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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
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2023
Most common paperPaper3
Typical marks3

Common command terms

  • Explain
  • Describe

Scoring notes

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

Recent exam appearances

November 2023Paper3 ["HL"] · TZ225(b)[ 3 ]B3.1.3—Maintaining concentration gradients
November 2023Paper3 ["HL"] · TZ125(b)[ 3 ]B3.1.3—Maintaining concentration gradients
Practice this objective

Coverage 2023–2023 · Updated 15 Jul 2026

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.
ConceptIB Biology HL