11th Class Biology Excretory Products and their Elimination Osmoregulation.

The regulation of solute movement, and hence, water movement, which follows solutes by osmosis, is known as osmoregulation. Osmosis may be defined as a type of diffusion where the movement of water occurs selectively across a semipermeable membrane. It occurs whenever two solutions, separated by semipermeable membrane (the membrane that allows water molecules to pass but not the solutes) differ in total solute concentrations, or osmolarity. The total solute concentration is expressed as molarity or moles of solute per litre of solution. The unit of measurement for osmolarity is milliosmole per litre (mosm\[{{L}^{1}}\]). If two solutions have the same osmolarity, they are said to be isotonic. When two solutions differ in osmolarity, the solution with higher concentration of solute is called hypertonic, while the more dilute solution is called hypotonic. If a semipermeable membrane separates such solutions, the flow of water (osmosis) takes place from a hypotonic solution to a hypertonic one.

Osmoconformers are the animals that do not actively control the osmotic condition of their body fluids. They rather change the osmolarity of body fluids according to the osmolarity of the ambient medium. All marine invertebrates and some freshwater invertebrates are strictly osmoconformer. Osmoconformers show an excellent ability to tolerate a wide range of cellular osmotic environments.

Osmoregulators, on the other hand, are the animlas that maintain internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited osmoregulators. Most vertebrates are strict osmoregulators, i.e. they maintain the composition of the body fluids within a narrow osmotic range. The notable exception, however, are the hagfish (Myxine sp., a marine cyclostome fish) and elasmobranch fish (sharks and rays).

Osmoregulators must either eliminate excess water if they are in hypotonic medium or continuously take in water to compensate for water loss if they are in a hypertonic situation. Therefore, osmoregulators have to spent energy to move water in or out and maintain osmotic gradients by manipulating solute concentrations in their body fluids.