Bronchial tree

The right main bronchus is wider, shorter (about 2.5 cm) and more vertical (about 25 degrees from the tracheal axis, against 45 degrees on the left), and the carina sits slightly left of the midline, so the right main bronchus is almost a straight continuation of the trachea. In an erect adult the object falls into the right lower lobe, classically the posterior basal segment; when supine it settles in the superior segment of the right lower lobe or the posterior segment of the right upper lobe. In children under about 3 years the two bronchi leave the trachea at nearly equal angles and foreign bodies are found on the left almost as often.

Cartilage changes from C-shaped rings in the trachea and main bronchi to irregular plates in the lobar and segmental bronchi and disappears once the airway is under about 1 mm, which defines a bronchiole. Submucosal glands stop with the cartilage. Goblet cells thin out through the bronchioles and are absent from the terminal bronchiole, where non-ciliated club cells take over secretion. Cilia persist into the respiratory bronchioles and are gone by the alveolar ducts. Bronchioles are therefore held open only by the elastic recoil of the surrounding lung, which is why they collapse in emphysema and narrow so readily in asthma and bronchiolitis.

The carina lies at the sternal angle, the T4/T5 intervertebral disc, in the cadaver and at end-expiration, descending to T6 in deep inspiration or on standing. It is the most sensitive cough-trigger zone in the airway. A subcarinal angle above 90 to 100 degrees is abnormal and points to left atrial enlargement, usually from mitral stenosis, or to subcarinal lymphadenopathy from lung cancer, lymphoma, sarcoidosis or tuberculosis; at bronchoscopy a widened, fixed carina suggests nodal spread of a carcinoma.

In the Weibel model the trachea is generation 0 and the airways divide about 23 times. Generations 0 to 16, trachea to terminal bronchiole, only conduct gas: their volume, about 150 mL or 2 mL/kg, is the anatomical dead space. Generations 17 to 23, respiratory bronchioles, alveolar ducts and alveolar sacs, carry alveoli and exchange gas; they hold about 3 L, and gas moves through them by diffusion rather than bulk flow because the combined cross-sectional area is enormous. The acinus is the unit distal to one terminal bronchiole.

The right lung has 10: apical, posterior and anterior in the upper lobe; lateral and medial in the middle lobe; superior, medial basal, anterior basal, lateral basal and posterior basal in the lower lobe. The left has 8 in most texts (up to 10 in some) because the apical and posterior segmental bronchi share a stem (apicoposterior) and the heart displaces the medial basal segment so that the medial and anterior basal bronchi fuse (anteromedial basal). The lingula, with superior and inferior segments, is the left counterpart of the right middle lobe. Each segment has its own bronchus and artery centrally, with veins in the intersegmental planes, so it can be resected on its own.

Aspirated material goes where gravity takes it at the moment of aspiration, with a right-sided bias from the bronchial asymmetry. Lying supine, as in an unconscious, post-ictal or intoxicated patient, it falls into the superior segment of the right lower lobe and the posterior segment of the right upper lobe. Sitting or standing, it reaches the posterior basal segment of the right lower lobe. Lying on one side, the dependent lower lobe is involved. Anaerobic lung abscess with an air-fluid level follows one to two weeks later in these same segments.