Control of Plasma Osmolarity Quiz

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Which statement best describes the difference between osmolarity and osmolality?

A Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of solvent ✓
B Osmolarity is independent of temperature, whereas osmolality varies significantly with temperature
C Osmolarity counts only particles that cross cell membranes, whereas osmolality counts only those that cannot
D Osmolality is used exclusively in plasma, whereas osmolarity is used only for urine
E Osmolarity measures only electrolytes, whereas osmolality measures all solutes including glucose and urea

The answer is A: Osmolarity is osmoles per litre of solution; osmolality is osmoles per kilogram of solvent

Explanation

Why A is correct

See Plasma Osmolarity and Plasma Osmolality. Option A is correct. To understand the difference, start with the osmole, a unit that counts the number of particles in solution that contribute to its concentration. One mole of sodium chloride in solution gives two osmoles, because it separates into a sodium ion and a chloride ion. Osmolarity (written with an R) is the number of osmoles per litre of solution. Osmolality (written with an L) is the number of osmoles per kilogram of solvent (the liquid in which the solutes are dissolved, which in the body is water). A larger number for either means a more concentrated solution. In practice they give almost the same value for body fluids such as plasma, because a litre of water has a mass of about a kilogram, which is why the two terms are often used loosely, but they are not identical. The difference matters because a volume (litres) can change with temperature, whereas a mass (kilograms) does not, so osmolality is the more stable measure. Plasma osmolarity is the concentration that the hypothalamic osmoreceptors monitor.

Osmolarity: The number of osmoles per litre of solution.
Osmolality: The number of osmoles per kilogram of solvent.
In practice: The two values are almost the same in plasma, but osmolality is more stable because mass does not change with temperature.

Why the other options are incorrect

B. Osmolarity is independent of temperature, whereas osmolality varies significantly with temperature: This is reversed. The volume of a litre of solution changes with temperature, whereas the mass of a kilogram of solvent does not, so if either measure is sensitive to temperature it is osmolarity.
C. Osmolarity counts only particles that cross cell membranes, whereas osmolality counts only those that cannot: Both measures count all the osmotically active particles in the solution, whether or not they cross membranes. Counting only particles that cannot cross a membrane describes tonicity, which is a different concept.
D. Osmolality is used exclusively in plasma, whereas osmolarity is used only for urine: Both terms can be applied to any body fluid, including plasma and urine. The difference is that one is per litre and the other per kilogram, and not the fluid they are used for.
E. Osmolarity measures only electrolytes, whereas osmolality measures all solutes including glucose and urea: Both measures include all osmotically active particles, including glucose and urea as well as electrolytes. The difference between them is the unit of measurement, not the types of solute counted.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
Which of the following best describes the primary action of antidiuretic hormone (ADH) on the kidney in regulating plasma osmolarity?
2
Which change directly stimulates thirst through the brain's osmoreceptors?

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