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The pulmonary interstitium can be considered as an interfused tissue, which consists of a supportive connective tissue network of varying density, lymphatic vessels, blood vessels (with capillary microstructure) and tissue fluid. To understand the arrangement of the pulmonary interstitium, anatomical knowledge of the secondary pulmonary lobule is necessary. The secondary pulmonary lobule is the basic functional unit of the lung tissue, which is peripherally bound by a connective tissue septa containing lymphatics and small pulmonary veins. It is ventilated by a secondary pulmonary bronchiole and blood supply is provided by a secondary pulmonary arteriole, which together enters the lobule centrally and then branch out together. On cross-sectional imaging, the secondary pulmonary lobule has a polygonal shape, and its size usually ranges from 10 mm to 25 mm. Each lobule contains further subunits called pulmonary acini, which are supplied by the terminal bronchioles and arterioles, ventilated by the respiratory bronchiole, forming the primary pulmonary lobules.