With the inhalation of air, it passes through the nasal passage where particulate impurities are removed and the air is humidified, before passing into the trachea, down the bronchi, into the bronchioles finally reaching the alveoli and air sacs.
In the alveolar, the oxygen is absorbed into the bloodstream by diffusion through its capillaries and carried to the heart by the pulmonary artery, while the waste products like carbon dioxide are diffused out of the blood that comes to the lungs from the pulmonary vein, and out the very same path the air comes in through.