The tracheobronchial tree is the anatomical and functional segment of the respiratory system that conducts air from the upper airways to the lung parenchyma. It is composed of the trachea and the intrapulmonary airways, including the bronchi, bronchioles and respiratory bronchioles. Different histological characteristics are seen at each level and serve specific purposes.
The trachea and bronchi (from the Greek bronkhos, meaning “windpipe”) have cartilaginous walls. Bronchi undergo multiple divisions and eventually give rise to the terminal bronchioles, which by definition, lack cartilage. The most distal respiratory bronchioles and alveoli are in charge of gas exchange.