Osmoregulation

Osmoregulation is the control of water and dissolved-solute balance in an organism or cell. It keeps internal osmotic conditions within ranges compatible with biological function.

Osmosis: The net movement of water across a selectively permeable membrane toward the side with higher effective solute concentration. Water moves across membranes according to osmotic gradients that organisms must counter or exploit.
Kidney: An organ that filters blood, removes wastes, and regulates water and electrolyte balance in vertebrates. Kidneys provide the central long-term control of water and ion excretion in many animals.
Tonicity: The ability of a solution to cause a cell to gain or lose water, determined by nonpenetrating solutes. Tonicity predicts cell-volume changes when cells encounter different external solutions.
Osmoconformer: An organism whose internal osmolarity remains close to that of its surrounding environment. Unlike osmoregulators, osmoconformers tolerate or track external osmotic conditions.
Osmotic pressure: The pressure required to prevent net water movement across a selectively permeable membrane because of a solute concentration difference. It describes the force that water gradients impose on cells and tissues.
Nephron: The microscopic functional unit of the kidney, consisting of a filtering corpuscle and a tubule. Its segmented tubule reabsorbs water and solutes while forming urine.
Osmolarity: The concentration of osmotically active particles per litre of solution. It quantifies particle concentration, a key determinant of water movement.
Osmoregulator: An organism that maintains internal osmotic conditions substantially independently of its environment. This is the organism-level strategy exemplified by active osmoregulation.
Aquaporin: A membrane channel protein that allows water to cross cell membranes rapidly. Aquaporins provide regulated routes for water movement across membranes.
Contractile vacuole: A membrane-bound organelle that collects and expels excess water from some freshwater protists. It prevents water entering by osmosis from swelling and rupturing the cell.

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