Gas Exchange & Transport

AQA Specification

Surface area to volume ratio & exchange surfaces

Surface Area : Volume Ratio Calculator

As organisms increase in size, their SA:V ratio decreases — making simple diffusion insufficient. Explore this relationship below.

SA:V ratio decreases as size increases

Surface Area
6
cm²
Volume
1
cm³
SA : V Ratio
6:1

Principles of Exchange

Fick's Law: Rate of diffusion ∝ (SA × ΔConcentration) / Thickness

Large surface area

More area = more molecules can cross simultaneously. Achieved by folding (villi, alveoli, cristae).

Thin barrier

Short diffusion distance = faster exchange. Alveolar walls are just one cell thick (~0.5μm).

Concentration gradient

Maintained by blood flow (perfusion) and ventilation. Steeper gradient = faster diffusion.

Moist surface

Gases dissolve in moisture before crossing membranes. Mucus and surfactant maintain this.

Exchange Surfaces

AQA exam tip: When asked about adaptations of exchange surfaces, always link back to Fick's Law. Structure your answer: name the adaptation → explain how it increases SA, reduces thickness, or maintains the gradient → state the effect on rate of diffusion.