The effects of H+ upon the gills of freshwater fish

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Abstract
The structure and function of the gills of freshwater fish are briefly summarized and the responses to low pH are reviewed, with particular reference to the salmonid fishes. Major influences are seen upon ion and acid–base regulatory mechanisms at the gills and upon mucous secretion and gill structure. Ionic imbalances which can be responsible for death at low pH are caused by disturbances to both the active transport and the diffusional losses of Na+ and Cl−. The disturbances, while not identical for the two ions, are similar to the extent that the undirectional fluxes of both are dependent upon the severity and duration of the acid exposure and upon the external calcium concentration. Calcium also has an important influence on the net flux of H+ across the gills. A model is proposed for the interaction of Ca2+ and H+ on gill function. The physiological importance of mucous secretion at low pH is discussed and the issue of whether acid-intolerant species can be successfully adapted for life at low pH is examined.

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