terminal bronchiole

The terminal bronchiole is the very last stretch of pure plumbing in the lung — the final airway that only conducts air, just before the part of the lung where oxygen actually crosses into the blood begins. It is the end of the conducting zone.

Each terminal bronchiole gives rise to the respiratory bronchioles beyond it, which start to bear alveoli. Everything from the trachea down to the terminal bronchiole forms the conducting airways: this air-filled tubing carries breath but takes no part in gas exchange, and so makes up part of the anatomical dead space.

The lung unit fed by a single terminal bronchiole — together with all the gas-exchanging structures beyond it — is called the acinus, the basic functional building block of the lung. Damage focused at this level is characteristic of certain forms of emphysema.