Gas exchange in the fish swimbladder.

The fish swimbladder acts as a device to adjust for neutral buoyancy at various depths. High gas pressures, corresponding to the ambient hydrostatic pressure, are encountered, most of which is made up by O2 and N2. To prevent gas loss, the swimbladder wall is made impermeable by guanine crystals in its wall. Gas deposition is made possible by lactic acid production in the swimbladder epithelium, which increases blood gas partial pressures of inert gases (salting-out effect), O2 (Bohr and Root effects) and CO2 (conversion from HCO3-). The hairpin counter-current blood flow in the rete mirabile enhances this partial pressure increase to the tremendous values, up to several 100 atm, encountered in deep sea dwellers. Flow balance in the rete capillaries is found to be crucial, and salt back-diffusion to be advantageous, for the concentrating efficiency in the rete mirabile.