Gill Na+/H+ and Cl-/HCO3- exchange systems evolved before the vertebrates entered fresh water.
D000136:Acid-Base Equilibrium; D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D005075:Biological Evolution; D002712:Chlorides / Q000378:metabolism*; D005399:Fishes / Q000378:metabolism*; D005880:Gills / Q000378:metabolism*; D006196:Hagfishes / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D007474:Ion Exchange; D012621:Seasons; D012623:Seawater; D012964:Sodium / Q000378:metabolism*; D013045:Species Specificity
D001639:Bicarbonates; D002712:Chlorides; D000641:Ammonia; D006859:Hydrogen; D012964:Sodium