Digestive System 4a

• Alimentary canal


  Muscular tube that runs from the mouth to anus (approx. 9 m long), digests food and absorbs fragments through lining into blood
• Organs of the digestive system

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anus
• Accessory digestive organs


  • Teeth*
  • Tongue*
  • Gallbladder*
  • Salivary glands*
  • Liver*
  • Pancreas*
• Digestive processes


  • Ingestion
  • Propulsion
  • Mechanical breakdown
  • Digestion
  • Absorption
  • Defecation
• Peristalsis


  Major means of propulsion of food that involves alternating waves of contraction and relaxation
• Segmentation


  Local constriction of intestine that mixes food with digestive juices
• Peristalsis vs Segmentation


  Peristalsis is the major means of propulsion, segmentation mixes food with digestive juices
• Enteric nervous system


  GI tract's own nervous system, also called the "gut brain", contains more neurons than the spinal cord
• Enteric nervous system reflexes

  • Short reflexes (mediated by enteric nerve plexuses, respond to stimuli in GI tract)
  • Long reflexes (respond to stimuli arising inside or outside of gut)
• Parasympathetic system


  Enhances digestive process
• Sympathetic system


  Inhibits digestion
• Concepts regulating digestive activity

  • Digestive activity is provoked by mechanical and chemical stimuli
  • Effectors of digestive activity are smooth muscle and glands
  • Neurons and hormones control digestive activity
• Saliva


  Cleanses mouth, dissolves food chemicals for taste, moistens food and compacts into bolus, begins breakdown of starch with amylase
• Saliva is mostly water (97-99.5%), slightly acidic (pH 6.75-7.00), and contains electrolytes, salivary amylase, lingual lipase, and immune system components
• Salivary gland control


  1500 ml/day can be produced, minor glands keep mouth moist, major glands activated by parasympathetic, sympathetic stimulation inhibits salivation, smell/sight of food or upset GI can stimulate
• Deglutition (swallowing)


  Involves coordination of 22 muscle groups and two phases: buccal phase (voluntary tongue contraction) and pharyngeal-esophageal phase (involuntary)
• If the epiglottis did not close during deglutition

  Food would enter the airway, causing choking
• If the upper esophageal sphincter did not close following deglutition

  Food would reflux back into the pharynx