An osmoregulatory animal is generally in an osmotic steady state even though there may be hourly and daily variations in osmotic balance. The concentration of internal salts and water is maintained relatively constant. The intake and outflow of water and salts are equal. Such osmotic homeostasis is maintained at the cost of metabolic energy, obtained from ATP.
Figure: Osmotic homeostasis in osmoregulatory animal
The need for the evolution of efficient osmoregulatory mechanism gave chance for animal speciation and diversification. The resourcefulness of evolutionary adaptations can be realised if we consider the adaptations of arthropods and vertebrates. If they had not evolved suitable osmoregulatory mechanism how they have been so successful in both terrestrial and aquatic environments - the environments that are osmotically hostile and difficult. You will learn about such adoptions in the following sections of the unit.