Structural Pathway of Air in the Lungs

  • The lung contains over 300,000 branching airways,
    • largest – trachea and
    • smallest, -respiratory  a diameter of 0.5 millimeters
  •  airways (trachea to the respiratory bronchiole)
    • average of 23 airways generations (
  • bronchi are distinguished from bronchioles
    • presence of cartilage
  • conducting branches
    • bronchi and bronchioles, to terminal bronchiole,
  • respiratory bronchioles have
    • alveoli
      • transport,
      • function in respiration (
  • airways – two zones that reflect the differences in
    airway resistance and flow. T

    •  large airways, with diameters greater than 2 mm
      • 75% of the airway resistance
    • small airways, with diameters of less than 2 mm,
      • large number of small airways,
      • overall cross-sectional area is > large airways.
        • 25% of the airway resistance,
  • High-resolution CT (1 .0- 1 .5-mm collimation)
    • allows identification of
      • airways 1-2 mm in diameter and
      • vessels 0. 1 -0.2 mm in diameter
    • The secondary pulmonary lobule
      • supplied by three to five terminal
        bronchioles (aka Reid lobule)
      • terminal bronchiole and artery supplying the lobule are
        • center
        • intralobularbronchioles cannot be identified because the
          thickness of their walls is less than 0. 1 5 mm.


Mainstem Large and Medium Sized Bronchi

Diamters of the bronchi
This diagram of the airways reveals the approximate diameters of the airways. Note that the left main stem bronchus is thinner than right whose length is truncated by the take off of the right upper lobe bronchus.
Ashley Davidoff
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


Secondary Lobule

Contains up to 30 acini

Secondary Lung Lobule
Normal lung histology
This image is a panoramic view of the lung showing secondary lobules and interlobular septa. Within the interalveolar septae, one sees small venules and lymphatics.Courtesy Armando Fraire MD. 32649b
code lung pulmonary alveoli alveolus secondary lobule interlobular septa vein lymphatic histology
interstitium interstitial
The Secondary Lobule
The secondary lobule is housed in a connective tissue framework in which run the lymphatic and venular tributaries . Together these 3 structures form the interlobular septum.
The lobar arteriole enters the framework, accompanied by the lobar bronchiole, and they all run together and form the interlobular septa. This structure measures between .5cms and 2cms and is visible on CT scan.
It is important in clinical radiology since many of the structures can be identified in health, and more particularly in disease, enabling the identification and characterization of many pathological processes.
Courtesy Ashley Davidoff MD
lungs-0036-low res
Histology of the Large and Medium Sized Airways
Airways are lined by a pseudostratified ciliated columnar epithelium interspersed with mucus secreting goblet cells
Ashley Davidoff
As the medium sized airways progress to to the small airways they lose many of the goblet cells, become a simple epithelium and remain ciliated
Ashley Davidoff
As the terminal bronchiole transitions to the respiratory bronchiole the mucosa becomes non ciliated and cuboidal
Ashley Davidoff
At the level of the membranous airways (respiratory bronchiole, alveolar duct, alveolar sac and alveoli, the mucosa becomes mostly a simple squamous epithelium
Ashley Davidoff
The cellular composition of the upper and lower respiratory tracts that serves as the primary barrier. Epithelial cells (ECs) that span the entire length of the respiratory tract (RT) are lined with basal cells that are attached to the basement membrane. Squamous ECs make up the beginning (nasal) and ends (alveoli) of the RT, ciliated and non-ciliated columnar epithelia makeup the upper RT and the large bronchi, while cuboidal epithelia line the small bronchi and bronchioles. Surface liquid that overlays the ECs consists of mucus secreted from mucus producing cells, airway liquids secreted from secretory cells, neutralizing immunoglobulins, and antimicrobials. Resident leukocytes such as dendritic cells, γδ T cells, and innate lymphoid cells line the mucosa while alveolar macrophages are found in the lower airways and alveoli. The bronchial smooth muscle cells underlying the RT from the basal end provide structural support and elasticity to the airways.
LeMessurier K.S et al Respiratory Barrier as a Safeguard and Regulator of Defense Against Influenza A Virus and Streptococcus pneumoniae Front. Immunol., 04 February 2020
The Alveolus –
The Buck Ends Here
The alveolus is lined by a simple epithelium – one cell layer thick. There are two types of lining cells; Type 1 pneumocytes are squamous cells that cover 90% of the surface of the inner lining of the lung , and type II cuboidal pneumocytes that are in fact much more numerous than Type I. They are involved in the production of surfactant . In the lumen there are resident macrophages which play a crucial role in the immune system. The mucosa is grounded by a basement membrane and a lamina propria, and connected to the lamina propria and basement membrane of the surrounding capillary. The alveolus is lined by a thin layer of surfactant. (teal blue)
Ashley Davidoff


References and Links