Respiratory tract. Computer artwork showing the various stages and structures of the human respiratory tract. Air is drawn in through the oral and/or nasal cavities, and passes down the trachea, into the bronchi and then through the bronchioles into the alveoli. Once in the alveoli, the oxygen in the air dissolves in the damp lining of the alveolar sacs and diffuses through their walls into the blood system. On exhaling, carbon dioxide passes out of the blood into the alveoli and is expelled in the opposite direction.