9.1.1—Structure of human gas exchange system

The human gas-exchange route is a branching conducting system that delivers inhaled air to alveoli. Air enters through the nose or mouth, travels through the trachea, bronchi and bronchioles, and reaches the alveoli, where the conducting route ends and the exchange surface begins.

1. Nose or mouth → trachea: air enters the body and passes into the main airway.
2. Trachea → bronchi: the trachea divides into the left and right bronchi, directing air toward each lung.
3. Bronchi → bronchioles: each bronchus branches repeatedly into narrower bronchioles.
4. Bronchioles → alveoli: bronchioles carry air to many alveoli, the small sacs that provide the gas-exchange surface.
5. Exhalation reverses the route: air leaves the alveoli, returns through bronchioles → bronchi → trachea, and exits through the nose or mouth.

Trachea, bronchi and bronchioles are conducting airways: they route air toward and away from the lungs. Alveoli are the exchange surface and are closely associated with capillaries. This card establishes the route only; detailed diffusion across the alveolar barrier belongs to 4621, and airway mucus/cilia functions belong to 4619.