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Osmoregulation is a process which maintains the homeostasis of an organism’s water content through the regulation of the osmotic pressure of body fluids. As organisms began to move out of marine habitats to other environments such as land, seashores and estuaries, osmoregulation grew in importance. Osmoregulation is crucial because in order to survive, organisms must retain the appropriate concentration of both water and solutes. It is through osmoregulation that organisms are able to balance water loss and solute uptake and vice versa.

In general, most organisms must tolerate the need to deal with osmotic challenges. They can tolerate these changes by one of two ways:

  1. Conform: For species which conform, such as marine fish, this means that the changes which their bodies undergo mirror the conditions of their environments. Organisms which conform can either be stenohaline or euryhaline. A stenohaline species, such as a goldfish, cannot survive in conditions where the water has high salinity levels or high salinity fluctuations. Conversely, euryhaline species such as salmon are able to tolerate changes in salinity.
  2. Regulate: For species which are regulators, they are able to maintain the proper osmotic balance of their bodies regardless of the solute and water concentrations of the environment. Regulators can either be a) hyperosmotic or b) hyposmotic.

Hyperosmotic regulation means that the concentration of solutes within the body is greater than the solute concentration of the environment. For example, fish living in freshwater environments are hyperosmotic regulators and must combat the challenges of osmotic loss and high water uptake. Thus, these organisms have evolved chloride cells for example, which allow them to uptake chloride ions quickly as they are continuously being lost in freshwater environments.

Alternatively, in hyposmotic regulation organisms faces the challenges of too much water loss and salt gain, through the skin for example. Organisms such as reptiles and mammals on land are hyposmotic regulators. The salt-gland which exists in birds and reptiles is a method which these organisms use to combat these challenges. 

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Excretion of Nitrogen

Describe various strategies for excretion of nitrogenous wastes employed in human and non-human chordates.