Exchanging surfaces

• Exchange surfaces are surfaces that are adapted to maximise the efficiency of gas and solute (a substance dissolved in a liquid) exchange across them. 
• Large surface area

  • A large surface area allows more of a substance to diffuse at the same time.
• Ventilation

  • In animals, if a gas is exchanged, the surface is ventilated (through breathing) to maintain a high concentration gradient and increase the rate of exchange.
• Blood supply

  • Where substances are exchanged through blood in animals, exchange surfaces are densely packed with blood vessels:
  • These blood vessels replenish the blood supply to maintain a high concentration gradient by bringing in new blood as diffusion starts to even out the concentrations.
• Thin membrane

  • A thin membrane reduces the diffusion distance.