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B3.1.3—Maintaining concentration gradients

Concentration gradients for respiratory gases are maintained by ventilation, blood flow and changing oxygen demand during activity or altitude exposure.

Syllabus
First assessment 2025
Objective
B3.1.3
Level
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper3
Typical marks2–4

Common command terms

  • Explain
  • Describe

Scoring notes

Common mistake
Referring only to oxygen concentration at altitude instead of oxygen partial pressure.

Recent exam appearances

May 2024Paper2 ["SL"] · TZ16(b)[ 4 ]B3.1.3—Maintaining concentration gradients
November 2018Paper3 ["SL"] · TZ02(b)[ 2 ]B3.1.3—Maintaining concentration gradients
November 2014Paper3 ["SL"] · TZ06(a)[ 2 ]B3.1.3—Maintaining concentration gradients
May 2014Paper3 ["SL"] · TZ26(b)[ 3 ]B3.1.3—Maintaining concentration gradients

Coverage 2014–2024 · Updated 15 Jul 2026

Ventilation and Blood Flow Preserve Opposite Gradients

At an alveolus, ventilation replaces air while perfusion replaces blood. Together they keep alveolar O₂ above venous-blood O₂ and venous-blood CO₂ above alveolar CO₂.

alveolar air high O₂ → O₂ diffuses into blood → haemoglobin binding removes dissolved O₂
venous blood high CO₂ → CO₂ diffuses into alveolus → expiration removes it

Reduced ventilation lowers alveolar O₂; reduced perfusion leaves no blood to load. Either mismatch lowers whole-system exchange even if the membrane is intact.

SL Summary: One Diffusion Model

Feature Lung Leaf
area many alveoli broad blade + mesophyll walls
path thin barrier thin leaf + air spaces
gradient air + blood flow CO₂ use + stomata

Spirometry measures ventilation; a potometer estimates water uptake; a leaf cast measures stomatal density. Match each method to its direct measurement.

Maintaining concentration gradients

[ 4 ]

Describe how a concentration gradient of oxygen is maintained between the lungs and blood capillaries.

Concept essentials

  • Ventilation refreshes alveolar air so oxygen remains high and carbon dioxide remains low.
  • Blood flow maintains gradients by bringing deoxygenated blood to alveoli and removing oxygenated blood.
  • Exercise increases ventilation because cellular respiration uses more oxygen and produces more carbon dioxide.
  • High altitude lowers oxygen partial pressure and can reduce haemoglobin saturation.