Fig. 5. Hypothetical model showing the various proton-translocating mechanisms identified so far in swimbladder gas gland cells of the European eel. Acidic metabolites (lactic acid and CO₂) are produced in the glycolytic pathway and in the pentose phosphate shunt. This acid is secreted at the basolateral membranes to reduce the effective gas-transport capacity of the blood via the Root effect and the salting out effect (see Pelster, 1997; Pelster and Randall, 1998). The decrease in gas-transport capacity induces an increase in gas partial pressures in the blood, which provides the necessary partial pressure gradient towards the swimbladder lumen so that gases can enter the swimbladder by diffusion. The mechanisms of lactate transport have not yet been characterized in detail. CA, membrane-bound carbonic anhydrase.