Gill Gas Exchange: How Oxygen Diffuses from Water into Blood

Fish gills extract dissolved oxygen from water via countercurrent exchange, maximizing diffusion efficiency.

Structural Adaptations

• Gill filaments (primary lamellae): Project from gill arches; highly vascularized.
• Secondary lamellae: Microscopic plates on filaments; site of gas exchange. Blood-water barrier <1 µm thick.
• Countercurrent flow: Blood flows opposite to water across lamellae → maintains favorable PO₂ gradient along entire length.

Diffusion Process

1. Water enters mouth → passes over gills → exits operculum.
2. O₂ diffuses from water (higher PO₂) → blood (lower PO₂) down partial pressure gradient.
3. CO₂ diffuses blood → water (higher solubility, faster diffusion).
4. Hemoglobin in erythrocytes binds O₂ (cooperative binding).

Efficiency

Countercurrent exchange achieves ~80% O₂ extraction from water (vs. ~25% in mammalian lungs from air). Water holds 30× less O₂ than air per volume, making this efficiency critical.

Regulation

• Branchial motor neurons adjust lamellar perfusion (recruitment/derecruitment).
• Catecholamines, cortisol, hypoxia modulate vascular resistance.