Body fluid osmolality

Body fluid osmolality refers to the concentration of dissolved particles, primarily sodium, glucose, urea, and electrolytes, in blood, urine, or other fluids, measured in milliosmoles per kilogram (mOsm/kg). It is a colligative property critical for maintaining fluid homeostasis, where the body regulates it via antidiuretic hormone (ADH, or arginine vasopressin) released in response to osmoreceptor signals in the hypothalamus; high osmolality triggers ADH to promote renal water reabsorption and concentrate urine, while low osmolality suppresses ADH for dilution. Normal serum levels range from 275-295 mOsm/kg, with deviations signaling imbalances like dehydration (elevated) or overhydration (reduced). In disease, altered osmolality contributes to conditions such as diabetes insipidus, kidney dysfunction, hyperglycemia, or toxin exposures (e.g., methanol via osmolar gap). Though not a molecular target like receptors or enzymes, it is clinically modulated by osmotic agents like mannitol for cerebral edema or monitored in fluid disorders. Dysregulation poses therapeutic challenges, including risks of cell shrinkage, edema, or organ damage from rapid shifts.

Other names

Plasma osmolalitySerum osmolalityBlood osmolalityUrine osmolality

Osmotic diuretics alter osmolality to reduce brain swelling by drawing water out of tissues