OSMOREGULATION IN VARIOUS ANIMALS (for class 11 student and NEET aspirants)

 OSMOREGULATION IN VARIOUS ANIMALS

-Malay Nayak

All animals need to regulate water and salts in their environment and eliminate nitrogenous wastes. All these functions are inter-related, but they have different names. Regulation of water and salts is termed osmoregulation and elimination of nitrogenous wastes is termed excretion. Both excretion and osmoregulation bring about homeostasis, which is vital for life.

         In vertebrates, kidneys perform both excretion and osmoregulation, whereas invertebrates have different types of
excretory and osmoregulatory organs.

OSMOREGULATION
The regulation of solute movement, and hence, water movement, through 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 a semipermeable membrane (the membrane that allows water molecules to pass, but not the solutes) differ in total solute concentration, 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 (mOsmol/L).

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 and osmoregulators

Osmoconformers are the animals that do not actively control the osmotic concentration 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 osmoconformers. However, hagfish is a vertebrate osmoconformer. Osmoconformers
show an excellent ability to tolerate a wide range of cellular osmotic environments.

Osmoregulators, on the other hand, are the animals that maintain an internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited 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 elasmobranchs (sharks and rays). Osmoregulators must either eliminate excess water if they are in a hypotonic medium, or continuously take in water to compensate for water loss, if they are in a hypertonic situation. Therefore, osmoregulators have to spend energy to more water in or out, and maintain osmotic gradients by manipulating solute concentrations in their body fluids.

Osmoregulation in freshwater animals

Osmolarity of freshwater is generally much less than 50 mOsmol/L while the freshwater vertebrates have blood osmolarity in the range of 200 to 300 mOsmol/L The body fluids of fresh water animals are generally hypertonic to their surrounding environment.

The problems faced by these animals will be

1. Loss of body salt to the outside
2.  Entry of excess of water.

The freshwater animals prevent the net gain of water and net loss of body salts by several means such as:

• Freshwater animals (including freshwater fish) do not drink water to reduce the need to expel excess water.
• Protozoa (Amoeba, Paramoecium) have contractile vacuoles that pump out excess water.
• Water uptake and salt loss are minimized by a specialized body covering (Subcutaneous fat layer of scaleless fish and scales over the body of fish or crocodiles).



source- campbell biology


• Specialized cells, called ionocytes or chloride cells in the gill membrane of freshwater fish can import Na+ and Cl- from the surrounding water (surrounding water has less than 1mm NaCl and plasma concentration has more than 100 mM)

Osmoregulation in marine animals

Seawater usually has an osmolarity of about 1000 mOsmol/L, while osmolarity of human blood is about 300 mOsmol/L.
The osmoregulatory problems in marine situation are opposite to those in freshwater environment. Marine bony fish have the body fluids hypotonic to seawater, and thereby, they tend to lose water from the body through permeable surfaces
(gill membranes, oral and anal membranes). To compensate for the water loss, marine bony fish drink seawater. However, drinking seawater results in a gain of excess salts. The ionocytes or chloride cells of the gill membrane of marine bony fish
help to eliminate excess monovalent ions from the body fluid to the seawater. Divalent cations are generally eliminated with faeces. Hilsa, salmon and other fish that migrate between seawater and freshwater, when in ocean, drink and excrete
excess salt through the gill membrane. A number of hormones play a key role in this switching over process.
In general, the body fluids of marine invertebrates, ascidians and the hagfish are isosmotic to seawater. In elasmobranchs (sharks and rays) and coelacanths  (lobe finned fish), osmolarity of the body fluids is raised by accumulating certain
organic substances (osmolytes). Retention of osmolytes in body fluids reduces the osmoregulatory challenges. The
best known examples of such organic osmolytes are urea and trimethylamine oxide (TMAO). Body fluids of sharks and coelacanths are slightly hyperosmotic to seawater due to retention of urea and TMAO, but hypotonic to seawater, as they maintain far lower concentration of inorganic ions in the body fluids.

Osmoregulation in terrestrial animals

Land animals are always subject to osmotic desiccation, like the marine animals. Terrestrial air-breathing animals constantly lose water through their respiratory surfaces which may even be fatal. Humans, for example, die if they lose around 12
percent of the body water. Therefore, water loss must be compensated by drinking and eating moist food.
Animals utilise various means to minimise this water loss, such as the waxy coating of the exoskeleton of insects, the shell of the land snails, and the multiple layers of dead, keratinised skin cells covering most terrestrial vertebrates.

Kangaroo rats, lose so less water, that they can recover 90 percent of the loss by using metabolic water (water derived from different cellular metabolic processes).
Many desert animals are nocturnal to avoid the heat of day-time, a behavioural adaptation that minimizes dehydration. Camels produce dry faeces and concentrated urine. When water is not available, the camels do not produce urine, but store urea in tissues, and solely depend on metabolic water. When water is available, they rehydrate themselves by drinking upto 80 litres of water in 10 minutes.