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B3.1.2—Properties of gas-exchange surfaces

Efficient gas-exchange surfaces combine large area, thin permeable walls, moisture and maintained gradients so gases diffuse rapidly between air and blood.

Syllabus
First assessment 2025
Objective
B3.1.2
Level
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–3

Common command terms

  • State
  • Describe
  • Explain

Scoring notes

Common mistake
Listing alveolar features without linking each feature to faster diffusion.

Recent exam appearances

May 2025Paper1A ["SL"] · TZ213[ 1 ]B3.1.2—Properties of gas-exchange surfaces
May 2021Paper2 ["SL"] · TZ17(b)[ 7 ]B3.1.2—Properties of gas-exchange surfaces
May 2019Paper2 ["SL"] · TZ14(b)[ 3 ]B3.1.2—Properties of gas-exchange surfaces
November 2012Paper2 ["SL"] · TZ01(c)[ 1 ]B3.1.2—Properties of gas-exchange surfaces
May 2010Paper1 ["SL"] · TZ227[ 1 ]B3.1.2—Properties of gas-exchange surfaces
Practice this objective

Coverage 2010–2025 · Updated 15 Jul 2026

Concept essentials

  • Large surface area increases the amount of gas that can diffuse at once.
  • Thin exchange walls reduce the distance oxygen and carbon dioxide must cross.
  • Moist surfaces allow respiratory gases to dissolve before diffusion.
  • Ventilation and blood flow maintain steep concentration gradients across alveoli.
ConceptIB Biology SL