Ventilation surfaces must possess certain properties to allow for adequate amounts of gas exchange:

• Surface area: A large surface are will allow sufficient concentrations of gases to be exchanged

• Moisture: It is easier for gases to diffuse across membranes when they are dissolved in a solution

• Absorptive: Surfaces must be adequately permeable to enable the movement of respiratory gases

• Rich blood network: Gas exchange surfaces must be connected to a dense capillary network

• Thin tissue layer: Ventilation surfaces are lined by a single layer epithelium to optimise gas exchange
  

Alveoli are the site of gas exchange within the lungs and are composed of two types of alveolar cells (pneumocytes)

• Type I pneumocytes are squamous (flattened) and extremely thin – they function as the gas exchange surface
  

• Type II pneumocytes are responsible for the secretion of pulmonary surfactant, which reduces surface tension in the alveoli
  

Pulmonary surfactant prevent the sides of the alveolus adhering to each other by reducing surface tension

• As an alveolus expands with gas intake, the surfactant becomes more spread out, increasing surface tension and slowing the rate of expansion
  

• This ensures that all alveoli inflate at roughly the same rate and to proportionate dimensions – optimising gas exchange