![This is blank.](https://digitalhistology.org/wp-content/themes/histology-unified/imgs/trans.png)
Alveolar duct
Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x
![Alveolar ducts <p>Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x</p>](https://digitalhistology.org/wp-content/uploads/2018/06/O.R.I.R.AD_.P01.png)
Alveolar ducts
Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x
![Knobs <p>Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x</p>](https://digitalhistology.org/wp-content/uploads/2018/06/O.R.I.R.AD_.P02.png)
Knobs
Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x
![Associated alveoli <p>Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x</p>](https://digitalhistology.org/wp-content/uploads/2018/06/O.R.I.R.AD_.P03.png)
Associated alveoli
Alveolar ducts are formed by a series of rings which support the openings of alveoli. When cut in cross section, each ring appears as a pair of knobs bordering the alveolar opening. The rings (and, therefore, the knobs too) are lined by simple cuboidal epithelium, without cilia or club cells, that overlies a thin layer of connective tissue and smooth muscle fibers. 100x
![Alveolar sac > <p>An alveolar sac is a group of alveoli (blue arrows) that are all attached to a single supporting ring (black arrows).</p>](https://digitalhistology.org/wp-content/uploads/2018/06/O.R.I.R.AD_.P04.png)
Alveolar sac >
An alveolar sac is a group of alveoli (blue arrows) that are all attached to a single supporting ring (black arrows).
![Next image <p>The next image is similar to the area outlined by the rectangle.</p>](https://digitalhistology.org/wp-content/uploads/2018/06/O.R.I.R.AD_.P05.png)
Next image
The next image is similar to the area outlined by the rectangle.