The secondary lobule is the basic functional unit of the lung. Understanding its anatomy is key to interpreting the imaging results of interstitial lung diseases (ILDs) and is based on the type of secondary-lobule involvement.
Secondary pulmonary lobules measure between 1 and 2.5 cm in diameter. Each lobule is made up of up to a dozen acini and 30-50 primary pulmonary lobules. Each secondary lobule is supplied by a terminal bronchiole and a pulmonary artery branch and surrounded by a lobular septum within which pulmonary veins and lymphatics are located. These surrounding septa are very thin and only a few of them will be seen under healthy conditions.6
The lymphatic system is comprised of two parts:6