[Extrarenal salt excretion in oceanic birds].

Oceanic birds feed mainly on marine invertebrates which are isosmotic with sea water. The kidneys of birds have a low concentrating ability and are unable to produce urine with a sodium concentration more than 300mmol/l. How do these birds avoid dehydration? The excess salt is eliminated by the secretory work of a paired gland in the head known for centuries to anatomists as nasal glands or supraorbitary glands, and now that their function is elucidated called salt excreting glands or simply salt glands. They connect through a duct with the nasal cavity. Oceanic birds expel by the nares a clear and colourless watery liquid that contains primarily Na(+) and Cl(-), at high and constant concentration, with a small amount of K(+) and HCO(3)(-). These salt glands do not function continually as the kidney does, becoming active only in response to an osmotic load. The salt gland of oceanic birds is one of the most efficient systems of ionic transport in the living world.