• Lung Acinus

• • Etymology
    • Derived from the Latin word “acinus,” meaning berry or grape, referencing the grape-like clusters of alveoli.
  • AKA and Abbreviation
    • Also known as “primary lobule.”
  • What is it?
    • The acinus is the smallest functional unit of the lung responsible for gas exchange because it integrates multiple structures required for efficient oxygen-carbon dioxide exchange, including respiratory bronchioles, alveolar ducts, and alveoli. While the alveolus is the site of gas exchange, it lacks the additional structural and functional components needed for the coordinated delivery and removal of gases.
    • It is called the “primary lobule” because it represents the first distinct functional subunit within the respiratory division of the lung, with all elements necessary for complete respiratory function.
    • It includes all structures distal to the terminal bronchiole, comprising respiratory bronchioles, alveolar ducts, and alveoli.
  • Principles
    • Parts
      • Terminal bronchiole: Precedes the acinus.
      • Respiratory bronchioles: Initiate gas exchange.
      • Alveolar ducts: Lead to alveolar sacs.
      • Alveoli: Primary sites of oxygen and carbon dioxide exchange.
    • Size
      • Diameter: 6-10 mm.
      • Each acinus contains 2000-5000 alveoli.
    • Shape
      • Grape-like clusters of alveoli surrounding alveolar ducts.
    • Position
      • Distal to the terminal bronchioles within secondary pulmonary lobules.
    • Character
      • Highly vascularized structures with thin-walled alveoli.
    • Time
      • Functional throughout life but susceptible to damage from environmental or pathological factors.
    • Blood supply
      • Supplied by the pulmonary arteries.
    • Venous Drainage
      • Drained by the pulmonary veins.
    • Lymphatic drainage
      • Lymphatic vessels drain into interlobular septa lymphatics.
    • Nerve Supply
      • Autonomic innervation from the pulmonary plexus.
    • Embryology
      • Development begins in the pseudoglandular phase (~16 weeks gestation) and matures through the alveolar phase (~36 weeks gestation to early childhood).
    • Histology
      • Lined by simple squamous epithelium.
      • Supported by elastic fibers for recoil.
      • Structurally integrated into the secondary pulmonary lobule, with each acinus contributing to the overall function of its lobule.
      • Each secondary pulmonary lobule contains approximately 3 to 25 acini, with variation based on location within the lung (smaller near the pleura and larger near the hilum).
    • Physiology and Pathophysiology
      • Facilitates gas exchange through diffusion.
      • Vulnerable to damage in diseases like emphysema, pneumonia, and interstitial lung diseases.
  • Applied Anatomy to Radiology
    • CXR
      • The acinus itself is not directly visible but contributes to imaging patterns.
      • Acinar Shadows: Suggest filling of multiple acini within a secondary pulmonary lobule, appearing as ill-defined, nodular, or confluent opacities (1-2 cm) caused by fluid, pus, blood, or cells, commonly seen in conditions like pneumonia or pulmonary edema.
    • CT
      • The acinus itself is rarely imaged directly unless it is diseased.
      • Observations on CT often represent changes at the secondary pulmonary lobule level:
        • Acinar Shadows: Correlate with areas of consolidation or ground-glass opacity where groups of acini are filled or obliterated by pathological processes.

A structural unit of the lung which is distal to the terminal bronchiole and contains respiratory bronchioles, alveolar ducts, and alveoli, which are all involved in gas exchange.

 

The  Acinus

Artistic Rendering of an Acinus

Acinus Subtended by a Single  First Order Terminal Respiratory Bronchiole
3-30 Acini (usually 20-30)   per Secondary Lobule Measures 2-5mm

Normal acini are not visible on imaging, whereas abnormal acini may be visualized as small, rounded opacities referred to as “air space nodules”. (Etesami)

 

• 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
        • 2-5mms in diameter.

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.”  or “air space nodules” 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

Acinar Shadows Pulmonary Hemorrhage GPA aka Wegener’s Granulomatosis

 

Other Artistic Examples of Normal Acini in the Lung

 

The Acinus in Health and in Emphysema

• Parallels with Human Endeavors
  • Analogous to the smallest units in networks, such as individual cells in a hive or nodes in a neural network, which collectively drive function and efficiency.
  • Art and Sculpture
    • Bacchus (Roman) or Dionysus (Greek) is frequently depicted in art and sculpture as the god of wine, surrounded by grapes, vines, and symbols of revelry. These works often explore themes of abundance, indulgence, and the duality of creation and destruction.
    • Caravaggio’s “Bacchus”: Portrays the god with a goblet of wine and grapevines, symbolizing indulgence and life’s transformative powers.

• • • Michelangelo’s “Bacchus”: A sculpture representing the god in a youthful and indulgent pose, symbolizing the duality of creation and destruction.

View Image

• • Biblical
    • Grapes are significant in Biblical themes as symbols of abundance, blessing, and judgment. For example, in Revelation 14:19-20, “the great winepress of the wrath of God” uses grapes as a metaphor for divine justice and retribution.
    • “The Vine and the Branches” in the Bible (John 15:1-8): Grapes symbolize spiritual connection and sustenance, illustrating the relationship between individuals and their divine source.
    • Song of Songs: Grapes and vineyards are used as metaphors for love, vitality, and abundance, reflecting deep emotional and spiritual connections.
  • Literature

• • • The title “The Grapes of Wrath” draws from the Biblical imagery in Revelation 14:19-20, symbolizing both suffering and eventual justice. In Steinbeck’s novel, grapes metaphorically represent the hardships of oppressed workers and their potential for unified resistance, akin to the acini collectively sustaining lung function.

• • • “A Raisin in the Sun” by Lorraine Hansberry takes its title from Langston Hughes’ poem “Harlem,” where a deferred dream is likened to a grape drying into a raisin. The imagery reflects lost potential and unfulfilled aspirations, drawing a parallel to damaged or non-functional acini within the lung.
  • Quotes from Famous People
    • Homer (circa 8th century BCE) (The Odyssey): “Wine can of their wits the wise beguile, make the sage frolic, and the serious smile.”
    • Plato (427–347 BCE): “No thing more excellent nor more valuable than wine was ever granted mankind by God.”
    • Benjamin Franklin (1706–1790): “Wine is constant proof that God loves us and loves to see us happy.”
    • Ernest Hemingway (1899–1961): “Wine is the most civilized thing in the world.”

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.)