ABSTRACT
Anatomy The airways consist of multiple tubes connected in both series and parallel fashion that can be classified into two sections according to function: the conducting zone and the respiratory zone. The conducting zone encompasses the oronasopharynx, larynx and trachea, which divides into the right and left main bronchi at the level of the second thoracic vertebrae. These subsequently subdivide into the lobar (secondary) bronchi, segmental (tertiary) bronchi and then terminal bronchioles (diameter <1 mm). This section of the lung is known as the conducting zone as its rigid, largely cartilaginous structure facilitates the passage of inspired air to the respiratory zone and the ciliated epithelium humidifies and filters air before gas exchange occurs. The first 16 divisions or orders of branching airways make up the minority of the lung volume and its blood supply is from the bronchial circulation that originates from the left side of the heart.
