The secondary pulmonary lobule, also known the pulmonary lobule, is considered the functional unit of the lung, and is key to HRCT terminology.
The terminology used to describe the fundamental gas-exchange units of the lung can be confusing. The inconsistent descriptions in part reflect historical change and varying context (imaging versus pathology vs bronchoscopy). See the History and etymology section for more.
Although a primary pulmonary lobule has been defined, it serves little utility - the secondary pulmonary lobule is more useful, since there are clear imaging and histopathology correlates. For all intents and purposes, the terms "secondary pulmonary lobule", "primary pulmonary lobule", and "pulmonary lobule" are used synonymously 5.
Secondary pulmonary lobules represent a cluster of up to 30 acini 9 supplied by a common distal pulmonary artery and bronchiole 4-6. These clustered acini are bounded by interstitial fibrous septa (interlobular septa) which outline an irregular polyhedron of varying size between 1 and 2.5 cm 5. Peripheral lobules are larger and cuboidal/pyramidal, while central lobules tend to be smaller and hexagonal 6.
Each lobule has been conceptually divided into three components 6:
- interlobular septal structures
- centrilobular structures
- lobular parenchyma
- continuous with other interstitial components of the lung 6:
- subpleural peripheral interstitium (peripheral connective tissue)
- peribronchovascular interstitium (axial connective tissue)
- septa thicker and more numerous along the lung periphery, normally measuring ~0.1 mm in subpleural regions 6
- contain the distal pulmonary veins and lymphatics - draining structures
- "centrilobular" refers to the central aspect of the lobule - the approximate location of the feeding pulmonary artery, bronchiolar branches, afferent lymphatics, and surrounding (peribronchovascular) interstitium 6
- no consistent level/type of bronchiole or artery that supply each lobule
- artery and bronchioles tend to branch in unbalanced dichotomous fashion, i.e. into one dominant and one diminuitive branch
- lobular bronchiole 6
- the supplying bronchiole ("lobular bronchiole") represents a preterminal bronchiole, measuring ~1 mm
- the airway progressively branches into terminal and respiratory bronchioles within each lobule
- on CT, bronchiole visibility depending on its wall thickness:
- lobular bronchioles may be visible, although at lower limit of CT resolution
- terminal and respiratory bronchioles usually not visible (<0.1 mm thick)
- lobular pulmonary artery 6
- supplying artery measures ~1 mm, appears as a central dot within the lobule
- arterial branches appear as linear branching structures down to 0.2 mm in diameter, located as close as 3 mm from pleural surfaces
- peribronchovascular interstitium 6
- connective tissue sheath surrounding the afferent structures of the lobule
- refers to the elements located between outer septa and the centrilobular structures: the pulmonary acini and intralobular septa ("septal fibers") which contain the pulmonary capillary bed 6
- pulmonary acinus 6,7
- each lobule contains 3-30 acini 6,9
- each acinus comprises between 1,500-4,000 alveoli
- intralobular septa 6
- fine stromal tissue surrounding the alveoli which contains the pulmonary capillary bed
- not visible by CT in healthy tissue
There are two sets of lymphatics:
- central: runs with the arteries in the peribronchovascular interstitium
- peripheral: in the interlobular septa and drains to the subpleural plexus
History and etymology
Understanding of pulmonary anatomy has evolved considerably, and many concepts and terms have been updated or adapted over time.
The pulmonary lobule, with many smaller subcompartments, was first described by Thomas Willis in the 17th century 6,7. George Rindfleisch (1817-1905) later redefined the pulmonary lobule as a septa-lined structure supplied by a pulmonary bronchiole, with branching alveolar ducts forming sublobular "acini" 6,7.
Subsequently Rudolph Kolliker (1817-1905) showed that small airways exist as a continuum from the purely conductive terminal bronchiole, to partially-conductive respiratory bronchiole (both bronchiolar epithelial and alveolar walls), to alveolar ducts leading to alveoli 6,9. Thus, Rindfleisch's definition needed refinement.
The classic description of pulmonary lobules was proposed by William Snow Miller (1858-1939) 6,10. He considered the "primary lobule" to be the fundamental unit of the lung, defined as the structures distal to the respiratory bronchiole i.e. those exclusively devoted to gas-exchange. By contrast, Miller described the "secondary pulmonary lobule" as higher level structures outlined by interstitial septa, similar to Rindfleisch.
The concept of the secondary pulmonary lobule has been popularized due to its clear correlation with pathology and CT findings, as described by Heitzman et al 5.
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