The characteristics that make lungs an efficient respiratory surface are:

1. Thin: The walls of the alveoli or air sac are very thin resulting in the quick diffusion of
gases. They help in the absorption of oxygen inside the blood by inhalation process and also remove the carbon dioxide from the lungs by exhaling it to the outside.

2. Moist: The air sacs are composed of mucus that makes them moist which helps the gases to dissolve easily before diffusing.

3. Large surface area: In the lungs of human beings, its surface area is almost similar to the
surface area of the tennis court that helps in the diffusion of gases. The alveoli or air sacs result
in increasing the surface area of the lungs for the absorption of oxygen.

4. Good blood supply: The air sacs or alveoli are composed of a large capillary network that helps in the exchange of large volumes of gases. Thus greater the flow of blood in the lungs the greater would be the exchange of gases.