A level Biology exam revision resources written by A level Examiners


The gas exchange surface in mammals is the alveoli, which are the air sacs inside the lungs. There are approximately 700 million alveoli present in humans - if these were flattened out they would have an equal surface area to a tennis court. Each alveolus is a small air sac with a single cell thick wall and a moist inner surface. There are networks of blood capillaries surrounding each alveolus which are so dense that the alveoli are surrounded by an almost continuous sheet of blood. The distance between air in the alveolus and blood in the capillary can be less than 1mm.

The exchange that takes place at the alveoli is by simple diffusion. Figure 1, below, shows the exchange of oxygen and carbon dioxide that occurs between blood and the air in the alveoli.

Figure 1 : Summary of Exchange of O2 and CO2 between Alveolar Air and Blood
Exchange in mammals

The gas exchange surface of the alveoli are able to effectively exchange oxygen and carbon dioxide because they possess the following:

  • Large surface area to volume ratio. (i.e. very large surface area in proportion to volume)
  • Moist surfaces (gas exchange takes place in solution)
  • Thin walls (this makes diffusion distances small, diffusion is rapid as diffusion rate directly proportional to 1/distance)
  • Rich vascular supply (the gas exchange surface is linked to a mass flow system i.e. circulatory system)
  • Diffusion gradients maintained across the gas exchange surface (by lung ventilation)

The exchange at the alveoli is a continuous process. This is because the concentration gradient is actively maintained by the flow of blood through the capillaries and the ventilation of the alveoli.

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