Adaptations for gas

• Surface area to volume ratio

  Determines how much gas exchange can occur.
• Organisms exchange gases such as oxygen and carbon dioxide with the atmosphere via a gas exchange surface
• When an organism doubles in size, its volume (and therefore also its oxygen requirements) is cubed, but the surface area is only squared
• There is always a balance to be struck between exchanging gases and water loss in terrestrial organisms
• Surface area


  • 1 x 1 x 6 sides = 6mm²
  • 2 x 2 x 6 sides = 24mm²
  • 4 x 4 x 6 sides = 96mm²
• Volume


  • 1x1x1=1mm³
  • 2x2x2=8mm³
  • 4x4x4=64mm³
• Surface area to volume ratio

  • 6:1
  • 3:1
  • 1.5:1
• As organisms increase in size

  Their volume increases more than their surface area, so a specialised gas exchange surface is required.
• To achieve max rate of diffusion, a respiratory surface must have:

  • Large SA:V
  • Thin for short diffusion pathways
  • Permeable to allow respiratory gases to pass
  • Moist to allow a medium for gasses to diffuse across
  • Maintain a concentration gradient