The respiratory system includes the lungs and a system of passages that link the site of gas exchange with the external environment. It is customary to divide the respiratory system into 2 principle regions:

1. a conducting portion, consisting of the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.

2. a respiratory portion, consisting of respiratory bronchioles, alveolar ducts, and alveoli.

Initially, the patency of the airways is maintained by the presence of hyaline cartilage (larynx, trachea, extrapulmonary and intrapulmonary bronchi). Thereafter the patency of the airways is maintained by the elasticity of the lungs.

There are changes in the epithelial lining of the respiratory tree as one proceeds from the nasal cavity to the alveoli of the lungs:

Pseudostratified Columnar => Simple Columnar =>

Simple Cuboidal => Simple Squamous.