Secondary bronchus

A secondary bronchus is lined by a pseudostratified columnar epithelium with cilia and goblet cells. The lamina propria contains smooth muscle and cartilage plates. Longitudinally oriented elastic fibers are still present beneath the epithelium, however, they are not readily demonstrable at this magnification. 300x

The layer of circularly-oriented smooth muscle fibers is prominent.

Cartilage plates are present in secondary bronchi, but they do not persist into the bronchiole. Secondary bronchi also possess mixed glands, however, they are not present in this section.

Alveoli are visible surrounding the secondary bronchus indicating that this passageway is intrapulmonary. However, their lumens are not continuous with those of the secondary bronchus and thus this passageway is not capable of gas exchange.

Numerous, small bronchial vessels (filled with red blood cells) are present in the wall of the passageway. These vessels provide nutritive support, but are not involved with gas exchange.

A portion of a pulmonary vein is visible in the lower right, surrounded by alveoli. This vessel is conveying oxygenated blood out of the lung.