Lung structure
300 million alveoli, 1/3mm in diameter => 50-100 m2 surface area
Z0 - Trachea
Z1 - Bronchi
Z4 - Bronchioles
Z16 - Terminal bronchioles
Z17-19 - Respiratory bronchioles
Z20-22 - Alveolar ducts
Z23 - Alveolar sac
Z0 to Z16 - Conducting airway
Z17 to Z23 - Transitional and respiratory zone
Alveolar are polyhedral, not spherical, and not all surface areas are available for gas exchnge.
=> Total surface area can only be estimated.