth muscles of GI tract walls
• Transit time


  Amount of time it takes food to pass the length of the gastrointestinal tract
• Food disassembly


  1. Mechanical and chemical processes deconstruct food into polymers (macromolecules: CHO, lipids, protein, nucleic acid)

  2. Further breakdown into monomers (sugars, fatty acids, amino acids, nucleotides)
• Steps of food disassembly

  1. Ingestion (oral cavity)

  2. Propulsion (pharynx, esophagus, stomach)

  3. Mechanical breakdown (stomach)

  4. Chemical digestion (accessory organs)

  5. Absorption (small intestine)

  6. Elimination (large intestine)
• Layers of GI tract

  • Mucosa, submucosa, muscularis externa, serosa
  • Each contains tissue type that plays specific role in food breakdown
• Mucosa


  • Folds increase surface area for absorption
  • Secretion of mucus, digestive enzymes, hormones
  • Absorption & protection
• Epithelium


  • Oral cavity, pharynx, esophagus, anus: stratified squamous
  • Stomach, small & large intestine: simple columnar epithelium rich in mucus secreting cells, lot's of absorption
• Submucosa


  • Contains large blood vessels & lymphatic vessels
  • Rich supply elastic fibers that enable stomach to regain normal shape after storing large meal
  • Binds mucosa to muscularis externa
• Muscularis externa


  • Smooth muscle cells involved with: mechanical processing, movement materials along digestive tract, peristalsis, segmentation
• Oral cavity to stomach

  1. Oral cavity, pharynx, esophagus

  2. Mechanical processing, moistening, mixing with salivary secretions

  3. Muscular propulsion of materials into the esophagus

  4. Transport of materials to the stomach
• Functions of oral cavity

  • Sensory analysis of food before swallowing
  • Mechanical processing through actions of teeth, tongue, palatal surfaces
  • Lubrication by mixing with mucus and saliva
  • Limited digestion of CHO and lipids
• Mastication


  • Breaks down tough connective tissues in meat and plant fibers in vegetables
  • Saturates food with salivary secretions/enzymes
  • Food is shredded and moistened with salivary secretions
  • Bolous is formed and swallowed
• Saliva


  • 99.4% water, 0.6% electrolytes, buffers, glycoproteins, antibodies, enzymes
  • Buffers keep pH ~7.0
  • Mucins give saliva lubricating action
  • Antibodies control oral bacteria
  • Enzymes - salivary amylase
  • Waste products - urea and uric acid
• Functions of saliva


  • Moistens & lubricates materials in mouth for swallowing
  • Initiates digestion of complex CHO via salivary amylase
  • Lingual lipase secreted by glands of tongue for breakdown of lipids
• Pharynx


  Passage way for solid food, liquids, air from oral cavity to esophagus
• Esophagus


  • Conveys solid food and liquid to stomach
  • Coordinated activity of tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups
  • Esophageal sphincter: end of esophagus (prevents heartburn)
• Motility - Propulsion


  • Swallowing (voluntary)
  • Peristalsis (involuntary): alternate waves of contraction/relaxation to squeeze food from one organ to the next
• Stomach


  • Empty: 50mL
  • Capacity: 2-4L of food storage
• Functions of stomach


  • Storage of ingested foods
  • Mechanical breakdown of ingested foods via secretions
  • Chemical breakdown of ingested foods via secretions
  • Mixing and propulsion of chyme into small intestine
• Peristalsis


  Alternate waves of contraction/relaxation to squeeze food from one organ (e.g., esophagus) to the next (e.g., stomach)
• Major Subdivisions of the Digestive Tract

  • Oral Cavity, Teeth, Tongue
  • Pharynx
  • Esophagus
  • Stomach
  • Small Intestine
  • Large Intestine
• Mechanical processing, moistening, mixing with salivary secretions

  Oral Cavity
• Muscular propulsion of materials into the esophagus

  Pharynx
• Transport of materials to the stomach

  Esophagus
• Chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions

  Stomach
• Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions

  Small Intestine
• Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions

  Large Intestine
• Accessory Organs of the Digestive System

  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas
• Stomach capacity


  • Empty: 50mL
  • Capacity: 2-4L of food storage
• Stomach


  • Ingested substances combine with secretions of glands of stomach --> chyme
  • Storage of ingested foods
  • Mechanical breakdown of ingested foods via segmentation
  • Chemical breakdown via disruption of chemical bonds in food material (acid & enzymes)
  • Production of intrinsic factor (glycoprotein required for absorption of Vitamin B12 in small intestine)
• Stomach Anatomy - 4 Regions


  • Esophagus
  • Cardia
  • Fundus
  • Body
  • Pyloric antrum
  • Pylorus
• Layers of the Stomach Wall

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa
  • Longitudinal muscle
  • Circular muscle
  • Oblique muscle
  • Muscularis mucosae
  • Lamina propria
  • Mucous epithelium
  • Gastric pit (opening to gastric gland)
  • Myenteric plexus
  • Lymphatic vessel
  • Artery and vein
• Glandular Secretions


  • Fundus and Body: Gastric Glands (secretes mucus, intrinsic factor, hydrochloric acid)
  • Pylorus: Pyloric Glands (secretes mucous, gastrin, somatostatin)
• Intrinsic Factor


  Required for intestinal absorption of vitamin B12, needed to produce mature erythrocytes (rbc)