Gas Exchange

• Property of surface: Thin (one cell thick)

  Reason: Short distance to diffuse
• Property of surface: Large surface area

  Reason: Many molecules can diffuse at once
• Property of surface: Moist

  Reason: Cells die if not kept moist
• Property of surface: Well ventilated

  Reason: Concentration gradients for oxygen and carbon dioxide are kept up by regular fresh supplies of air
• Property of surface: Close to blood supply

  Reason: Gases can be carried to/from the cells that need/produce them