![This is blank.](https://digitalhistology.org/wp-content/themes/histology-unified/imgs/trans.png)
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
![Respiratory bronchiole <p>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 </p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-1-ping.png)
Respiratory bronchiole
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
![Alveolar duct > <p>The respiratory bronchiole transitions to the alveolar duct, where the wall consists entirely of alveoli.</p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-2-ping.png)
Alveolar duct >
The respiratory bronchiole transitions to the alveolar duct, where the wall consists entirely of alveoli.
![- Knobs > <p>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. </p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-3-ping.png)
- Knobs >
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 > <p>Alveoli form an integral part of the respiratory bronchiole (black arrows) and alveolar duct (blue arrows). </p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-4-ping.png)
Alveoli >
Alveoli form an integral part of the respiratory bronchiole (black arrows) and alveolar duct (blue arrows).
![Non-associated alveoli > <p>Alveoli associated with similar adjacent passageways surround this respiratory bronchiole and alveolar duct.</p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-5-ping.png)
Non-associated alveoli >
Alveoli associated with similar adjacent passageways surround this respiratory bronchiole and alveolar duct.
![Alveolar sac > <p>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.</p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-6-ping.png)
Alveolar sac >
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.
![Pulmonary artery > <p>These vessels accompany the respiratory passageway carrying deoxygenated blood to the alveoli.</p>](https://digitalhistology.org/wp-content/uploads/2019/03/O.R.I.R.AD_.03A-7-ping.png)
Pulmonary artery >
These vessels accompany the respiratory passageway carrying deoxygenated blood to the alveoli.
![Image source > <p>This image was taken of a slide in the University of Michigan collection.</p>](https://digitalhistology.org/wp-content/uploads/2019/03/Michigan-source-ping-2-2.png)
Image source >
This image was taken of a slide in the University of Michigan collection.