osmoregulation

Osmoregulation is the physiological process by which organisms regulate the water and electrolyte balance of their bodies to maintain osmotic homeostasis. It involves controlling the gain and loss of water and inorganic ions to keep body fluids at a relatively constant osmolarity, despite fluctuations in the external environment.

In vertebrates, the kidneys play a central role. Blood is filtered to form urine, which is then modified by tubular reabsorption and secretion. The kidney’s medullary gradient enables the production of concentrated or diluted urine, conserving water when needed. Hormones such as antidiuretic hormone (ADH or vasopressin) increase water reabsorption in the collecting ducts; aldosterone promotes sodium reabsorption and potassium excretion, helping adjust plasma osmolality and blood volume. Osmoreceptors in the hypothalamus sense plasma osmolarity and signal these responses, often via the renin–angiotensin–aldosterone system.

Organisms use different strategies to cope with osmotic challenges. Osmoregulators maintain a relatively constant internal osmolarity regardless of external changes, whereas osmoconformers adjust their internal osmolarity to match their surroundings. Freshwater fish are hyperosmotic to their environment and tend to gain water and lose salts, producing dilute urine and actively taking up salts through the gills. Marine fish are hypoosmotic and typically drink seawater, excreting excess salts through gill epithelia and kidneys. In insects, Malpighian tubules and rectal complexes manage water and ion balance, often by active transport to conserve water.

In plants, osmotic balance is linked to cell turgor and water relations, achieved through solute accumulation, water uptake, and regulation of transpiration. Osmoregulation thus encompasses strategies across life to sustain cellular function and organismal pressure–volume homeostasis.