Glossary
Osmolality.
Direct answer
Osmolality states how many osmotically active particles are dissolved in one kilogram of solvent, expressed in osmoles per kilogram. This quantity determines in which direction water moves across a semi-permeable membrane, and therefore how fluid is distributed between cells and their surroundings.
Osmolality and osmolarity are not the same: the first refers to the mass of the solvent, the second to the volume of the solution. Only particles that do not cross the membrane freely count towards the effect. Urea, for instance, raises the measured value but shifts very little water between compartments.
In the horse, blood osmolality tends to stay low during prolonged exercise, because sweat removes an electrolyte such as sodium in large amounts along with the water. In humans it rises under the same conditions and triggers thirst earlier. The same measurement therefore tells a different story in each species.
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