The human lung comprises 24 generations of dichotomously branching tubes known as bronchi [1]. Functionally, these generations can be categorized as: (1) conducting airways which are non-alveolated and comprise the first 16 generations, and (2) the acini which consist of flexible, alveolated airways and are responsible for gas exchange. The alveoli are the most important units of the human respiratory system and provide large surface area (about 70–80 m2) for efficient gas exchange; oxygen diffuses into the blood through the alveolar epithelium, whereas carbon dioxide diffuses in the opposite direction from the blood to the lung.

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