When we inhale atmospheric air, through the nose it go to the trachea and then via bronchus and bronchioles it ultimately go to the alveoli where exchange of different gas takes place.Alveoli is the sac filled with air and it is functional unit of lung. Gaseous exchange occur here.This alveoli separated from blood by a membrane called Alveo-capillary membrane.It is the barrier between air and blood. Different gaseous overcome this barrier by Diffusion method. 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 the pulmonary capillary blood.So, we can see that oxygen and carbondioxide, this 2 major gas exchanged from alveoli to blood and from blood to alveoli respectively by diffusion in the lung. This exchange happening maximum in base of the lung and minimum in apex of the lung.

Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs. It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli.