Osmotic concentration
Osmotic concentration, formerly known as osmolarity,[1] is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L (pronounced "osmolar"), in the same way that the molarity of a solution is expressed as "M" (pronounced "molar"). Whereas molarity measures the number of moles of solute per unit volume of solution, osmolarity measures the number of osmoles of solute particles per unit volume of solution.Cite error: A <ref> tag is missing the closing </ref> (see the help page).[2]
Penetrating solutes can diffuse through the cell membrane, causing momentary changes in cell volume as the solutes "pull" water molecules with them. Non-penetrating solutes cannot cross the cell membrane; therefore, the movement of water across the cell membrane (i.e., osmosis) must occur for the solutions to reach equilibrium.
A solution can be both hyperosmotic and isotonic.[2] For example, the intracellular fluid and extracellular can be hyperosmotic, but isotonic – if the total concentration of solutes in one compartment is different from that of the other, but one of the ions can cross the membrane (in other words, a penetrating solute), drawing water with it, thus causing no net change in solution volume.
Plasma osmolarity vs. osmolality
Plasma osmolarity can be calculated from plasma osmolality by the following equation:[3]
Osmolarity = osmolality * (ρsol − ca)
where:
- ρsol is the density of the solution in g/ml, which is 1.025 g/ml for blood plasma.[4]
- ca is the (anhydrous) solute concentration in g/ml – not to be confused with the density of dried plasma
According to IUPAC, osmolality is the quotient of the negative natural logarithm of the rational activity of water and the molar mass of water, whereas osmolarity is the product of the osmolality and the mass density of water (also known as osmotic concentration).[1]
In simpler terms, osmolality is an expression of solute osmotic concentration per mass of solvent, whereas osmolarity is per volume of solution (thus the conversion by multiplying with the mass density of solvent in solution (kg solvent/litre solution).
where mi is the molality of component i.
Plasma osmolarity/osmolality is important for keeping proper electrolytic balance in the blood stream. Improper balance can lead to dehydration, alkalosis, acidosis or other life-threatening changes. Antidiuretic hormone (vasopressin) is partly responsible for this process by controlling the amount of water the body retains from the kidney when filtering the blood stream.[5]
See also
References
- D. J. Taylor, N. P. O. Green, G. W. Stout Biological Science
- ^ a b McNaught, A. D.; Wilkinson, A.; Chalk, S. J. (1997). IUPAC. Compendium of Chemical Terminology (the "Gold Book") (2nd ed.). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. Retrieved 23 January 2022.
- ^ a b Cite error: The named reference
Widmaierwas invoked but never defined (see the help page). - ^ Page 158 in:Martin, Alfred N.; Patrick J Sinko (2006). Martin's physical pharmacy and pharmaceutical sciences: physical chemical and biopharmaceutical principles in the pharmaceutical sciences. Phila: Lippincott Williams and Wilkins. ISBN 0-7817-5027-X. [1]
- ^ Shmukler, Michael (2004). Elert, Glenn (ed.). "Density of blood". The Physics Factbook. Retrieved 2022-01-23.
- ^ Earley, LE; Sanders, CA (1959). "The Effect of Changing Serum Osmolality on the Release of Antidiuretic Hormone in Certain PAtients with Decompensated Cirrhosis of the Liver and Low Serum Osmolality". Journal of Clinical Investigation. 38 (3): 545–550. doi:10.1172/jci103832. PMC 293190. PMID 13641405.
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