Intrapulmonary bronchi

Intrapulmonary bronchi are the airways that lie within the substance of the lung parenchyma, as distinguished from the extrapulmonary (main) bronchi that reside outside the lungs at the hilum. They represent the continuation of the bronchial tree after the main bronchi enter the lung at the hilum and branch into progressively smaller airways.

Intrapulmonary bronchi include the lobar bronchi, segmental bronchi, and intrasegmental bronchi that course within the lung tissue. They are characterized by the presence of cartilaginous support in their walls, which distinguishes them from bronchioles.

The cartilaginous framework becomes progressively less complete as the airways branch distally, transitioning from C-shaped rings to irregular cartilage plates.

Intrapulmonary bronchi are surrounded by lung parenchyma and maintain close threedimensional relationships with the pulmonary arteries and veins at each level of branching.

The branching pattern follows a combination of regular and irregular dichotomies, with the number of divisions from the segmental bronchus to the terminal bronchiole varying from approximately 9 to 23 generations.

Once the airways lose their cartilaginous support entirely and reach an internal diameter of approximately 2 mm or less, they are classified as bronchioles (membranous bronchioles), which then continue as terminal bronchioles and respiratory bronchioles. The intrapulmonary bronchi also contain submucosal glands and a relatively thick wall with smooth muscle, features that diminish as the airways transition to bronchioles.