 the gill epithelium is the site of gas exchange , ionic regulation , acid - base balance , and nitrogenous waste excretion by fishes . 
 the last three processes are controlled by passive and active transport of various solutes across the epithelium . various environmental pollutants ( e.g. , 
 heavy metals , acid rain , and organic xenobiotics ) have been found to affect the morphology of the gill epithelium . associated with these morphological pathologies , 
 one finds alterations in blood ionic levels , as well as gill na , k - activated atpase activity and ionic fluxes . 
 such physiological disturbances may underly the toxicities of these pollutants . 
 in addition , the epithelial transport steps which are affected in the fish gill model resemble those described in the human gut and kidney , sites of action of a variety of environmental toxins.imagesfigure 1 . 
 afigure 1 . 
 bfigure 3 . 
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