Physiology · Cell and membrane
Cell volume regulation !
- Regulation
- For completeness
Cells guard against swelling or shrinking by moving ions and small molecules out or in, and water follows them.
Updated 2026-11-01
Also called: Regulatory volume decrease, Organic osmolytes
What it is
Cells hold many proteins and anions that cannot leave and that attract water; without the Na⁺/K⁺-ATPase keeping Na⁺ out, they would swell and burst. When the surrounding tonicity changes, a cell first swells or shrinks like an osmometer and then corrects its volume by moving K⁺, Cl⁻ and organic osmolytes. The brain is where this adaptation matters most, because it is enclosed by the skull.
How it works, step by step
- Extracellular tonicity falls (for example, hyponatraemia from SIADH).
- Water enters the cell and it swells within minutes.
- Stretch opens K⁺ and Cl⁻ channels; the ions leave and water follows them out.
- Over hours to days the cell also exports organic osmolytes (taurine, glutamate, myo-inositol) and reaches a near-normal volume with fewer solutes.
- If tonicity then rises quickly, the adapted cell loses water abruptly and shrinks.
Regulation
- +Na⁺/K⁺-ATPase: Pumps Na⁺ out and prevents swelling from trapped anions.
- −Ischaemia or lack of ATP: Stops the pump: Na⁺ enters with water and the cell swells (cytotoxic oedema).
- +Sustained hypertonicity: Induces ion uptake and osmolyte synthesis that restore volume.
- −Sustained hypotonicity: Opens K⁺ and Cl⁻ channels and releases osmolytes; volume returns towards normal.
Clinical notes
- Acute hyponatraemia (hours): cerebral oedema with headache, vomiting and seizures; the brain has not yet shed osmolytes.
- Too-rapid correction of chronic hyponatraemia: adapted neurons dehydrate and osmotic demyelination appears days later.
- Hypernatraemia: neurons shrink and bridging veins may tear; the brain makes osmolytes, so lowering Na⁺ must also be gradual.
Test yourself
Think of the answer, then tap the question to see it.
Why is hyponatraemia that develops over hours more dangerous to the brain than one over weeks at the same value?
Within hours neurons have not exported osmolytes and swell inside the skull; over weeks they have shed solutes and their volume is near normal.
Why does ischaemia make cells swell?
Without ATP the Na⁺/K⁺-ATPase stops; Na⁺ enters and, with intracellular anions trapped, water follows: cytotoxic oedema.
What explains osmotic demyelination after raising Na⁺ too fast?
Adapted cells hold fewer solutes; when tonicity rises abruptly they lose water suddenly and oligodendrocytes are injured.
Related topics
- Osmosis, osmolarity and tonicity
- Body fluid compartments
- Primary and secondary active transport
- IV fluids: tonicity and where they go
- Cerebral blood flow, brain metabolism and intracranial pressure
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 3.1 The Cell Membrane (OpenStax (Rice University), 2022); 26.2 Water Balance (OpenStax (Rice University), 2022); Low blood sodium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 4). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.