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Plasma osmolality is a measure of the concentration of substances such as sodium, chloride, potassium, urea, glucose, and other ions in human blood. It is calculated as the osmoles of solute per kilogram of solvent. Normal osmolality in plasma is about 280 - 303 milli-osmoles per kilogram. It is affected by changes in water content.

Contents 1 Composition 2 Regulation 3 Clinical Relevance 4 References

Plasma osmolality can be approximated with the following formula:

US units (sodium as mEq/l, BUN (Blood Urea Nitrogen) and glucose as mg/dl)

Plasma osmolality (mOsm/kg) = 2([Na+] + [K+]) + ([BUN]/2.8) + ([Glucose]/18)

SI units (all variables in mmol/l):

Plasma osmolality (mOsm/kg) = 2[Na+] + [Urea] + [Glucose][1]

Osmolality 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.

As 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). Therefore, plasma osmolality is a guide to intracellular osmolality. This is important, as it shows that changes in ECF osmololity have a great affect on ICF osmolality - changes that can cause problems with normal cell functioning and volume. If the ECF was to become too hypertonic, water would readily fill surrounding cells, increasing their volume and potentially lysing them (cytolysis).

1. ^ Worthley LI, Guerin M, Pain RW (1987) "For calculating osmolality, the simplest formula is the best" Anaesthesia and Intensive Care 15 2:199-202