Art of Small Airways

 

Small Airways
The diagram allows us to understand the the components and the position of the small airways starting in (a) which is a secondary lobule that is fed by a lobular bronchiole(lb) which enters into the secondary lobule and divides into terminal bronchioles (tb) which is the distal part of the conducting airways, and  at a diameter of 2mm or less .  It divides into the respiratory bronchiole (rb) a transitional airway which then advances into the alveolar ducts(ad) and alveolar sacs (as)   Diseases isolated to the small airways do not affect the alveoli and hence there is peripheral sparing Ashley Davidoff MD TheCommonVein.net lungs-0749
Overview of the Anatomy of the Lungs with a Focus on the Bronchioles
This image shows the division of the airways in the lungs classified as large airways and small airways.
A large airway is considered any airway larger than 2mm, and therefore includes all the airways involved with transport of air except for the terminal bronchiole. Included as seen in image a, are the trachea, mainstem bronchi, lobar bronchi segmental and subsegmental airways and the 3 subsequent divisions of subsegmental bronchi and bronchioles till the last transporting airway – the respiratory bronchiole which is usually about 2mm and is considered a small airway Image (a) shows the airways starting in the trachea and continuing to the mainstem bronchi, lobar bronchi, segmental bronchi, and subsegmental bronchi.
Image b shows the structures that make up the small airways starting with the terminal bronchiole (tb) followed by the respiratory bronchiole (rb) alveolar duct, (ad) and alveolar sacs (as)
Image (c) shows the histologic makeup of the large airways that include a pseudostratified ciliated columnar epithelium with mucus secreting goblet cells a muscular layer (red) and a prominent cartilage layer (white) In the larger bronchioles (d) the epithelium remains as a pseudostratified, ciliated, columnar epithelium with prominent muscular layer (red).  The columnar epithelium transitions to a stratified ciliated cuboidal epithelium by the terminal bronchiole s (f) both still with a muscular layer.  The respiratory epithelium transitions from a cuboidal epithelium to a squamous epithelium (f)  with alveoli and type I and II pneumocytes starting to branch (g) 
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Overview of the Anatomy of the Lungs
Image a shows the airways starting in the trachea and continuing to the mainstem bronchi, lobar bronchi, segmental bronchi, and subsegmental bronchi,. The subsegmental bronchi have 3 subsequent generations until the bronchiole is reached. The terminal bronchiole is the last of the transporting airways and is considered the most proximal small airway with a diameter of 2mm or less, and it gives rise to the respiratory bronchiole which is the feeding airway for the acinus . The acinus is the functional unit of the lung.
Image b is a 3D reconstruction of a CT scan showing the proximal airways from the trachea to the segmental airways.
Image c shows the structures that make up the acinus and the other parts of the small airways, starting with the respiratory bronchiole (rb) . The diagram in d, shows the detail of the small airways that participate in gas exchange, including the respiratory bronchiole, (rb) alveolar duct, (ad) and alveolar sac (as)
Image e shows the secondary lobule made from about 20-30 acini, arising from a single lobular bronchiole accompanied by a single pulmonary arteriole (pa).. Structure that surround and enclose the secondary lobule include the pulmonary venule, (red) lymphatics,(yellow) and a fibrous septum (pink).
Ashley Davidoff MD TheCommonVein.net
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Pneumonia
The collage provides a perspective of purulent accumulation in the small airways and the alveoli that results in consolidation. A process that increases the density of the lungs to a net “white” regional density will result in a consolidation and in this case when the fluid is infected it is labelled “pneumonia” The net result on CT is air bronchograms within the non aerated dense lung tissue.
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The collage provides a perspective of disease of the small airways and the alveoli that results in ground glass appearance on Xray. A process that increases the density of the lungs to a net “gray” regional density will result in a ground glass opacity whether it is inflammation of the walls ((second column) fluid within the lumen of the small air ways and alveoli (3rd column) or whether it is fibrosis in the walls of the small airways or alveolar septa (last column alveoli. The net result on CT is a ground glass opacity (bottom row). In fibrosis there are secondary changes which include bronchiolectasis in this case, but other associated changes may include reticulations or centrilobular nodules
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Advancing Acute Eosinophilic Pneumonia
As the disease advances the small airways, and alveoli, get progressively filled with eosinophils, inflammatory cells and fluids resulting in consolidation.  This image reveals progressive filling of the small airways, (a) alveoli, (b) and secondary lobules (c) with eosinophils and products of inflammation resulting in multi-segmental consolidations (d), in the  lung bases, with air bronchograms at the right base (e), and less dense consolidation at the left base (f) 
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Alveolar and Interalveolar Interstitial Infiltration with Eosinophils and Inflammatory Exudate – Ground Glass Changes
The ground glass changes are a combination of the cellular and exudative inflammatory response in the small airways, alveoli, interalveolar septa and interstitium, and thickened alveolar septum
The diagram shows the abnormal secondary lobule (a) The involved components include the small airways(b) alveoli and interalveolar interstitium (c) and the thickened interlobular septum (d) surrounding the secondary lobule due to an inflammatory process, cellular infiltrate and congestion of the venules and lymphatics in the septum.  An anatomic specimen of a secondary lobule from a patient with thickened interlobular septa and interstitial thickening is shown in image e, and is overlaid in red (f) . A magnified view of an axial  CT of the lungs in a patient with acute eosinophillic pneumonia shows thickened interlobular septa and centrilobular nodules (g) The inflammatory changes in the aforementioned structures result in an overall increase in density of the lung manifesting as ground glass changes (g) and overlaid in red (h) 
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Small Airways Infiltration with Eosinophils and Inflammatory Exudate – Centrilobular Nodules
The diagram shows the small airways of the lung including the respiratory bronchiole, alveolar ducts and alveolar sacs in coronal (a) and in cross section (b) and correlated with an anatomic specimen of a secondary lobule that contains a thickened interlobular septum .  The respiratory bronchiole is overlaid in maroon (d), next to the arteriole.  Images e and f are magnified views of a CT of the lungs in a patient with acute eosinophillic pneumonia and the centrilobular nodules reflecting small airway disease are highlighted in f.
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A collage shows the normal small airway(a) alveoli (b) and secondary lobule (c) and the changes in the airways in acute eosinophillic pneumonia.  There is filling of the the small airways(d) alveoli (e) are filled with inflammatory changes in the interalveolar septa (e) and thickening of the interlobular septa (f) The CT findings include consolidation at the lung bases (g)with thickening of the interlobular septa, centrilobular nodules,  and ground glass opacity (g)
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A collage shows the normal small airway(a) alveoli (b) and normal CT 9c) and small airways infiltrated with eosinophils, (d) the alveoli infiltrated with eosinophils(e) and the radiological findings with aiirbronchograms within a consolidation (f) and thickening of the interlobular septa, centrilobular nodules and ground glass opacity (g)
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Small airways with infiltration of eosinophils
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Small Airway Fibrosis and Luminal Narrowing or Obstruction 
The diagram shows fibrotic changes around and within the small airways including the terminal bronchiole, respiratory bronchiole and the alveolar duct.  In this instance the increase density from the fibrotic tissue would result in ground glass changes in the parenchyma and solid centrilobular nodules.  Obstruction of the small airways  would result in air trapping.
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Small Airway Fibrosis
The diagram shows fibrotic changes around the small airways including the terminal bronchiole, respiratory bronchiole and the alveolar duct.  In this instance the increase density from the fibrotic tissue would result in ground glass changes in the parenchyma and ground glass centrilobular nodules.  Since the airways are patent there would be no air trapping.
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Hypersensitivity Pneumonitis is a disease that most commonly affects the midlung field Next in frequency are the upper lung field and lastly diffuse involvement. Anatomically the small airways and the alveoli are affected with inflammation and granulomas are present
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Inflammatory response in the small airways attracts cellular interstitial infiltrates which surround the bronchiole. The diagram shows a bronchiole surrounded by an acute cellular inflammatory response
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