Solute back-diffusion raises the gas concentrating efficiency in counter-current flow

We have extended the counter-current model of the rete mirabile of the fish swimbladder to include the effects of inert gas secretion into the swimbladder as well as solute back-diffusion in the rete capillaries. (1) Gas secretion attenuates the inert gas concentrating efficiency of the rete, i.e. its ability to produce high inert gas partial pressures in blood at the swimbladder pole. The maximum attainable gas secretion rate depends on the salting-out effect, i.e. on the ratio of solubility in venous and arterial blood of the rete. (2) Solute back-diffusion leads to a significant increase in the concentrating efficiency, and this is due to the salting-out effect produced by the solute when it diffuses back into the arterial capillary blood. This enhancement is particularly prominent when gas and solute permeabilities of the rete vessels are high. (3) Estimates for physiologic parameters in the European eel suggest that lactate back-diffusion may contribute significantly to the gas concentrating efficiency of the rete mirabile.