The gill of fishes is modified for gas exchange, thereby providing a site for net movement of salts and water down their respective gradients. Specialized cells in the gill epithelium are joined by tight junctions of variable depth and express a variety of transporters and channels. These cells mediate NaCl extrusion in marine fishes and NaCl uptake in freshwater fishes. These transport steps also provide pathways for the extrusion of ammonia and acid vs. base equivalents.