The airway begins at the mouth or nose, and accesses the trachea via the pharynx. The trachea branches into the left and right main bronchi at the carina, situated at the level of the second thoracic vertebra. The bronchi branch into large bronchioles, one for each lobe of the lung. Within the lobes, the bronchi further subdivide some 20 times, ending in clusters of alveoli.
The epithelial surfaces of the airways contain cilia. Inhaled particles stick to mucus (secreted by goblet cells) which is continuously removed from the airways by these cilia. The airway epithelium also secretes a watery fluid upon which the mucus can ride freely. The production of this fluid is impaired by the disease cystic fibrosis. Macrophages are present in the airways. These cells protect the airways from infection by engulfing inhaled particles and bacteria.