Animals as well as plants contain a large amount of water both in their cells and in their extra cellular fluids.
In an animal’s body water may enter as the result of drinking, of food intake, of metabolic reactions that yield water as their by products, and of osmosis caused by concentrations gradients across the exposed semipermeable membranes of the animal.
Water may leave the organism in the form of excreta, through evaporation from exposed body surfaces, through sweat or other glands, by osmosis through the skin, and by exocytosis.
It is vitally important to an organism that its water content, both intracellular and extracellular, be nearly constant, or at least that it should vary only within a moderately small range, so that its tissues or its body fluids may not be diluted or concentrated beyond the limits of its tolerance .
This more or Jess steady state of water contents in the body can only be maintained when there is an equality between the amount of water entering the body and the amount of water leaving the body, that is, water in= water out.
The process by which the movement of water and its volumes in the body are regulated is known as osmoregulation. The term osmoregulation was coined by Hober in 1902.
Like water, the concentration of many dissolved substances in the body fluids and in the blood must also remain within narrow limits of tolerance because they regulate the movement of water as a result of osmosis.
These substances that are needed for proper functioning of tissues and cells of the body are controlled by a variety of mechanisms whose activities fall under the study of ionic regulation.
Since water and ionic regulation are inseparable activities in organisms, by extension the term osmoregulation is sometimes used to include both activities.
Different organisms have different limits of tolerance both for water and salt gain and for water and salt loss because of different environmental conditions.
Animals whose body fluids and blood have the same concentration as that of surrounding medium (isotonic) never face the problem of osmoregulation as long as they live in such a medium.
Most marine organism, for examples have the same concentration of the body fluids as present in the sea water.
Animals which live in a medium of a lower salt concentration (hypotonic) have to face the difficulty of dilution of blood and body fluids due to endosmosis and hence they have evolved special mechanisms to get rid of the excess water that has entered their body and diluted their body fluids.
Animals which live in a medium of high salt concentration (hypertonic) have to face the difficulty of excessive exosmosis which may cause the shrinkage of the hpdy fluids and blood.
To avoid this they have to evolve regulatory mechanisms. In terrestrial animals water is lost by evaporation from the general body surfaces and hence mechanisms to conserve water are developed in them.
Aquatic animals that have only a limited tolerance to changes in the osmotic concentrations of the external environment or are restricted to a narrow range of salinity, usually to full sea water, are called stenohaline.
Most animals fall into this category. Animals which can tolerate a wider range of osmotic concentrations or are tolerant of a variety of salinities, are called euryhaline.
When aquatic animals are transferred from their natural environments to different waters or salinities, they are usually put under conditions of osmotic stress.
Some cannot survive such a change and, therefore, in nature are prevented from migrating to areas where the salinity of the water is greater or less than that of their natural habitats.
Others have proved more able to endure and survive considerable degrees of osmotic stress, either as osmoconformers or osmorgulators.
Osmo- conformers are those animals which are osmotically labile (dependent) and whose body-fluid concentrations change with the medium and that, having a high tissue tolerance, can survive such changes as long as their basic metabolic functions can proceed effectively at the dilutions or concentrations to which they are thus subjected.
Osmoregu- Iators are those animals which are osmotically stable (independent) and are able to maintain their internal osmotic concentration at constant level (or nearly so) despite of changes in that of their external environments.
Gradations between these two extremes of ability and stability in respect to internal concentration are frequent, and an organism may conform to external conditions in one situation and regulate in another.
But, in general, osmoconformers can tolerate greater variations in their internal environments than can osmoregu- lators and osmoaegulators can tolerate greater variations in their external environments than can osmoconformers.
Some animals change in volume (due to alterations of water content) as the external osmotic concentration changes.
These are known as volume conformers. Others which maintain a constant volume despite of external osmotic changes are known as volume regulators.
Animals tend to maintain an optimum osmotic concentration for a given environment. Many animals, upon return to normal environment after a period of dehydration, take up water, and after a period of hydration, lose water, until the osmoconcentration reaches the “optimum” for the particular animals.