Hagfishes (Myxine glutinosa, Cyclostomata) are the only vertebrates whose body fluids are isoosmotic to seawater due to high sodium and chloride concentrations in the plasma. The unique mode of fluid and electrolyte handling of Myxine has been studied by micropuncture methods at the single nephron level. In the archinephric duct, which assumes the function of the tubular apparatus of higher vertebrates, no fluid or sodium is reabsorbed from the glomerular filtrate. Calcium, glucose and magnesium are reabsorbed whereas potassium and phosphate are excreted. The intravascular volume of M. glutinosa is mainly regulated by the filtration rate which greatly increases with increasing arterial as well as venous pressure.

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