Osmolality

Osmolality is a measure of the solute concentration of a solution, defined as the number of osmoles of solute per kilogram of solvent (mOsm/kg). It reflects the potential of solutes to draw water across semi-permeable membranes and to influence cell volume.

In physiological contexts, osmolality is often preferred over osmolarity because it depends on solvent mass rather than solution volume, and it is less affected by small volume changes. It is typically expressed in milliosmoles per kilogram (mOsm/kg). Normal human plasma osmolality ranges about 275–295 mOsm/kg.

Measurement can be direct, using an osmometer that relies on freezing point depression or vapor pressure, or calculated from concentrations of major solutes: calculated osmolality ≈ 2[Na+] + glucose/18 + BUN/2.8 (in mOsm/kg). The difference between measured and calculated osmolality is called the osmolar or osmolal gap and can indicate the presence of unmeasured osmoles such as alcohols or toxins.

Physiologically, osmolality is tightly regulated by water balance. Osmoreceptors in the hypothalamus detect changes and trigger thirst and antidiuretic hormone (ADH) release to conserve water and restore osmotic equilibrium. Hyperosmolality can cause cellular dehydration, while hypoosmolality can lead to cellular swelling.

Notes: the term osmolality refers to total osmotically active particles, whereas tonicity describes the effect on cell volume and depends on membrane permeability; some solutes (notably urea) can cross cell membranes and contribute to osmolality without changing tonicity.