Digestive System

• Digestion


  The breaking down of carbohydrate, protein and fat molecules into products small enough to be absorbed into the blood and cells
• Six basic activities of the digestive system

  • Ingestion of food and water
  • Mechanical digestion of food
  • Chemical digestion of food
  • Movement of food along the alimentary canal
  • Absorption of digested food and water into the blood and lymph
  • Elimination of material that is not absorbed
• Mechanical digestion


  • Physical breakdown of food particles to increase surface area for more effective chemical digestion
• Chemical digestion


  • Large, complex molecules broken down into smaller, simpler molecules which can be absorbed into the bloodstream using enzymes
• Products of chemical digestion

  • Monosaccharides (e.g. glucose)
  • Peptides
  • Amino acids
  • Fatty acids and glycerol
  • Nucleotides
• Alimentary canal


  A continuous tube from the mouth to the anus that works with accessory organs like the pancreas, liver and gall bladder to digest and absorb nutrients
• Mechanical digestion in the mouth


  1. Action of jaws and teeth

  2. Chewing (mastication)
• Chemical digestion in the mouth

  Food mixed with saliva containing salivary amylase enzyme
• Bolus


  The rounded lump of chewed food
• Teeth


  • Incisors (biting/cutting)
  • Canines (tearing)
  • Premolars (crushing and grinding)
  • Molars (crushing and grinding)
• Peristalsis in the oesophagus


  Circular muscle contractions behind the bolus narrowing the tube, pushing the food along
• Stomach


  • J-shaped, enlarged section of the alimentary canal
  • Mechanical digestion achieved by waves of muscular contraction along stomach wall
  • Oblique layer of muscle enables 'churning' of food with gastric juices to form chyme
  • Pyloric sphincter prevents contents moving until pushed by peristalsis
• Gastric juice


  Secreted by gastric glands in the stomach lining, contains hydrochloric acid, mucus, and the enzyme pepsinogen
• Conversion of pepsinogen to pepsin


  Hydrochloric acid in gastric juice activates pepsinogen into the enzyme pepsin which breaks down proteins
• Regions of the small intestine


  • Duodenum
  • Jejunum
  • Ileum
• Pancreatic juice


  Secreted by the pancreas, helps neutralise stomach contents and contains enzymes like amylase, trypsin, and lipases
• Bile


  Produced by the liver, stored in the gall bladder, and secreted into the small intestine to emulsify fats
• Intestinal juice


  Secreted by glands in the small intestine lining, contains enzymes that complete digestion of carbohydrates, proteins and lipids
• Small intestine absorption


  • Large surface area achieved by long length, folded lining, villi, and microvilli
  • Nutrients absorbed through blood capillaries and lacteal lymph vessels
• Villi


  Finger-like projections on the small intestine lining that increase surface area for absorption
• Elimination


  The removal of undigested material (faeces) from the body, not the removal of metabolic wastes (excretion)
• Components of faeces


  • Water
  • Undigested food (cellulose)
  • Bacteria
  • Bile pigments
  • Remains of cells from the alimentary canal lining
• The large intestine absorbs water and vitamins, stores faeces, and facilitates defecation
• The mouth breaks food into small particles by chewing (mastication) and saliva containing salivary amylase begins starch digestion
• The oesophagus transports food from the mouth to the stomach
• The stomach's mechanical digestion churns food with gastric juice, and its chemical digestion uses pepsin to break down proteins
• The small intestine's mechanical digestion uses muscular contractions and bile salts to churn food, while its chemical digestion uses pancreatic and intestinal enzymes to further break down nutrients
• The small intestine absorbs simple sugars, amino acids, fatty acids, glycerol, vitamins, and mineral nutrients through the villi