Osmotic and ionic regulation in fish and the role of gills

Fish have evolved special homeostatic mechanisms to maintain osmotic and ionic balance. Gills play an important role in these mechanisms. Oxygen and carbon dioxide, many ions, salts and water are exchanged between blood and external environment, passing through a thin epithelium, which covers primary and secondary lamellas of the gills. Water passes through the membrane by diffusion, while ions and salts use passive or active transport depending on the species and living environment. The fish can be divided into four groups or strategies of osmoregulation and into five groups or strategies of ionic regulation. Osmoregulation and ionic regulation are most intensive in diadromous and brackish species.