Concentration gradients for respiratory gases are maintained by ventilation, blood flow and changing oxygen demand during activity or altitude exposure.
Coverage 2014–2024 · Updated 15 Jul 2026
Net gas diffusion continues when ventilation, blood flow or photosynthetic uptake continually removes gas from one side of the surface.
If both sides approach the same concentration, random movement continues but net transfer falls. Renewal of air or fluid restores the difference that drives net movement.
Trace the chain: gas enters; a gradient drives diffusion; ventilation, circulation or metabolism renews the gradient.
Fresh air reaching an alveolus keeps oxygen higher in the air than in incoming blood, sustaining oxygen uptake.
A concentration gradient is not maintained by diffusion itself; another flow or reaction must renew it.
This objective is assessed through structured response, commonly using Explain / Describe.
Explain / Describe
Build the answer around this relationship: Ventilation refreshes alveolar air so oxygen remains high and carbon dioxide remains low.
Referring only to oxygen concentration at altitude instead of oxygen partial pressure.
Representative question
Describe how a concentration gradient of oxygen is maintained between the lungs and blood capillaries.
Core gas-exchange answers link exchange surfaces to diffusion gradients. For animals, exchange surfaces are explained by diffusion properties, ventilation, and blood flow. For plants, leaves allow carbon dioxide entry and oxygen/water vapour exit while controlling water loss through stomata. Spirometry, transpiration, and stomatal density data provide evidence of gradient and surface-area effects.
