] + /18 + /2.8 where and are measured in mg/dL. To calculate plasma osmolality use the following equation (typical in the US): In medical lab reports, this quantity often appears as "Osmo, Calc" or "Osmo (Calc)." According to the international SI unit use the following equation :Ĭalculated osmolarity = 2 Na + Glucose + Urea (all in mmol/L)Īs Na+ is the major extracellular cation, the sum of osmolarity of all other anions can be assumed to be equal to natremia, hence x2 ≈ + Įlevation may be associated with stroke mortality. This ADH secretion may occur in excessive amounts from the posterior pituitary gland, or from ectopic sources such as small-cell carcinoma of the lung. Syndrome of inappropriate ADH secretion occurs when excessive release of antidiuretic hormone results in inappropriately elevated urine osmolality (>100 mOsmol/L) relative to the blood plasma, leading to hyponatraemia. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma. This will result in increased water reabsorption, more concentrated urine, and less concentrated blood plasma.
In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH). Osmolality of blood increases with dehydration and decreases with overhydration.
Many poisons, medications and diseases affect the balance between the ICF and ECF, affecting individual cells and homeostasis as a whole. If the ECF were to become too hypotonic, water would readily fill surrounding cells, increasing their volume and potentially lysing them ( cytolysis). This is important, as it shows that changes in ECF osmolality have a great effect on ICF osmolality - changes that can cause problems with normal cell functioning and volume. Therefore, plasma osmolality is a guide to intracellular osmolality. Clinical relevance Īs cell membranes in general are freely permeable to water, the osmolality of the extracellular fluid (ECF) is approximately equal to that of the intracellular fluid (ICF). 400 mOsm/L) that could be fatal to some mammals. Plasma osmolarity of some reptiles, especial those from a freshwater aquatic environment, may be lower than that of mammals (e.g. Normal human reference range of osmolality in plasma is about 275-299 milli-osmoles per kilogram. For this reason, both terms are often used interchangeably, even though they refer to different units of measurement. In practice, there is almost negligible difference between the absolute values of the different measurements. Therefore, bedside calculations are actually in units of osmolarity, whereas laboratory measurements will provide readings in units of osmolality. Osmometer ( freezing point depression osmometer, or vapor pressure depression osmometer)ĭerived from lab data that were measured in solutions (Na, Glu, Urea) Technically, the terms can be compared as follows: Origin
However, at low concentrations (below about 500 mM), the mass of the solute is negligible compared to the mass of the solvent, and osmolarity and osmolality are very similar. Otherwise, one litre of plasma would be equivalent to one kilogram of plasma, and plasma osmolarity and plasma osmolality would be equal. For a given solution, osmolarity is slightly less than osmolality, because the total solvent weight (the divisor used for osmolality) excludes the weight of any solutes, whereas the total solution volume (used for osmolarity) includes solute content. In contrast, osmolality is independent of temperature and pressure. Osmolarity is affected by changes in water content, as well as temperature and pressure. It works on the method of depression of freezing point. Osmolality can be measured on an analytical instrument called an osmometer.