The trachea divides into two primary bronchi, the left primary and the right primary bronchi.
On entering the lungs each primary bronchus divides into two secondary bronchi.
The secondary bronchi will then divide into two tertiary bronchi.
The tertiary bronchi divide into the first two bronchioles. After 13 other divisions, the terminal bronchioles give rise to the respiratory bronchioles, then to alveolar ducts and finally to the alveoli.
The trachea, bronchi and bronchioles up to the last division of the terminal bronchiole constitute the conducting zone. The blood supply to these parts are mainly to supply the muscle layers of the airways. Because no gas exchange takes place in these airways, the conducting zone constitutes the Anatomical Dead Space.
Primary bronchi have Incomplete Cartilagenous Rings while the secondary and tertiary bronchi have Cartilagenous Plaques. Both of these help in keeping the bronchi open during inspiration.
The cartilage disappears in the terminal bronchioles, and in these structures collapse and closure of the opening is prevented by the muscular layer and interstitial tension.
Though with each generation of divisions the diameter of the conducting system is reduced, the overall resistance is reduced as the total cross sectional area at any on point is larger than at the preceding division.
The movement of air from the nose to the terminal bronchioles is accomplished by Bulk Flow. At the terminal bronchiole the forward velocity of bulk air is negligible and Gas Diffusion becomes the predominant process.