When we inhale atmospheric air, through the nose it go to the trachea and then via bronchus and bronchioles it ultimately go to the alveolus where exchange of different gas takes place. Gaseous exchange occur here. This process is called diffusion. In diffusion method gaseous go from high concentrated area to low concentrated area. So, Oxygen which is more in concentration in alveolus and low in blood, go to the blood from alveolus and carbon-di-oxide which is more in concentration in pulmonary capillary & low in alveolus go to the alveolus from pulmonary capillary blood.
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