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Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is, it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the osmotic pressure of a solution, the more water tends to move into it. Pressure must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water.
Organisms in aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion (getting rid of metabolic wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood) through organs such as the skin and the kidneys.
Regulators and conformers 
Two major types of osmoregulation are osmoconformers and osmoregulators. Osmoconformers match their body osmolarity to their environment actively or passively. Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater.
Osmoregulators tightly regulate their body osmolarity, which always stays constant, and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gills actively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very hypotonic (dilute) urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes salt out from the gills. Most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e.g., salmon. Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.
Some marine fish, like sharks, have adopted a different, efficient mechanism to conserve water, i.e., osmoregulation. They retain urea in their blood in relatively higher concentration. Urea is damaging to living tissue so, to cope with this problem, some fish retain trimethylamine oxide. This provides a better solution to urea's toxicity. Sharks, having slightly higher solute concentration (i.e., above 1000 mOsm which is sea solute concentration), do not drink water like fresh water fish.
Osmoregulation in plants 
While there are no specific osmoregulatory organs in higher plants, the stomata are important in regulating water loss through evapotranspiration, and on the cellular level the vacuole is crucial in regulating the concentration of solutes in the cytoplasm. Strong winds, low humidity and high temperatures all increase evapotranspiration from leaves. Abscisic acid is an important hormone in helping plants to conserve water — it causes stomata to close and stimulates root growth so that more water can be absorbed.
Plants share with animals the problems of obtaining water but, unlike in animals, the loss of water in plants is crucial to create a driving force to move nutrients from the soil to tissues. Certain plants have evolved methods of water conservation.
Xerophytes are plants that can survive in dry habitats, such as deserts, and are able to withstand prolonged periods of water shortage. Succulent plants such as the cacti store water in the vacuoles of large parenchyma tissues. Other plants have leaf modifications to reduce water loss, such as needle-shaped leaves, sunken stomata, and thick, waxy cuticles as in the pine. The sand-dune marram grass has rolled leaves with stomata on the inner surface.
Halophytes are plants living in marshy areas (close to sea). They have to absorb water from such a soil which has higher salt concentration and therefore lower water potential(higher osmotic pressure). Halophytes cope with this situation by activating salts in their roots. As a consequence,the cells of the roots develop lower water potential which brings in water by osmosis. The excess salt can be stored in cells or excreted out from salt glands on leaves. The salt thus secreted by some species help them to trap water vapours from the air, which is absorbed in liquid by leaf cells. Therefore this is another way of obtaining additional water from air, e.g. glasswort and cord-grass.
Mesophytes are plants living in lands of temperate zone, which grow in well watered soil. They can easily compensate the water lost by transpiration through absorbing water from the soil. To prevent excessive transpiration they have developed a water proof external covering called cuticle.
Osmoregulation in animals 
Kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney tabules, which is controlled by hormones such as antidiuretic hormone (ADH), aldosterone, and angiotensin II. For example, a decrease in water potential of blood is detected by osmoreceptors in hypothalamus, which stimulates ADH release from pituitary gland to increase the permeability of the wall of the collecting ducts in the kidneys. Therefore a large proportion of water is reabsorbed from fluid to prevent a fair proportion of water from being excreted.
A major way animals have evolved to osmoregulate is by controlling the amount of water lost through the excretory system.
Osmoregulation in protists 
Amoeba make use of contractile vacuoles to collect excretory waste, such as ammonia, from the intracellular fluid by diffusion and active transport. As osmotic action pushes water from the environment into the cytoplasm, the vacuole moves to the surface and disposes the contents into the environment.
Osmoregulation in bacteria 
Vertebrate excretory systems 
Waste products of the nitrogen metabolism 
Ammonia is a toxic by-product of protein metabolism and is generally converted to less toxic substances after it is produced then excreted; mammals convert ammonia to urea, whereas birds and reptiles form uric acid to be excreted with other wastes via their cloacas.
Achieving osmoregulation in vertebrates 
Four processes occur:
- filtration — fluid portion of blood (plasma) is filtered from a nephron (functional unit of vertebrate kidney) structure known as the glomerulus into. Bowman's capsule or glomerular capsule (in the kidney's cortex) and flows down the proximal convoluted tubule to a "u-turn" called the Loop of Henle (loop of the nephron) in the medulla portion of the kidney.
- reabsorption — most of the viscous glomerular filtrate is returned to blood vessels that surround the convoluted tubules.
- secretion — the remaining fluid becomes urine, which travels down collecting ducts to the medullary region of the kidney.
- excretion — the urine (in mammals) is stored in the urinary bladder and exits via the urethra; in other vertebrates, the urine mixes with other wastes in the cloaca before leaving the body (frogs also have a urinary bladder).
See also 
- E. Solomon, L. Berg, D. Martin, Biology 6th edition. Brooks/Cole Publishing. 2002