Lungs vs. Gills: Comparative Gas Exchange Efficiency in Air and Water

Lungs – Specialized for Air

• Large internal surface area (alveoli) + thin barrier → efficient O₂ uptake from air (21% O₂, high diffusion coefficient).
• Ventilation by tidal breathing; dead space limits efficiency but high O₂ gradient compensates.
• Ineffective in water: water's O₂ content ~30× lower, diffusion coefficient ~10,000× lower → cannot meet metabolic demand.

Gills – Specialized for Water

• Countercurrent exchange (blood flows opposite to water) → maintains gradient along entire filament length.
• Highly vascularized, thin filaments/lamellae → massive surface area/volume.
• Efficient extraction (up to 80% O₂ removal from water).
• Collapse in air → surface tension forces filaments together, eliminating surface area; no ventilation mechanism.

Evolutionary Match

Each system is exquisitely adapted to its medium. No respiratory organ is universally "better"; efficiency is context-dependent. Some amphibians and fish (lungfish, bichirs) have both, using lungs in hypoxic water/air and gills in normoxic water.