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The terminal bronchioles are a continuation of the bronchi and are the last divisions of the conducting airways.
Terminal bronchioles are confusingly named, as they are not the final branches but rather the distal bronchioles that do not bear alveoli. The first 19 divisions from the main bronchi are conducting airways up until the final terminal bronchioles. Terminal bronchioles often are reached by the 5th or 6th division from the main bronchi 1. Each terminal bronchiole and its branches constitutes a lung acinus. Terminal bronchioles become respiratory bronchioles when cilia start to disappear and alveoli start to bud off.
Terminal bronchioles have a diameter of <1 mm. They do not contain cartilage in their walls like bronchi. The epithelium is comprised of simple columnar ciliated cells 2.