• The Acinus
  • is a functional unit of the lung which is involved in gas exchange
• It is
  • distal to the terminal bronchiole and
  • contains
    • respiratory bronchioles,
    • alveolar ducts, and
    • alveoli,
  • which are
    • all involved in gas exchange.
  • Normal acini are not visible on imaging,
  • whereas
    • abnormal acini may be visualized as small, rounded opacities referred to as
      • “air space nodules”.
  • The acinus is also defined as the portion of  lung  that is
    • distal to the terminal bronchiole and is
    • supplied by the
      • respiratory bronchiole and
      • usually measures  about
        • 6-10mms in diameter. (Osborne)

It is the functional unit of the lung

The terminal bronchiole precedes the acinus. (Webb)

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.

Respiratory System Anatomy Samuel Chen

Imaging

 

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.)