Osmotic concentration

[2] This value allows the measurement of the osmotic pressure of a solution and the determination of how the solvent will diffuse across a semipermeable membrane (osmosis) separating two solutions of different osmotic concentration.

A microosmole (μOsm) (also spelled micro-osmole) is one millionth of an osmole.

Both sodium and chloride ions affect the osmotic pressure of the solution.

[2] [Note: NaCl does not dissociate completely in water at standard temperature and pressure, so the solution will be composed of Na+ ions, Cl- ions, and some NaCl molecules, with actual osmolality = Na+ concentration x 1.75]

[2] The osmolarity of a solution, given in osmoles per liter (osmol/L) is calculated from the following expression:

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.

Plasma osmolarity, the osmolarity of blood plasma, can be calculated from plasma osmolality by the following equation:[4] where: 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).

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.

Similarly, it is said to be hypoossmolar if the osmolarity, or osmatic concentration, is too low.

For example, if the osmolarity of parenteral nutrition is too high, it can cause severe tissue damage.