Osmoregulation, Osmoconformers and Osmoregulators

Excretory organs do far more than simply remove waste — they are central to homeostasis, the maintenance of a stable internal environment, because the composition of the blood (and so of every tissue) is set far more by what the excretory organs choose to retain than by what the mouth happens to ingest. A key part of this job is osmoregulation: controlling the concentration of dissolved solutes and the overall water balance of the body.

Organisms fall into two broad groups depending on how they relate to the salinity of their surroundings. Osmoconformers simply let their internal body-fluid osmolarity match (be isoosmotic with) the external environment; most marine invertebrates behave this way because seawater's osmolarity is already close to what their body fluids need. Osmoregulators, by contrast, actively keep their internal environment's solute concentration independent of the external medium, regardless of whether the surroundings are fresh water, seawater or dry land — freshwater organisms and all terrestrial organisms must be osmoregulators, since neither pure fresh water nor air offers a ready-made isoosmotic environment. Some marine osmoregulators go further still: marine birds such as the albatross, which spend their lives at sea and can only drink salty water, have specialised salt glands near the nostrils that actively pump excess salt back out of the body; sea turtles and marine iguanas have similar salt-excreting glands. …