Postnatal growth and size of the pulmonary acinus and secondary lobule in man

The acinus and the secondary lobule are important anatomic, functional, and pathologic units in the human lung; there are, however, few studies that specifically describe their postnatal growth and size. Normal lungs from 15 children and adults dying without evidence of pulmonary disease were fixed and dried using a standardized technique and were radiographed after tantalum or silver nitrate bronchoacinography. The acini and secondary lobules were identified and the diameters of these terminal respiratory units determined. The mean diameter of the pulmonary acinus at 1 month was 1 mm; 1 year, 2.5 mm; and at 2 years, 3 mm. The mean acinar diameter had increased to 6 mm at 12 years and reached a normal adult range of 6-10 mm during late adolescence. Acinar size seemed to be related to both height and age. The mean diameter of the secondary lobule at birth measured 3 mm, increased to 5 mm by the end of the first year of life, and by the fourth year measured 9 mm; at 12 years it measured 15 mm, and in late adolescence and adult life it ranged between 13 and 20 mm. A knowledge of the size of the acinus and secondary lobule has a practical application in radiologic practice and will be important in regional structural analysis of the pulmonary parenchyma with computed tomography.