Osmoregulation

Water moves across selectively permeable membranes in response to differences in solute concentration, a process called osmosis. Cells may gain or lose water when their surroundings are respectively more dilute or more concentrated than their internal fluid. Osmoregulation comprises mechanisms that manage water and solutes, including electrolytes such as sodium and potassium, to keep internal conditions within suitable ranges. The processes also interact with waste removal and, in vertebrates, fluid volume and blood pressure.

Osmoregulatory strategies vary across animals. An osmoregulator maintains internal osmotic conditions within a range even when the surrounding medium differs, whereas an osmoconformer's internal osmotic concentration more closely follows that of its environment. These are broad descriptions, not a complete account of an animal's physiology: species can regulate some solutes more actively than others, and tolerance depends on the particular species and conditions.

In humans and other mammals, the kidneys filter blood and adjust how much water and solute are reabsorbed or excreted. Water intake, partly driven by thirst, and renal water loss are coordinated with the body's needs. Vasopressin, also known as antidiuretic hormone (ADH), is released from the posterior pituitary and promotes water reabsorption in the distal nephron. The renin–angiotensin–aldosterone system also influences blood volume and sodium handling. Together, these mechanisms help stabilize body-fluid composition; they do not operate independently of other physiological controls.

The surrounding water creates different challenges for freshwater and marine fish. Freshwater is generally more dilute than a fish's body fluids, so water tends to enter and salts can be lost. Many freshwater bony fish compensate by producing large amounts of dilute urine and actively taking up ions, including across the gills. Marine bony fish face water loss to the more concentrated seawater; many drink seawater, absorb water in the gut, and excrete excess salts, including through the gills. These patterns are examples rather than universal rules: different fish lineages use distinct strategies, and some species can adjust when moving between salinities.