Concentration gradients for respiratory gases are maintained by ventilation, blood flow and changing oxygen demand during activity or altitude exposure.
Coverage 2023–2023 · Updated 15 Jul 2026
Animal gas-exchange surfaces maintain steep concentration gradients by continuously renewing the external medium and transporting gases away or toward the internal side.
Dense networks of blood vessels give a large contact area, and continuous blood flow brings oxygen-poor blood to the surface while carrying oxygenated blood away. This prevents the two sides from approaching equilibrium.
Ventilation renews air over lung surfaces and water over gill surfaces. Together, ventilation and circulation keep oxygen higher in the environment than in incoming blood and carbon dioxide higher in incoming blood than in the environment.
Fresh alveolar air has a higher oxygen partial pressure than venous blood arriving in adjacent capillaries; continuous airflow and blood flow preserve this difference while oxygen diffuses.
Diffusion follows a gradient but does not maintain it. Without ventilation and continuous flow, net exchange slows as concentrations approach equilibrium.
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.
