Background: The trachea begins at the cricoid cartilage of the larynx and ends when it bifurcates to form the primary bronchi. The bronchial tree is composed of airways located outside of the lungs (the primary bronchi, extrapulmonary bronchi) and airways located inside of the lungs: the intrapulmonary bronchi (lobar [secondary] and segmental [tertiary] bronchi), bronchioles, terminal bronchioles, and respiratory bronchioles. The bronchial tree divides 15 to 20 times before reaching the level of terminal bronchioles. Tracheobronchial airway walls have abundant cartilage, extracellular matrix, and smooth muscle, as well as submucosal glands. Epithelial cells at tracheobronchial surfaces include mucous cells, mucin-packed goblet-shaped mucous cells, serous cells, and multiciliated cells that all contribute to airway surface liquid hydration and mucociliary defense. Bronchioles have progressively thinner airway walls, with many of the same epithelial cell types found in the tracheobronchial airways, including basal cells. Neuroepithelial cells and club cells are enriched in bronchioles, where they have progenitor, chemosensory, and detoxification functions. Distal airways include club cell–lined respiratory bronchioles that terminate at type 1 and type 2 pneumocyte-lined alveoli. The respiratory portion of the respiratory system is composed of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. Respiratory bronchioles are the first region of the respiratory system where exchange of gases can occur. Respiratory bronchioles are similar in structure to terminal bronchioles in that their epithelium is a simple cuboidal epithelium rich in club cells and some ciliated cells. However, this epithelium is broken up by the presence of thin-walled, pouch-like structures known as alveoli. Alveolar ducts do not have walls of their own; they are merely a continuous sequence of alveoli. An alveolar duct that arises from a respiratory bronchiole forms branches. Each of the resultant alveolar ducts usually ends as a blind outpouching composed of two or more small clusters of alveoli, in which each cluster is known as an alveolar sac