Plasma osmolality

Overview
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.

Composition
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+ K] + [Urea] + [Glucose]

Clinical Relevance
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 hypotonic, water would readily fill surrounding cells, increasing their volume and potentially lysing them (cytolysis).