9.1.5—Airway secretory and ciliated cells

Gas exchange occurs at the alveolus–capillary interface by simple diffusion. The direction of each gas is set by its partial-pressure gradient: oxygen moves from alveolar air into the blood, while carbon dioxide moves from blood into the alveolar air.

1. Oxygen gradient: air in the alveoli has a higher oxygen partial pressure than arriving deoxygenated blood, so oxygen diffuses across the moist alveolar surface into the capillary.
2. Carbon-dioxide gradient: blood arriving at the alveolar capillary has a higher carbon-dioxide partial pressure than the alveolar air, so carbon dioxide diffuses in the opposite direction and is exhaled.
3. Short exchange path: thin squamous alveolar epithelium and the closely adjacent capillary wall make the diffusion barrier short, helping gases cross efficiently.
4. Large exchange area: many alveoli provide a large total surface across which oxygen and carbon dioxide can move.
5. Gradient maintenance: ventilation refreshes alveolar air and blood flow carries oxygenated blood away while bringing more deoxygenated blood, helping preserve the gradients.

This is passive diffusion, not ATP-driven transport: gas moves down its partial-pressure gradient across the thin, moist exchange surface. A large area or strong blood supply helps only when the alveolar barrier and gradients remain effective. The card does not introduce a quantitative diffusion equation or later clinical mechanisms; airway mucus/cilia protection is a separate function.