Is plasma osmolality the same as plasma tonicity?
No! While plasma hypertonicity implies hyperosmolality, hyperosmolar plasma is not necessarily hypertonic. The effective osmolality, often termed plasma tonicity, denotes the concentration of osmoles in plasma that do not move freely across the cell membrane. Such osmoles can generate concentration gradients across cell membranes and in turn drive shifts in water between the extra- and intracellular compartments. While sodium and glucose as effective osmoles contribute to tonicity, urea and ethanol are ineffective osmoles as they freely cross cell membranes. High concentrations of these latter molecules confer hyperosmolality without affecting tonicity. In the absence of ethanol or other unexpected solutes, the effective osmolality is simply the measured osmolality minus blood urea nitrogen (BUN) (mg/dL)/2.8.