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Osmolarity)
Osmolality, in biology and chemistry, is a measure of moles of solute per kg of water. (Osmolarity is similar but measures moles per litre of solvent.) Osmolality increases with dehydration and decreases with overhydration.
In normal people, increased osmolality in the blood will stimulate secretion of ADH (antidiuretic hormone). This will result in increased water reabsorption, more concentrated urine, and less concentrated plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.
Normal osmolality in blood plasma is about 385 mosm/L, this is contributed to mostly by sodium, chloride, potassium, urea and glucose, as well as other ions and substances in the blood.
An increase in any one of these osmotically active compounds will cause an increase in osmolality of the blood. For instance, a diabetic whose glucose is 25 mmol/L instead of the normal 5 mmol/L, will have an osmolality 20 mosm/L higher than normal.
Measurement of osmolality
Osmolality can be measured by determining the change in freezing point compared to deionised water (which has an osmolality of 0 mosm/L). It can also be calculated from levels of specific solutes measured in the blood.
If there is a difference between measured and calculated osmolality, this may be because of an unknown osmotically-active substance present in the blood.
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