9.1.2—Tissue distribution in gas exchange system

The gas-exchange system distributes different tissues along the airway-to-alveolus route. Conducting airways carry and protect the air stream; alveoli and their capillary networks form the specialised exchange region.

• Trachea: c-shaped cartilage rings keep the main airway open while allowing flex during breathing; ciliated epithelium and goblet cells help protect the route.
• Bronchi: the two main branches remain supported by cartilage and lined with ciliated epithelium; they divide the airflow toward the lungs.
• Bronchioles: narrower tubes lack cartilage but contain smooth muscle, so their diameter can regulate airflow before it reaches the alveoli.
• Alveoli: thin squamous epithelium forms the air-facing exchange surface, and many alveoli together provide a large total area.
• Capillary network: each alveolus is closely surrounded by capillaries, keeping blood contact with the exchange surface and providing a short route for gas movement.

This distribution separates two linked jobs: supported, cleaned conducting tubes deliver air, while thin alveolar walls and close capillary contact favour efficient exchange. Branching increases the available exchange surface; short diffusion distance and continuous blood contact help that surface remain effective. Detailed image recognition and plan-diagram construction belong to 4617 and 4618.

Do not place cartilage in bronchioles or treat the trachea, bronchi and bronchioles as gas-exchange surfaces. Do not treat “large surface area” alone as sufficient: location, wall thickness and capillary contact also matter.