Anatomy
The airways of the lungs consist of the cartilaginous bronchi, membranous bronchi, and gas-exchanging bronchi termed the respiratory bronchioles and alveolar ducts. While the first 2 types function mostly as anatomic dead space, they also contribute to airway resistance. The smallest non-gas-exchanging airways, the terminal bronchioles, are approximately 0.5 mm in diameter; airways are considered small if they are less than 2 mm in diameter.[7]