Osmoregulation is the homeostatic process by which organisms balance the forces of diffusion and osmosis that are always at play between their bodily fluids and their surrounding environment. One of the creatures that best helps illustrate the ideas behind osmoregulation are fish. Imagine the body of a fish in water like two solutions of different concentration separated only by a thin, permeable membrane. These two solutions will naturally be of two different salt concentrations; a fish is a living creature that needs salts in its body to survive. However, depending upon the chemical nature of the water in which they live, fish have developed means by which to maintain their salt reserves.
Salt ions will always move by diffusion from the stronger, more concentrated solution to the weaker one while water molecules move by osmosis towards the stronger solution in order to dilute it. Chemically, this process is one of equilibrium on both sides - a natural effort towards balance. In order for the fish to thrive in its environment, it has evolved in time to combat this process so that its internal salt balance is not disturbed. Depending upon the aquatic environment the fish lives in, this process will differ.
Ocean water has a significantly higher concentration of salts than the fluids within marine fish; therefore, there is a need to take in as much water as possible by the fish in order to combat the fluid lost into the concentrated salt water. Marine fish, particularly the bony fishes, do this by actually drinking huge amounts of water and excreting as little urine as possible. The large amounts of salt taken in through this process isn't absorbed by the fish's body, rather; it is expelled through chloride cells, cells in the gills that help to get rid of excess salts. Most cartilaginous fishes do not have the ability to drink large amounts of water and as such, they have developed ways of keeping the osmotic concentration of their blood slightly higher than the sea water around them. This eliminates the dangers of diffusion from the salt water into the fish's body.
Freshwater fish use a process precisely the opposite of marine fish. In freshwater, there is a concentration of salts that is lower relative to that within the fish's body. Due to this, water outside the fish flows in, potentially diluting salt concentrations in the fish. Freshwater fish have developed kidneys that expell water very fast. Loss of salts is further lowered by absorbing salts from the urine before it is excreted and the chloride cells in freshwater fishes' gills act to absorb salt from the water around them.
—Osmoregulation in Humans & Other Organisms—
I primarily focus on fish because it is one of the most important physiological processes of its life. However, osmoregulation applies not only to fish, but to all organisms. In human beings and other creatures, osmoregulation refers to the regulation of the salt and water content in the blood and tissues of the body. In humans, the hypothalamus regulates both body temperature and fluid balance. When the hypothalamus detects that there is a shortage of water available in the body, the pituitary gland releases anti-diuretic hormone (ADH) which stops the kidneys from releasing water from the body in large concentrations. If there is an excess of water in the body, more fluid is released by the kidneys.
In closing, osmoregulation is the primary means for all living creatures to maintain their fluid balance according to environmental and internal factors. This is one of the most basic, yet most critical processes for the survival of any creature.
Manual of Fish Health. Dr. Chris Andrews, Adrian Exell, Dr. Neville Carrington. Copyright © 2003 Firefly Books.
SeaLife. Ed. Geoffrey Walker. Copyright © 1996 Smithsonian Institution Press.