Editing Anatomy and Physiology (section)

=== Structure ===

The upper airway (nose, pharynx, larynx) warms, humidifies, and filters air. The lower airway (trachea, bronchi, bronchioles, alveoli) conducts air and performs gas exchange. There are roughly 300 million alveoli, each wrapped in capillaries. '''Surfactant''', produced by type II alveolar cells, lowers surface tension and prevents alveolar collapse. Insufficient surfactant causes respiratory distress syndrome in the preterm newborn.

The right mainstem bronchus is shorter, wider, and more vertical than the left, so aspirated material and a malpositioned endotracheal tube most often go right.