The concentration terms for body fluids are not interchangeable: osmolarity, osmolality and tonicity are different measures.
What is an osmole?
An osmole is the number of moles of solute dissolved in a solution, adjusted for what the solute does in water.
For a non-dissociating solute, one osmole equals one mole.
A dissociating solute splits into several particles, so each mole gives more particles. Its osmole count is the number of moles multiplied by the number of particles formed. A solute that splits into two particles therefore contributes twice as many osmoles as moles.
How is osmolarity measured, and why is it not the clinical measure?
Osmolarity is the solute concentration per unit volume of solution.
Osmolarity is a colligative property. It follows the number of particles and does not depend on what the particles are. Only their count matters.
There is a weakness. Osmolarity is defined against volume, and volume changes with temperature and pressure. So osmolarity changes with them too.
Why do clinicians use osmolality instead?
Osmolality is the number of osmoles of solute per unit mass of solvent, per kg of solvent. It is colligative in the same way as osmolarity.
The difference is the reference. Osmolality rests on mass, and mass does not change with pressure or temperature. So osmolality stays stable when temperature and pressure change. Body fluids and their dissolved electrolytes are reported as osmolality.
How is plasma osmolality found?
Plasma osmolality can be calculated:
2 × [Na+] + [Glucose] + [Urea]
It can also be measured. The laboratory determines the freezing point depression of the plasma.
When do glucose and urea change the number?
In most people, glucose and urea contribute little to plasma osmolality. In diabetes mellitus and in reduced kidney function they contribute significantly.
Which solutes actually pull water?
Osmolality counts every particle, but not every particle moves water. 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. Only solutes that do not cross it count toward tonicity. These are the effective solutes: they are confined to one side of the membrane.
Can two solutions match in osmolality and still differ in effect?
Yes. The two measures include different solutes. A solution is iso-osmolar when its total osmolarity, effective and ineffective solutes together, equals that of body fluids. A solution is iso-tonic when its effective osmolarity equals that of body fluids. The first counts every solute; the second counts only those that stay on one side.
What does dextrose 5% do?
Dextrose 5% is iso-osmolar and hypotonic. Both statements hold at once.
Its total osmolarity matches that of body fluids, so it is iso-osmolar. Its solute is glucose, which crosses the cell membrane easily. So its effective osmolarity is lower than its total. The water in dextrose 5% remains in the blood and has a hypotonic effect.
Why is urea left out of tonicity?
Urea is a component of the plasma osmolality calculation. Urea is freely distributed across membranes. It is left out of the plasma tonicity formula.
What sets plasma tonicity?
Plasma tonicity is calculated as 2 × [Na+] + [Glucose].
This is the same glucose that crosses the cell membrane easily in dextrose 5%. The two statements are written as the facts give them, and the note does not reconcile them further.
Tonicity also depends on how the body’s water is divided into compartments, and on which solutes stay in each compartment. That raises the next question: how is body water divided between compartments, and what does that division do to tonicity? Total Body Water and Its Compartments answers it.