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The secondary pulmonary lobule (SPL) is the smallest functional and anatomical unit of the lung that is bounded by connective tissue septa and supplied by a single terminal bronchiole and its associated pulmonary artery branch.

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It is the key structural unit in interpreting high-resolution CT (HRCT) images, especially in interstitial and small airway diseases.

Anatomical structure

Component Description
Size Roughly 1–2.5 cm in diameter
Shape Polygonal or hexagonal in cross-section
Boundaries Surrounded by interlobular septa (fibrous septa containing pulmonary veins and lymphatics)
Central structure Centrilobular bronchiole and pulmonary artery (enter through lobular core)
Peripheral structures Pulmonary veins and lymphatics within septa
Acini Each SPL contains 3–5 acini (the site of gas exchange)

![Secondary pulmonary lobule (A: Centrilobular arteries and bronchioles with a diameter of approximately 1 mm; B: Interlobular septa with a thickness of approximately 0.1 mm; C: Pulmonary vein and lymphatic branch with diameters of 0.5 mm each; D: Acinus—never visible on CT scans)

Mueller-Mang, C., Ringl, H., Herold, C. (2017). Interstitial Lung Diseases. In: Nikolaou, K., Bamberg, F., Laghi, A., Rubin, G.D. (eds) Multislice CT. Medical Radiology(). Springer, Cham. https://doi.org/10.1007/174_2017_151](attachment:096127c7-b45e-48fc-9836-2b271b258c2d:63847_4_En_151_Fig1_HTML.webp)

Secondary pulmonary lobule (A: Centrilobular arteries and bronchioles with a diameter of approximately 1 mm; B: Interlobular septa with a thickness of approximately 0.1 mm; C: Pulmonary vein and lymphatic branch with diameters of 0.5 mm each; D: Acinus—never visible on CT scans)

Mueller-Mang, C., Ringl, H., Herold, C. (2017). Interstitial Lung Diseases. In: Nikolaou, K., Bamberg, F., Laghi, A., Rubin, G.D. (eds) Multislice CT. Medical Radiology(). Springer, Cham. https://doi.org/10.1007/174_2017_151

![Diagram shows anatomy and dimensions of secondary lobule and pulmonary acinus. Two secondary pulmonary lobules in the lung periphery are illustrated, with approximate dimensions of their components indicated.

Webb WR. Thin-Section CT of the Secondary Pulmonary Lobule: Anatomy and the Image—The 2004 Fleischner Lecture. Radiology. 2006;239(2):322-338. doi:https://doi.org/10.1148/radiol.2392041968](attachment:8e4cf53e-5193-4747-a6b3-111a50200171:image.png)

Diagram shows anatomy and dimensions of secondary lobule and pulmonary acinus. Two secondary pulmonary lobules in the lung periphery are illustrated, with approximate dimensions of their components indicated.

Webb WR. Thin-Section CT of the Secondary Pulmonary Lobule: Anatomy and the Image—The 2004 Fleischner Lecture. Radiology. 2006;239(2):322-338. doi:https://doi.org/10.1148/radiol.2392041968

Radiology

Understanding SPL anatomy is crucial for pattern recognition in HRCT, especially for:

HRCT appearance:

Structure Imaging Appearance (HRCT)
Interlobular septa Seen as thin linear opacities outlining lobules (best in periphery)
Centrilobular artery/bronchiole Typically not visible unless diseased
Lobular core Site of centrilobular nodules, tree-in-bud, or vascular markings
Lobular margins Best appreciated in subpleural regions (adjacent to fissures/pleura)

Nodular Patterns:

Pattern Distribution Examples
Centrilobular nodules Around lobular core, sparing septa ‣ , TB, RB-ILD
Perilymphatic nodules Along interlobular septa and subpleura Sarcoidosis, lymphangitic carcinomatosis
Random nodules Scattered throughout lobule Hematogenous metastases, miliary TB

Interstitial thickening:

Thickening Type Associated Diseases
Interlobular septal thickening Pulmonary edema, lymphangitic carcinomatosis
Intralobular lines Fibrosis, UIP/NSIP
Crazy paving pattern Septal + ground-glass opacities: alveolar proteinosis, PCP, ARDS

Airspace & Airway Disease