How are the lungs adapted for gas exchange?

-Large surface area to volume ratio: the lungs contain millions of microscopic air sacs called alveoli, which are the sites of gas exchange in the lungs. The large number of alveoli means that the lungs have a large surface area for gas exchange, proportional to the volume of the lungs: if you were to spread all of the alveoli flat, then the lungs would cover the area of a tennis court! This large surface area maximises gas exchange.
-Small diffusion distance: the walls of the alveoli are only one cell thick, which means that gases do not have to diffuse far to get to their destinations (oxygen from the air in the alveoli to the capillaries which cover the alveoli, and carbon dioxide the other way round).
-Large diffusion gradient: gases move from areas of high concentration of the gas to areas of low concentration. The difference between the high concentration and low concentration is called the “diffusion gradient”, and gas exchange is maximised if the diffusion gradient is large. There is a large diffusion gradient in the lungs as the oxygen concentration is high in the air in the alveoli, and the carbon dioxide concentration is high in the blood in the capillaries covering the alveoli.