in humans (animals)

• Alveoli

  • Dense network of capillaries around
  • Allows oxygen to pass across into the blood
  • Allows CO2 from blood to alveolus
  • Helps maintain a concentration gradient for diffusion across the alveoli
• Gas exchange in an alveolus

  1. Oxygen diffuses from high concentration in alveolus to low concentration in blood

  2. Carbon dioxide diffuses from high concentration in blood to low concentration in alveoli
• Alveoli and capillaries

  • Large surface area for gas exchange
  • Thin walls with short diffusion distance
  • Moist surfaces
  • Permeable surfaces
• There are only 2 layers of cells separating the oxygen in the alveoli from the blood
• The respiratory surfaces are moist, which helps the gases to pass through
• The moist, thin walls make the respiratory surfaces permeable
• Alveoli and capillaries

  • Surrounded by a good blood supply
  • Maintains a diffusion gradient to encourage gas exchange
• The gradient is maintained by the replenishing of the air in the lungs with fresh air
• The high oxygen concentration in the alveoli compared to the blood causes oxygen to diffuse into the blood
• The high carbon dioxide concentration in the blood compared to the alveoli causes carbon dioxide to diffuse into the alveoli to be exhaled
• Respiratory surfaces in humans (animals)


  • Large surface area
  • Thin (short diffusion distance)
  • Moist
  • Permeable
  • Good blood supply
  • Steep concentration (diffusion) gradients
• Gas exchange in humans


  1. Oxygen diffuses from alveoli to blood

  2. Carbon dioxide diffuses from blood to alveoli
• Alveoli


  • Microscopic air sacs in lungs
  • Site of gas exchange between alveoli and blood
• Adaptations of respiratory surfaces


  • Very large surface area with millions of alveoli
  • Cells are moist, making them permeable to gases
  • Only two layers of cells between alveolus and blood, so diffusion distance is very short
  • Good diffusion gradients maintained by breathing bringing oxygen onto the alveoli and carbon dioxide being removed
  • Good blood supply increases the surface area where alveoli and capillaries are in close contact
  • Blood returning to lungs is low in oxygen and high in carbon dioxide, further increasing the diffusion gradient
• In animals, respiratory surfaces usually have large surface areas, are thin (have a short diffusion distance), are moist, are permeable, have a good blood supply and have steep concentration (diffusion) gradients
• The main respiratory surfaces in plants are the cells surrounding the intercellular air spaces in leaves