Respiratory Surfaces

Characteristics of Respiratory Surfaces

  • All respiratory surfaces have a large surface area to increase the amount of gases that can be exchanged.
  • They are all thin, reducing the distance gases have to diffuse.
  • They are moist, allowing gases to dissolve into them before being transported elsewhere.
  • These surfaces are very well vascularised, meaning they have a rich supply of blood vessels, which help transport the gases to and from the surface.

Types of Respiratory Surfaces


  • In simple organisms, like worms, gases can diffuse directly through the skin.
  • The skin must be moist to allow gases to dissolve before diffusion.


  • Fish and many aquatic organisms use gills to extract oxygen from the water.
  • Water is taken in through the mouth and forced out over the gills.
  • Oxygen in the water diffuses into the blood vessels in the gills, while carbon dioxide diffuses out into the water.
  • The gills are ‘flapped’ to ensure a constant flow of fresh water over them.


  • In mammals, birds, and reptiles, lungs are the main respiratory surface.
  • Air is drawn into the lungs by diaphragm and rib muscle contractions and forced out by relaxation of these muscles.
  • Oxygen in the air diffuses across the alveoli walls into the blood vessels, while carbon dioxide diffuses out of the blood into the lungs to be exhaled.
  • The lining of the alveoli is covered with a moist mucus to allow gases to dissolve before diffusion and is highly vascular, with very close proximity to a dense network of blood capillaries, making it an efficient site for gas exchange.

Tracheal System

  • Insects have a network of air tubes called tracheae.
  • Air enters and exits through tiny pores on the insect’s body called spiracles.
  • Oxygen moves down the tracheae and diffuses directly into the cells; carbon dioxide diffuses out of the cells into the tracheae to be expelled.

Leaf Stomata

  • Green plants exchange gases with the atmosphere through the stomata (tiny pores) in their leaves.
  • The stomata can open and close to regulate the exchange of gases, taking in carbon dioxide and expelling oxygen.