How are the alveoli designed to maximise the exchange of gases?

(i) Alveoli are balloon-like structures with thin elastic walls, so they provide the maximum surface area for gaseous exchange.

ii) The alveoli are thin-walled and richly supplied with a network of tiny blood vessels called blood capillaries to facilitate the exchange of gases between blood and the air-filled alveoli. 

The characteristics of the lungs which make them efficient for gaseous exchange are-  

1. Thin: The walls of the air sacs are very thin so that gases can quickly diffuse through them. Oxygen is absorbed into the bloodstream and carbon dioxide is given out. 

2. Moist: The air sacs are moist with mucus lining so that gases can dissolve before diffusing. 

3. Large surface area: The alveoli help to increase the surface area for the absorption of oxygen. 

4. Good blood supply: The alveoli have a large capillary network so that large volumes of gas can be exchanged. The greater the flow of blood, the greater the exchange of gases.