Each gas acts independently of the others. The partial pressure of a gas is equal to its percentage of the total pressure. In the lungs these gases must be in solution to diffuse. Solubility of CO 2 is much higher than O 2 or N 2 . Despite the high partial pressure of N 2 very little dissolves in blood.
Alveolar air is a mixture of atmospheric and residual air.
Air is humidified in the lungs.