Air enters through the nose or mouth (which join to form the nasopharyngeal cavity). The air travels through the trachea which and the trachea splits into two bronchi. The air travels through the bronchi which split into smaller and smaller bronchioles. The tiny bronchioles and the air ends up in the miniscule alveoli, where the oxygen in the air diffuse into the bloodstream. Carbon dioxide diffuse from the bloodstream into the alveoli and the unwanted gas travels in reverse back to the nose and mouth, where it is breathed out.
