In expiration air first moves out of the large, conducting airways followed by air from progressively smaller airways.

Oxygen and carbon dioxide move in and out of respiratory bronchioles and alveoli by gaseous diffusion, not by airflow.

• The figure on the right illustrates the anatomy of the intrapulmonary airways and the gas exchanging units of the lung. Gas exchange is not evaluated by spirometry, but by other tests such as measurement of carbon monoxide gas transfer, blood gas analysis or stage II exercise testing.

• Flow in small airways is laminar, whereas in large airways it is turbulent.
• Flow encounters resistance.
• Poiseuille's Law: Resistance to laminar is proportional to length and inversely proportional to the fourth power of the radius of a tube .

• The figure illustrates the relationship of total cross-sectional area of the lung and airway generation. The structure maximises the cross-sectional area available for gas exchange.
• The total cross-sectional area of airway generations 10 - 15 is far greater than that of the larger proximal airways.
• Since resistance to flow is inversely proportional to the fourth power of the radius of the airway, it follows that most resistance to airflow is encountered in the small conducting airways.