ABSTRACT
Inhabiting a wide diversity of habitats, fish face great variabilities in dissolved O2 and ion concentration in the environment. They show a remarkable ability to uptake the O2 needs for aerobic metabolism from water and they keep ionic and osmotic levels independent of their habitat. The gills are the main organs for respiration, ionic regulation and acid-base equilibrium but some fish need to emerge from the water and take O2 from atmospheric air using air-breathing organs. In relation to ionic and osmoregulation of the extracellular fluid, besides the gills, the effector organs include the kidney, the intestinal tract and the body surface. This chapter will be restricted to the structure of gills considering the gas exchange (O2/CO2) and the cells related to ionic transport in the epithelium of the gills, with emphasis to freshwater fish.
