Making sense of any electrolyte or water imbalance begins with the concentration terms used for body fluids, because they are not interchangeable.
An osmole is the number of moles of solute dissolved in a solution. For a solute that does not dissociate, the osmole equals the mole; for a solute that dissociates during dissolution, the osmole equals the mole multiplied by the number of particles formed, so a solute that splits into two particles contributes twice as many osmoles as moles.
Osmolarity measures the solute concentration per unit of solution volume. It is a colligative property: it does not depend on the nature of the particle, only on the number of particles. Because it is defined against a volume, osmolarity changes with the temperature and pressure of the solution.
Osmolality measures the number of osmoles of solute dissolved in a unit of solvent mass — per kilogram of solvent. It is colligative in the same way, but because it rests on mass, and mass does not change with pressure or temperature, osmolality stays stable when temperature and pressure change. That is the reason body fluids and the electrolytes dissolved in them are reported as osmolality. The reported value can be calculated from the formula
or measured from the freezing point depression of plasma, determined in the laboratory.
Glucose and urea contribute little to plasma osmolality in most people, but their contribution becomes significant in diabetes mellitus and in reduced kidney function.
Tonicity, or effective osmolality, is the part of osmolality that actually moves water. Osmotic pressure is generated only by solutes that do not cross the semi-permeable membrane, so only those solutes count toward tonicity. This makes the two pairs of terms easy to confuse:
- An iso-osmolar solution has a total osmolarity — effective and ineffective solutes together — equal to that of the body fluids.
- An iso-tonic solution has an effective osmolarity equal to that of the body fluids.
Dextrose 5% shows why the distinction matters: it is iso-osmolar and hypotonic. Its total osmolarity matches the body fluids, but its solute is glucose, which crosses the cell membrane easily, so its effective osmolarity is lower. The water in this solution therefore remains in the blood and has a hypotonic effect.
Urea is one of the components of the plasma osmolality calculation, but it is freely distributed across membranes, so it is left out of the plasma tonicity formula:
Effective solutes are the ones confined to one side of a membrane, so tonicity depends on how the body’s water is divided into compartments and on which solutes stay in each.