Acinus

The Acinus

The acinus is defined as the portion of  lung  that is distal to the terminal bronchiole and is supplied by the respiratory bronchiole and usually measure  about 6-10mms in diameter. (Osborne)

It is the functional unit of the lung

The terminal bronchiole precedes the acinus. (Webb)

Acinus.
This diagram illustrates the acinus which consists of the respiratory bronchioles (rb 1, 2, 3) the alveolar duct (ad) the alveolar sac (as) and the alveoli. (a)
Courtesy Ashley Davidoff MD 42446b12
TheCommonVein.net

The bronchi proceed from the mainstem bronchus via 16 to 23 divisions into the terminal bronchioles.  Thereafter sac-like protrusions develop in the system, which allow gas exchange to start taking place. The first branch that is able to perform this gas exchange is called the respiratory bronchiole.  After three divisions the respiratory bronchioles become alveolar ducts and after further division become alveolar sacs.  Finally, at the terminal end of this pathway, are the alveoli.  The system that starts at the respiratory bronchiole and terminates at the alveoli is called an acinus, and it is functionally characterized by having the ability to both conduct air as well as enable gas exchange.

The acinus  averages about 7mm  (4-8mm)in diameter.  When it is filled with fluid, it can be visualized on a CXR as a 6mm nodular density called an “acinar shadow.”  As a structural entity it has little diagnostic utility.  We will expand on the pulmonary lobule in the next section which has greater implications for the imaging of the lung.  Functionally, however, the acinus can be considered as the unit of gas exchange in the lung.

Acinus.
The acinus with its arborizations is shaped more like a bunch of grapes.
Courtesy of: Ashley Davidoff, M.D 42650
TheCommonVein.net

Respiratory System Anatomy Samuel Chen

Imaging
Acinar Nodules in a patient with transbronchial spread of TB
Courtesy https://www.slideshare.net/

 

Anatomy of the Distal Airways in Color and in the Black and White of Radiology
The subsegmental medium sized airways give rise to the terminal bronchiole (tb) which gives rise to the membranous airways. These include in order, the respiratory bronchiole (rb), alveolar duct (ad) and alveolar sac (as)
Ashley Davidoff
TheCommonvein.net
Acini
Ashley Davidoff TheCommonVein.net
Acinus.
This diagram illustrates the acinus which consists of the respiratory bronchioles (rb 1, 2, 3) the alveolar duct (ad) the alveolar sac (as) and the alveoli. (a)
Courtesy Ashley Davidoff MD 42446b12
TheCommonVein.net
Normal Acinus and Emphysema
Image on the left shows normal size and appearance of terminal bronchioles and alveoli. On the right the image shows the effects on the respiratory bronchioles and when severe, on the alveoli as well
Ashley Davidoff MD
TheCommonVein.net

Links and References

Fleischner Society

acinus

Anatomy.—The acinus is a structural unit of the lung distal to a terminal bronchiole and is supplied by first-order respiratory bronchioles; it contains alveolar ducts and alveoli. It is the largest unit in which all airways participate in gas exchange and is approximately 6–10 mm in diameter. One secondary pulmonary lobule contains between three and 25 acini (,4).

Radiographs and CT scans.—Individual normal acini are not visible, but acinar arteries can occasionally be identified on thin-section CT scans. Accumulation of pathologic material in acini may be seen as poorly defined nodular opacities on chest radiographs and thin-section CT images. (See also nodules.)