An osmoregulator is an organism that can regulate or keep the solutes or salts of its body fluid at a higher or lower concentration than the concentration of solutes in the external medium, although this regulation may be limited at extremely high or extremely low external solute concentrations. If the solutes in the organism's body fluids are kept at a concentration higher than that of the external medium (e.g., lake water), this organism's body fluids are said to be hyperosmotic to the external medium and the organism is a hyperosmotic osmoregulator. If the solutes of the organism's body fluids are kept at a concentration lower than that of the external medium (seawater for example), this organism's body fluids are said to be hypoosmotic to the external medium and the organism is a hypoosmotic osmoregulator. If the solutes of the organism's body fluids are the same concentration as that of the external medium, the organism is isoosmotic and is at its isoosmotic point (see the Figure). An organism could be hyperosmotic in an external medium of low solute concentration and be isoosmotic in an external medium of higher solute concentration. Conversely, an organism could be hypoosmotic in an external medium of high solute concentration and be isoosmotic in an external medium of low solute concentration. Both of these situations could happen, for example, in a tidal marsh or other estuarine habitat because in these environments the solute concentrations of the water change with the tides, salinity being lower at low tide and higher at high tide as ocean water floods the estuary. In either case the organism is still an osmoregulator as long as the solute concentration of its body fluids are different than the solute concentration of the external medium. An organism could also be hyperosmotic in both low and high solute concentrations in the external medium. This is the case with sharks. They are always hyperosmotic because of the high concentration of urea in their bodies.
Organisms that are hyperosmotic tend to lose solutes to the external medium by diffusion and to gain water from the external medium by osmosis. The opposite is true of organisms that are hypoosmotic, which tend to gain solutes from the external medium by diffusion and lose water to the external medium by osmosis. Organisms that are isoosmotic are at equilibrium with the external medium and do not gain or lose solutes or water.
Hyperosmotic regulators decrease the loss of solutes from the blood to the external medium via chloride cells that actively take up the chloride ions that were lost to the external medium due to diffusion. In animals, chloride cells are located in the gills. Hyperosmotic regulators decrease the osmotic gain of water in the blood from the external medium by excreting urine that is hypoosmotic to the blood. Hyperosmotic regulators decrease the osmotic gain of water in their cells from the blood by joining amino acids into proteins. This decreases the gain of water into the cells because the cells become hypoosmotic to the blood when the solute concentration is decreased by joining many amino acids into fewer proteins, and this makes less water move into the cells from the blood. Although proteins are larger than amino acids, they do not increase the solute concentration as much as the many amino acids needed to make a protein. In these ways, regulators can maintain homeostasis in their blood and cells.
Animals that are hypoosmotic regulators decrease the gain of solutes that diffuse into the blood from the external medium by drinking the external medium and having efficient kidneys that can remove the solutes, pumping them into the urine so that the urine is hyperosmotic to the blood. Hypoosmotic regulators decrease the osmotic loss of water from the blood to the external medium by having body surfaces that are relatively impermeable. Hypoosmotic regulators decrease the osmotic loss of water from their cells to their blood by breaking proteins into amino acids. This decreases the loss of water from cells into the surrounding blood because the resulting high concentration of amino acids in the cells makes them hyperosmotic to the blood. This increases the gain of water into the cells from the blood and maintains homeostasis.
Almost all aquatic animals are physiological osmoregulators. Exceptions include soft-bodied animals such as worms and mollusks, which are osmoconformers. Some animals are capable of behavioral osmoregulation -- they behave in certain ways that regulate how fast or slowly they encounter the differing solute concentration of the external medium. For example, certain animals may seek out microhabitats with favorable salinity, and bivalved mollusks such as clams may close their shells when bathed by water with unfavorabale solute concentrations.