Physiology
O
Osmoregulation
/ /ˌɒzməʊˌreɡjʊˈleɪʃ(ə)n/ /
Etymology:
From Greek + Latin osmos (push) + regulare (to rule)
Quick Definition
The physiological process by which fish maintain the balance of water and salts in their body fluids despite the external osmotic environment.
📖 Full Definition
Osmoregulation is the active regulation of osmotic pressure (water and ion balance) in body fluids. Freshwater fish face the challenge of being hyperosmotic to their environment — water constantly floods in through the gills and skin, and salts leak out. They compensate by producing large amounts of dilute urine and actively absorbing ions through gill chloride cells. Marine teleosts face the opposite problem — they are hypoosmotic to seawater, constantly losing water and gaining salt. They drink seawater and excrete excess ions through the gills. Elasmobranchs (sharks, rays) maintain near-isoosmotic balance with seawater by retaining urea and TMAO in their blood. Osmoregulation is energetically costly, consuming 20–40% of resting metabolic rate.
💡 Examples & Context
Atlantic salmon undergo a dramatic transformation called smoltification before entering the sea, switching from freshwater-type to seawater-type osmoregulation under hormonal control.
🧪 Scientific Usage
Plasma osmolality (mOsm/kg) and plasma chloride are standard clinical parameters for assessing osmoregulatory status in farmed fish.