Fluid Physiology Concepts
Osmolality Measurement and Effects
- Osmolality is measured as osmoles of solute per kilogram of solvent, representing the total concentration of all dissolved particles in a solution 1, 2
- Serum osmolality can be estimated using the formula: 2 × Na (mEq/L) + BUN (mg/dL)/2.8 + glucose (mg/dL)/18 1, 3
- Normal plasma osmolality ranges from 275-295 mOsm/kg 1
- Osmolality is a critical indicator of hydration status, with values >300 mOsm/kg indicating dehydration 4
- All solutes in a solution contribute to osmolality, including electrolytes, glucose, and urea 1
Relationship to Tonicity and Clinical Implications
- Tonicity (effective osmolality) relates to both the effect on a cell of a fluid and the osmolality of the fluid 1
- Unlike osmolality, tonicity only accounts for solutes that cannot freely cross cell membranes 1
- For example, urea affects osmolality but not tonicity because it moves freely across cell membranes 1
- The tonicity of intravenous fluids is primarily affected by sodium and potassium concentration 1
- Serum osmolality is used to assess hydration status 4