Osmoles, osmolality and osmotic pressure: clarifying the puzzle of solution concentration

Nurses are routinely involved in the collecting and testing of urine and plasma, dialysis, the administration of intravenous fluids and the treatment of osmolar disorders, all of which require an understanding of solution concentration. This article discusses the various ways in which the concentration of solutions are stated, how they differ and why the different ways of expressing concentration are useful in human physiology. It also explains the similarities and differences between the terms used to describe solution concentration: tonicity, percentage concentration, density, specific gravity, molarity, osmolarity, osmolality and osmotic pressure. The terms osmolarity, osmolality and osmotic pressure appear routinely in textbooks used in undergraduate nursing courses but often are used incorrectly as synonyms. The usefulness and the appropriate context to use the different ways of expressing solution concentration is discussed. Osmolality (or osmolarity) should be used instead of osmotic pressure to describe the movement of water between compartments while the use of osmotic pressure should be reserved for situations where filtration and osmosis are operating together.