What is the main process of gas exchange in the body?

Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. O₂ and CO₂ are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion. Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2 for oxygen and pCO2 for carbon dioxide. Partial pressures of these two gases in the atmospheric air and the two sites of diffusion The data given in the table clearly indicates a concentration gradient for oxygen from alveoli to blood and blood to tissues