GLOSSARY

Hypotonic describes a lower tonic effect than the reference, requiring the reference and relevant membrane to be specified.

Osmosis is net water movement across a water-permeable barrier in response to differences in solute effects. Particle count alone is insufficient: membrane permeability to each solute matters. Freely crossing particles have different sustained effects on cell volume from confined particles. This explains why measuring a solution's total particle concentration is not always enough to predict cellular swelling or shrinking.

Osmolality expresses osmotically active particles per mass of solvent; osmolarity expresses them per volume of solution. Reporting is commonly per kilogram and per liter respectively, so labels and units should not be exchanged. Values can be close in dilute fluids without making the definitions identical. Check what was measured or calculated before comparing a beverage label with a laboratory result.

For sports use, first identify the compartment and observation time; a concentration change is not directly converted into total body water or a drinking prescription.

Tonicity, or effective osmolality, concerns solutes that do not freely cross the relevant membrane over the time considered and thereby affect cell volume. Urea contributes to measured osmolality but generally lacks the sustained action of a confined solute. Iso-osmotic and isotonic are therefore not necessarily synonymous. The reference membrane and time frame are part of the interpretation.