Osmoregulation is the active regulation of the osmotic pressure of an
organism's body fluids, detected by osmoreceptors, to maintain the
homeostasis of the organism's water content; that is, it maintains the
fluid balance and the concentration of electrolytes (salts in
solution) to keep the fluids from becoming too diluted or
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 nitrogen
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.
1 Regulators and conformers
2 In plants
3 In animals
4 In protists
5 In bacteria
6 Vertebrate excretory systems
6.1 Waste products of the nitrogen metabolism
6.2 Achieving osmoregulation in vertebrates
7 See also
Regulators and conformers
Movement of water and ions in saltwater fish
Movement of water and ions in freshwater fish
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, maintaining
constant internal conditions. They 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., Flounder.
Flounder have 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 damages living
tissues 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
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.
Hydrophytes are plants in water habitats. They mostly grow in water or
in wet or damp places. In these plants the water absorption occur
through the whole surface of the plant, e.g., the water lily.
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
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 waterproof external
covering called cuticle.
Kidneys play a very large role in human osmoregulation by regulating
the amount of water reabsorbed from glomerular filtrate in kidney
tubules, which is controlled by hormones such as antidiuretic hormone
(ADH), aldosterone, and angiotensin II. For example, a decrease in
water potential is detected by osmoreceptors in the hypothalamus,
which stimulates ADH release from the pituitary gland to increase the
permeability of the walls of the collecting ducts in the kidneys.
Therefore, a large proportion of water is reabsorbed from fluid in the
kidneys to prevent too much water from being excreted.[citation
A major way animals have evolved the ability to osmoregulate is by
controlling the amount of water lost through the excretory system.
Paramecium aurelia with contractile vacuoles.
Amoeba makes use of contractile vacuoles to collect excretory wastes,
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.
Bacteria respond to osmotic stress by rapidly accumulating
electrolytes or small organic solutes via transporters whose
activities are stimulated by increases in osmolarity. The bacteria may
also turn on genes encoding transporters of osmolytes and enzymes that
synthesize osmoprotectants. The
EnvZ/OmpR two-component system,
which regulates the expression of porins, is well characterized in the
model organism E. coli.
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
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 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
^ Wood, Janet M. (2011). "Bacterial Osmoregulation: A Paradigm for the
Study of Cellular Homeostasis". Annual Review of Microbiology. 65 (1):
ISSN 0066-4227. PMID 21663439.
^ Cai, SJ; Inouye, M (5 July 2002). "EnvZ-OmpR interaction and
osmoregulation in Escherichia coli". The Journal of Biological
Chemistry. 277 (27): 24155–61. doi:10.1074/jbc.m110715200.
E. Solomon, L. Berg, D. Martin, Biology 6th edition. Brooks/Cole
Salt and water balance in animals
Pressure: Renin–angiotensin system