Transition of bronchiole to alveolar duct

As a respiratory bronchiole transitions to an alveolar duct, alveoli increase in number and the wall of the respiratory bronchiole is reduced to a series of knobs forming the wall of an alveolar duct. The knobs are really cross-sectional views of the rings that support the alveoli that are attached to them. 150x

The respiratory bronchiole transitions to the alveolar duct, where the wall consists entirely of alveoli.

The wall of the alveolar duct is formed by contiguous rings, resembling chicken wire rolled into a tube. When sectioned, one ring is represented by a pair of knobs. The rings provide support for the attached alveoli. Each ring (or pair of knobs) is lined by a simple cuboidal epithelium (blue arrows) overlying a thin layer of connective tissue and strands of smooth muscle.

Alveoli form an integral part of the respiratory bronchiole (black arrows) and alveolar duct (blue arrows).

Alveoli associated with similar adjacent passageways surround this respiratory bronchiole and alveolar duct.

An alveolar sac is two or more alveoli attached to a pair of knobs (representing a single ring) that forms the wall of the alveolar duct.

These vessels accompany the respiratory passageway carrying deoxygenated blood to the alveoli.

This image was taken of a slide in the University of Michigan collection.