The airway consists of the entire pathway for air flow from the mouth or nose down to the alveolarsacs. The conducting airways consist of the oro- and nasopharynx, the larnyx, the trachea, the twomain bronchi, the five lobar bronchi (three on the right, two on the left), and the fifteen to twentydivisions of bronchi and bronchioles down to the level of the terminal bronchioles. The part of theairway that participates in gas exchange with the pulmonary capillary blood consists of therespiratory bronchioles, alveolar ducts, and the alveoli themselves.
The surface area provided by the respiratory bronchioles and alveoli for gas exchange is tremendous. It is estimated that the adult human lung has 300 million alveoli, with a total suraface areaapproximately equal in size to a tennis court.