Explanation

Alveoli (thin, irregular-walled and vascularized bag-like structure at the end of bronchiole) are the primary sites of exchange of gases. $O_{2}$ and C$O_{2}$ are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. Solubility of the gases as well as thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion. Reactivity of the gases does not affect the rate of alveolar diffusion.