Gli organismi di ambienti sia acquatici sia terrestri contengono la giusta concentrazione di soluzione e una certa quantità di acqua all'interno dei loro fluidi corporei. Questo comporta [[escrezione|escrezioni]]: per liberarsi dei rifiuti metabolici e di altre sostanze, come gli [[ormoni]] che sarebbero tossici se accumulati nel [[sangue]], vengono espulsi attraverso la [[pelle]] e i [[reni]]; mantenendo la corretta quantità di acqua e bilanciando la dissoluzione di soluti secondo l'osmoregolazione.
==Regolatori e conformazione==
==Regulators and conformers==
[[Image:Osmoseragulation Carangoides bartholomaei bw en2.png|right|thumb|400px|Movement of water and ions in saltwater fish]]
[[Image:Bachforelle osmoregulatoin bw en2.png|left|thumb|400px|Movement of water and ions in freshwater fish]]
Two major types of osmoregulation are osmoconformers and osmoregulators.
'''[[Osmoconformer]]s''' match their body osmolarity to their environment. It can be either active or passive. 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 [[active transport|actively uptake]] salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very [[Tonicity#Hypotonicity|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 [[gill]]s. 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]].
==Osmoregulation in plants==
Whilst there are no specific osmoregulatory organs in higher [[plants]] the [[stomata]] are important in regulating water loss through [[evapotranspiration]] and on the [[plant cell|cellular]] level the [[vacuole]] is crucial in regulating the concentration of solutes in the [[cytoplasm]]. Strong [[wind]]s, low [[humidity]] and high [[temperature]]s all increase evapotranspiration from leaves. [[Abscisic acid]] is an important [[plant hormone|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 [[plant nutrition|nutrients]] from the soil to tissues. Certain plants have evolved methods of water conservation. [[Xerophyte]]s 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 [[vacuole|vacuoles]] of large [[parenchyma]] tissues. Other plants have [[leaf]] modifications to reduce water loss, such as needle-shaped leaves, sunken [[stomata]], and thick, waxy [[cuticle]]s as in the [[pine]]. The [[Ammophila (Poaceae)|sand-dune marram grass]] has rolled leaves with stomata on the inner surface. [[Hydrophytes]] are plants in water habitats e.g. the [[water lily]].
==Osmoregulation in protists and animals==
[[File:Paramecium contractile vacuoles.jpg|thumb|Protist [[Paramecium]] aurelia with contractile vacuoles.]]
[[Amoeba]] make use of [[contractile vacuoles]] to collect excretory waste, such as [[ammonia]], from the intracellular fluid by both 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.
[[Kidneys]] play a very large role in human osmoregulation, regulating the amount of water in urine waste. With the help of [[hormones]] such as [[antidiuretic hormone]], [[aldosterone]], and [[angiotensin II]], the human body can increase the permeability of the collecting ducts in the kidney to reabsorb water and prevent it from being excreted.
A major way animals have evolved to osmoregulate is by controlling the amount of water excreted through the [[excretory system]].
==Vertebrate excretory systems==
===Waste products of 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; [[mammal]]s convert ammonia to urea, whereas [[bird]]s and [[reptile]]s form uric acid to be excreted with other wastes via their [[cloaca]]s.
===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).
== Voci correlate ==
* [[Apparato protonefridiale]]