The airways of the lung are tubular or pipe-like structures that conduct air through their lumens. The airways branch into tubes or pipes of increasingly smaller diameter, with larger bronchi dividing into smaller bronchi that branch into smaller bronchioles, which eventually lead into the air sacs or alveoli, where gas exchange occurs. The airways are accompanied by branches of the pulmonary artery that approximate their diameters in cross-section.

Bronchi are conducting airways more than 1 mm in diameter. Multiple plates of cartilage in their walls prevent their collapse, permitting them to vary in caliber. In addition to their larger caliber, the histologic features that distinguish bronchi from bronchioles (see Chapter 2) are the presence of respiratory epithelium (pseudostratified ciliated columnar epithelium), bronchial seromucinous glands, cartilage plates, and smooth muscle. There are approximately 9 to 12 generations of bronchi. The left and right mainstem bronchi branch from the trachea at the carina and enter the left lung hilum and right lung hilum, respectively. The left mainstem bronchus is longer and narrower, and has a greater angle than the right mainstem bronchus. The right upper-lobe bronchus branches off the right mainstem bronchus before it enters the hilum. The mainstem bronchi branch into the lobar bronchi, which in turn branch into the segmental bronchi of the bronchopulmonary segments, which in turn branch into generations of smaller bronchi.