Plasma osmolality


Larger numbers indicate a greater concentration of solutes in the plasma.

Osmolality can be measured on an osmometer, on the method of depression of freezing point.

Osmolarity is affected by changes in water content, as well as temperature and pressure. 

Osmolality is independent of temperature and pressure. 

For a given solution, osmolarity is slightly less than osmolality.

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. 

One  liter of plasma would be equivalent to one kilogram of plasma, and plasma osmolarity and plasma osmolality would be equal. 

At  low concentrations the mass of the solute is negligible compared to the mass of the solvent, and osmolarity and osmolality are very similar.

Technically, the terms can be compared as follows:

osmolality mOsm/kg

osmolarity mOsm/L

Bedside calculations are actually in units of osmolarity, whereas laboratory measurements will provide readings in units of osmolality. 

There  is almost negligible difference between the absolute values of the different measurements: both terms are often used interchangeably, even though they refer to different units of measurement.

Normal reference range of osmolality in plasma is about 275-299 milli-osmoles per kilogram.

In  general , the osmolality of the extracellular fluid (ECF) is approximately equal to that of the intracellular fluid (ICF). 

Plasma osmolality is therefore a guide to intracellular osmolality. 

Changes in ECF osmolality have a great effect on ICF osmolality, and can cause problems with normal cell functioning and volume. 

A too hypotonic ECF water would readily fill surrounding cells, increasing their volume and potentially lysing them.

Many medications, diseases and poisons affect the balance between the ICF and ECF, affecting individual cells and homeostasis as a whole.

The osmolality of blood increases with dehydration and decreases with overhydration. 

Normally an increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH), resulting  in increased water reabsorption, more concentrated urine, and less concentrated blood plasma. 

Oppositely, a low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

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. 

In addition, ADH secretion may occur in excessive amounts from the posterior pituitary gland, or from ectopic sources such as small-cell carcinoma of the lung.

Calculated osmolarity = 2 Na + Glucose + Urea (all in mmol/L)

As Na+ is the major extracellular cation, the sum of osmolarity of all other anions can be assumed to be equal to natremia, hence [Na+]x2 ≈ [Na+] + [anions]

To calculate plasma osmolality 2[Na+] + [Glucose]/18 + [ BUN ]/2.8 where [Glucose] and [BUN] are measured in mg/dL.

The osmolar gap is the difference between the measured osmolality and the calculated osmolarity. 

Clinically the osmolar gap is used to detect the presence of an osmotically active particle that is not normally found in plasma, usually a toxic alcohol such as ethanol, methanol or isopropyl alcohol.

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