Osmolarity

 

Osmolarity is the measure of solute concentration per unit VOLUME of solvent.

Its not the same as tonicity! Osmolarity takes into account ALL of the solute concentrations, not just the ones that cant cross the semipermeable membrane.

 

Osmolality

 

Osmolality is the measure of solute concentration per unit MASS of solvent.

You never measure osmolarity in practice, because water changes its volume according to temperature (but mass remains the same, and so it is more convenient and consistent)

 

Osmolality is the same in the ICF and the ECF.

Both inside and outside, the osmolality is 285-290 mOsm/Kg.

Stubbornly, in spite of the membrane being water-impermeable lipid, water moves across the cell membrane well enough; more on this later....

 

Tonicity

 

Tonicity is the measure of the osmotic pressure gradient between two solutions.

Unlike osmolarity, tonicity is only influenced by solutes that cannot cross this semipermeable membrane, because these are the only solutes influencing the osmotic pressure gradient.

Thus, you can have iso-osmolar solutions which are not isotonic.

 

Iso-osmolar solutions which are not isotonic: 5% dextrose and intracellular fluid

 

5 percent dextrose is isoosmolar but hypotonic

5% dextrose, when infused, is iso-osmolar with the body fluid compartments. Its osmolality is the same as the osmolality of the cellular contents (about 300mOsm/L) However, because dextrose penetrates the cells so easily, it cannot contribute to tonicity. Thus, the infused dextrose is iso-osmolar but hypotonic.

Thus, a solution can be iso-osmolar and hypotonic, when the solute contributing to its osmolality is not a solute which can contribute to its tonicity. These osmoles, which don't contribute to the tonicity, are derisively termed "ineffective osmoles" by Brandis. Dextrose and urea are the main ineffective osmoles. In a diabetic patient, dextrose can still be an effective osmole.

 

 

Dextrose and urea are the main ineffective osmoles.

 

In a diabetic patient, dextrose can still be an effective osmole.

The major "effective" osmole is SODIUM.
Sodium and its anion harem contribute to 86% of the osmolality and 92% of the tonicity.

 

 

References

References

anaesthesiamcq.org, as always;

 

My source for some of this information has been Ohs Manual (6th edition), specifically chapter 84 by Simon Finfer and Anthony Delaney.

 

Most of this information derives from easily accessible physiology textbooks, such as Ganongs Review of Medical Physiology 23rd edition.