CB8.1 — Gas exchange surfaces (Edexcel 1SC0)

Features of efficient gas exchange surfaces

All gas exchange surfaces share these adaptations:

1. Large surface area — more space for diffusion.
2. Thin — short diffusion distance.
3. Moist — gases dissolve before diffusing across.
4. Good blood supply — maintains concentration gradient.
5. Ventilation — maintains concentration gradient of gases.

Alveoli (lungs)

• Human lungs contain ~700 million alveoli → huge surface area (~70 m²).
• Walls are one cell thick (squamous epithelium) → short diffusion distance.
• Dense capillary network → close contact with blood.
• Oxygen diffuses from alveolus (high O₂) into blood; CO₂ diffuses from blood (high CO₂) into alveolus.

Fish gills

• Gill filaments have lamellae → very large surface area.
• Counter-current flow: blood flows through lamellae in the opposite direction to water flowing over gills → maintains a concentration gradient for O₂ along the entire length → efficient extraction of O₂ from water.

Surface area to volume ratio calculation

(See CB1.3.) As organisms get larger, SA:V decreases → need specialised exchange surfaces (lungs, gills, villi).