Hypotonicity

Hypotonicity is the state in which a body fluid has a lower effective osmotic concentration, or tonicity, than the fluid on the other side of a cell membrane. In human physiology, the term commonly refers to reduced plasma tonicity. Because cell membranes are highly permeable to water, reduced extracellular tonicity causes water to move into cells, increasing cell volume .^1,2

Tonicity is determined by effective osmoles, solutes that do not readily cross the relevant cell membrane and can therefore sustain an osmotic gradient. Sodium and its accompanying anions are normally the major determinants of extracellular tonicity. In contrast, substances such as urea cross most cell membranes sufficiently readily that they contribute to measured plasma osmolality but have relatively little sustained effect on cell water distribution .^1,2

Hypotonicity is therefore not identical to low osmolality, or hypo-osmolality. Osmolality reflects the concentration of essentially all dissolved particles in a fluid, whereas tonicity considers the particles capable of producing sustained water shifts across cell membranes. A fluid can consequently have normal or elevated measured osmolality while having reduced tonicity if a substantial part of its osmolality is produced by ineffective osmoles .^1,2

Hypotonicity is also related to, but not synonymous with, hyponatremia. Most hyponatremia is hypotonic because sodium and its accompanying anions are major determinants of plasma tonicity, but a low measured sodium concentration can also occur with normal or increased tonicity in particular circumstances. It is reduced tonicity, rather than the sodium concentration itself, that drives water into cells and causes cellular swelling .¹