Primary pulmonary lobule
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Primary pulmonary lobule is not a term used with any frequency in HRCT, but is the reason the secondary pulmonary lobule has its name.
A primary pulmonary lobule is defined as the lung unit distal to the respiratory bronchioles. It is significantly smaller than an acinus, and is composed of alveolar ducts, alveolar sacs and alveoli.
It has been estimated that each secondary pulmonary lobule is composed of 30-50 primary lobules.