Describe three properties that all respiratory surfaces must have and explain why they must have them.

Respiratory surfaces must be thin (such as in alveoli which have walls that are 1 cell thick) to create a small diffusion distance for the gases to be exchanged through. They must also have a large surface area for diffusion, in the lungs achieved by the large number and shape of alveoli. Another important property is that they must be moist to allow the gases to dissolve and cross the membrane. The surface must be permeable to allow gases to pass through. Furthermore, having an extensive blood supply is vital, in the case of alveoli the capillary network ensures that oxygen rich blood can be transported away from the lungs. Finally, having a steep diffusion gradient is important and achieved in the alveoli by breathing. This ensures a higher concentration of oxygen is maintained in the alveoli than the capillaries so oxygen diffuses into the blood.