The exchange of O2 and CO2 between the respiratory organs and the atmosphere is called gaseous exchange. Gaseous exchange process takes place between alveoli and blood. First, O2-rich air from the atmosphere enters the alveoli of the lungs. This results in an increased O2 pressure within the alveoli (104 mm/Hg) and a lower O2 pressure in the alveoli wall (40 mm/Hg). As a result, O2 enters the bloodstream from the alveoli in the process of diffusion.
Again respiration produces CO2 in cells. This increases the CO2 pressure in the blood (46 mm/Hg) and lowers the CO2 pressure inside the alveoli (40 mm/Hg). As a result, CO2 enters the alveoli from the blood vessels in the diffusion process. The CO2 is then exhaled.