Digestive system

• Digestion


  The breakdown of food into simpler substances so that it can be used by the cells of the body
• Alimentary canal/gut


  • Stretches from the mouth to the anus
  • Organs involved in digestion: mouth, oesophagus, stomach, small intestines, large intestines, pancreas and liver

  
• Enzyme


  Proteins created by an organism to speed up the rate of reactions
• Mechanical digestion in the mouth


  1. Teeth bite, cut and grind food by chewing (mastication)

  2. Tongue mixes food with saliva and shapes it into a bolus
• Chemical digestion in the mouth


  Salivary amylase in saliva breaks down starch into maltose
• Swallowing


  1. Epiglottis closes over trachea

  2. Food enters oesophagus

  3. Peristalsis pushes food down into stomach
• Digestion in the stomach


  1. Gastric juice containing pepsin and rennin enzymes

  2. Hydrochloric acid kills bacteria and provides medium for enzymes

  3. Stomach walls churn and mix food with gastric juice (mechanical digestion)

  4. Rennin solidifies milk protein

  5. Food is changed to chyme and leaves stomach

  
• Food stays in the stomach up to three hours
• The small intestines are adapted for absorption, particularly the ileum

  
• Absorption


  The process by which digested products move from the gut to the bloodstream

  
• Substances absorbed in the small intestine


  • Monosaccharides
  • Amino acids
  • Fatty acids
  • Glycerol
  • Vitamins
  • Minerals
  • Water
• Absorption mechanisms


  • Water is absorbed by osmosis
  • Other substances are absorbed by diffusion and active transport
• Ileum


  • Very long (about 5m in an adult)
  • Inner surface has thousands of finger-like projections called villi
  • Villi have a very large surface area for absorption

  
• Villi


  • Finger-like projections on the inner surface of the small intestine
  • Epithelium is only one cell thick
  • Epithelial cells have tiny projections called microvilli which further aid absorption

  
• Assimilation


  The process by which products of digestion are used by the body

  
• Egestion


  The removal of undigested food from the body

  
• Excretion


  The removal of metabolic waste from the body