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Osmolality

From Academic Kids

Osmolality, in biology and chemistry, is a measure of moles of solute per kilogram of solvent. The similar measurement osmolarity measures moles per liter of solvent. If the solvent is water, these measurements are practically equivalent.

A related non-SI unit is the osmole, which is equal to the number of moles of osmotically active particles in an ideal solution; e.g. a mole of glucose is one osmole, whereas a mole of sodium chloride is two osmoles (one mole of sodium and one mole of chlorine).

Measurement of osmolality

Osmolality can be measured with an osmometer by determining the fall in freezing point when compared to deionised water (which has an osmolality of 0 mol/kg). It can also be estimated from levels of specific solutes.

If there is a difference between the measured and estimated osmolalities, this may be because of another osmotically-active substance that was not accounted for.

Osmolality and health

Osmolality of blood increases with dehydration and decreases with overhydration. In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH). This will result in increased water reabsorption, more concentrated urine, and less concentrated blood plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

Normal osmolality in plasma is about 280 - 303 milli-osmoles per kilogram. This is contributed to mainly by sodium, chloride, potassium, urea, and glucose, and additionally by other ions and substances in the blood.de:Osmolaritšt fr:Osmolalitť

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