Inspiration

Volume of thoracic cavity increases due to

• contraction of diaphragm
• intercostal muscle -> raises ribs

=> Intrathoracic pressure drops

=> Air flows in, to about terminal bronchiole

=> Past terminal bronioles, diffusion of gas becomes dominant mechanism for ventilation in the respiratory zone.

At terminal bronchioles, cross-sectional area increase and velocity of airflow greatly reduced

=> dust settles at the terminal bronchioles