Freshwater

Freshwater is a word that refers to bodies of water such as ponds, lakes, rivers and streams containing low concentrations of dissolved salts and other total dissolved solids. In other words, the word excludes salt water and brackish water.Freshwater is an important renewable resource, essential for the endurance of most terrestrial organisms, and is required by humans for drinking and farming, among several uses. The UN estimates that about 18 percent of the world's populace lacks access to safe drinking water. Freshwater can also be the output of desalinated seawater.

Freshwater creates a hypotonic environment for aquatic organisms. This is problematic for little organisms, whose cell membranes will burst if excess water is not excreted. Some protests accomplish these using contractile vacuoles, even as freshwater fish excrete overload water via the kidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity, fish have the ability to migrate between freshwater and saline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; these processes are hormonally controlled. The eel uses the hormone prolactin, while in salmon the hormone cortical plays a key position during this practice