1402 Physiology Digestion & Metabolism

• Digestive system


  The system responsible for breaking down food into smaller molecules that can be absorbed and used by the body

  
• Nutrients


  Building blocks for body tissues and a source of chemical energy
• Most food consists of molecules that are too large to be used by the cells of the body
• Digestion


  The process of breaking down large food molecules into smaller molecules that can be absorbed by the body
• Components of the digestive system

  • Gastrointestinal tract
  • Accessory digestive organs

  
• Gastrointestinal tract (GIT)


  A continuous tube that extends from the mouth to the anus, about 4.5 meters long

  
• Parts of the gastrointestinal tract

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine

  
• Accessory digestive organs


  • Teeth
  • Tongue
  • Salivary glands
  • Pancreas
  • Liver
  • Gallbladder

  
• Teeth and tongue


  Aid in the physical breakdown of food
• Other accessory digestive organs

  Produce or store secretions that aid in the chemical breakdown of food
• Basic digestive processes


  • Secretion
  • Digestion
  • Motility
  • Absorption

  
• Secretion


  The addition of fluids, enzymes and mucus into the lumen of the gastrointestinal tract

  
• About 7 liters of solutions containing enzymes are secreted daily into the lumen of the gastrointestinal tract

  
• Different gastrointestinal enzymes exhibit maximum activity at different pH levels
• Digestion


  The breakdown of large food molecules into smaller molecules that can be absorbed
• Mechanical digestion


  The physical breakdown of food by the teeth and the contractions of the stomach and small intestine
• Chemical digestion


  The splitting of large carbohydrate, lipid, and protein molecules into smaller molecules by digestive enzymes

  
• Digestive enzymes secreted in the gastrointestinal tract

  • Proteases
  • Carbohydrases
  • Lipases
  • Nucleases
• The enzymes induce hydrolysis, or chemical decomposition of the nutrients supplied with food

  
• This results in the formation of small molecules (monomers) that can be absorbed from the gastrointestinal tract lumen to the blood

  
• Examples of chemical digestion

  • Carbohydrate hydrolysis: Disaccharide maltose broken down into glucose
  • Protein hydrolysis: Polypeptide broken down into amino acids
  • Fat hydrolysis: Lipid (triglyceride) broken down into fatty acids and glycerol

  
• Motility


  The contractions of the smooth muscle in the gastrointestinal tract wall that mix food and digestive secretions, and propel them toward the anus
• Phasic contractions


  Periodic contractions followed by relaxation, found in parts of the gastrointestinal tract involved in mixing and propulsion
• Tonic contractions


  Contractions that maintain constant tension without regular relaxation, found in the upper stomach and gastrointestinal sphincters
• Sphincter


  A bundle of circular muscle that separates two adjacent regions of the gastrointestinal tract
• Types of smooth muscle cells in the gastrointestinal tract

  • Autorhythmic (pacemaker) cells
  • Contractile cells

  
• Autorhythmic cells


  Generate cycles of alternating depolarization and repolarization (slow wave potentials) that can trigger action potentials in contractile cells
• The amplitude of gastrointestinal smooth muscle contraction increases upon an increase in the frequency of spike potentials
• The frequency of slow wave potentials varies in different parts of the gastrointestinal tract, ranging from 3 to 12 per minute
• Acetylcholine increases the frequency of generation of spike potentials by autorhythmic cells, and stimulates gastrointestinal motility
• Adrenaline and noradrenaline produce the opposite effects on gastrointestinal motility
• Contractile smooth muscle cells

  Interconnected by gap junctions, allowing action potentials to spread and cause them to contract in unison
• Absorption


  The movement of the products of digestion from the lumen of the gastrointestinal tract into the blood or lymph

  
• Paracellular pathway


  Passive transport of small molecules between intestinal epithelial cells
• Transcellular pathway


  Active transport of nutrients across intestinal epithelial cells
• Mechanisms of nutrient absorption in the small intestine

  • Passive transport (simple diffusion, facilitated diffusion)
  • Active transport (primary active, secondary active)

  
• Most absorption of major nutrients takes place in the small intestine

  
• Mechanisms controlling gastrointestinal function

  • Neural
  • Endocrine
  • Paracrine
• Enteric nervous system (ENS)


  The "brain of the gut", consisting of 100 million neurons in the gastrointestinal tract wall
• Neural plexuses in the gastrointestinal tract wall

  • Myenteric plexus
  • Submucosal plexus