Osmoregulation Definition:

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os·mo·reg·u·la·tion

Osmoregulation is the process of regulating water potential in order to keep fluid and electrolyte balance within a cell or organism relative to the surrounding.

Supplement In biology, osmoregulation is important to organisms to keep a constant, optimal osmotic pressure within the body or cell. It is the way by which an organism maintains suitable concentration of solutes and amount of water in the body fluids.
An example employed by organisms is excretion (such as getting rid of metabolic wastes and other substances toxic to the body when they are in large amounts).

See also:
osmosis, solute Reference: animal and plant water regulation tutorial pages for related information in regards to evolutionary adaptations regarding osmoregulation.

Osmoregulation is the technical term for the physiological mechanism fish use to control the amount of salt and water in their bodily fluids. As the name suggests, it's based on osmosis which is the movement of dissolved stuff through a semi-permeable membrane from a strong concentration to a weaker one.

Freshwater fish are saltier than the water they live in, and their skin is semi-permeable. Since there's a big difference between the amount of salt on the inside of the fish and the amount of salt in the freshwater they live in, freshwater fish leak bodily salts and take in water.

This presents two problems for a freshwater fish:

Taking in salt and getting rid of water.

Osmoregulation process of the fish does not work well when the fish are stressed or diseased.

This "osmoregulatory dysfunction" means that fish find it difficult to get sufficient salts from the water and may have problems getting rid of excess water.

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