9.1.6—Airway and alveolar tissue functions

The gas-exchange system assigns different jobs to conducting airway tissues and alveolar exchange tissues. Each feature is useful because its structure fits a need: protect and control the air route, or provide a large, thin and well-supplied exchange surface.

• Ciliated epithelium + goblet cells + mucous glands → airway protection: mucus traps dust and microorganisms, while cilia move the mucus away from the lungs toward the mouth.
• Cartilage in the trachea and bronchi → supported airflow: strong but flexible rings/hoops help keep larger airways open while allowing movement during breathing.
• Smooth muscle in trachea, bronchi and bronchioles → airflow control: changing muscle tone changes airway diameter, helping regulate how much air enters or leaves.
• Elastic fibres in alveolar walls → filling and recoil: alveoli can stretch as they fill and recoil to help expel air, supporting the extensive exchange surface.
• Thin squamous epithelium in alveoli → short diffusion path: flattened, permeable cells form the exchange lining so gases can cross readily.
• Alveolar capillary network → exchange support: close blood contact allows gases to be carried to or from the exchange surface; it complements the thin alveolar lining rather than replacing it.

Cilia and mucus protect conducting airways; they are not the alveolar exchange barrier. Cartilage supports larger airways but is not a bronchiole feature. “Thin” explains the alveolar exchange surface, whereas “ciliated” explains airway clearance. This card synthesises feature→function relationships; detailed route, recognition and diffusion sequences remain in 4615–4619 and 4621.