Figure 1. Respiration in water-breathers from simple gaseous diffusion (1) to ventilatory and circulatory convection (5).

PwO₂, PeO₂, PcellO₂, Pw′O₂, PaO₂, PvO₂ correspond to the partial pressure of oxygen in water, in the extracellular medium, at cell level, within external chamber (e.g. branchial chamber), in arterial hemolymph, and in venous hemolymph. Today, the PO₂ in the blood and tissues of water-breathing animals (see [4] for a review), is remarkably low, ranging between 1 and 3 kPa and is largely independent of ambient PO₂. PcellO₂ results from the equilibrium between O₂ supply and O₂ consumption (90% by mitochondria). PwO₂>Pw′O₂>PaO₂>PeO₂>PcellO₂.