osmoregulation

osmoregulation The control of the water content and the concentration of salts in the body of an animal or protoctist (see osmoregulator). In freshwater species osmoregulation must counteract the tendency for water to pass into the animal by osmosis. Various methods have been developed to eliminate the excess, such as contractile vacuoles in protozoans and kidneys with well-developed glomeruli in freshwater fish. Marine vertebrates have the opposite problem: they prevent excessive water loss and enhance the excretion of salts by having kidneys with few glomeruli and short tubules (see also chloride secretory cell). In terrestrial vertebrates the dangers of desiccation are reduced by the presence of long convoluted renal tubules, which increase the reabsorption of water and salts.