Exchange substances

• • The lungs have millions of alveoli where gaseous exchange takes place
  • The lungs transfer oxygen to the blood and to remove carbon dioxide
• • The alveoli diffuse oxygen and carbon dioxide
  • They have a large surface area
  • They have a moist lining for dissolving gases
  • They have very thin walls
  • They have a good blood supply
• • Villi are found in the small intestine
  • They increase surface area so digested food more quickly into the blood
  • They have a single layer of surface cells
  • They have a good blood supply to help quick absorption
• • Oxygen diffuses from the water to the blood in the gills
  • Gills have thin plates called gill filaments which give the gills a large surface area
  • Gill filaments are covered in lamellae which increase surface area
  • Lamellae have thin surface layer of cells to minimise distance of diffusion
  • Blood and water flow in opposite directions which keep a large concentration gradient between the water and the blood
  • Concentration of oxygen in the water is higher than in the blood so as much oxygen as possible diffuses from water to blood
• • Carbon dioxide diffuses into air spaces in the leaf and into cells where photosynthesis happens - leaf adapted so this can happen easily
  • Stomata are underneath the leaf where carbon dioxide diffuses through
  • Oxygen and water vapour diffuse out of stomata
  • Guard cells control size of stomata and close stomata if too much water is being lost faster than being replaced by the roots
  • The leaf have a flattened shape to increase surface area
  • Walls of the cells inside the leaf have air spaces to increase area for the diffusion of carbon dioxide inside the cell