II. Definitions
- Osmolality
- Total solute concentration in a fluid compartment
- Tonicity
- Concentration of osmoles in a compartment that generate osmotic pressure gradient
- Tonicity impacts water traversal from one compartment to another
- Only solutes that cannot move freely between compartments generate an osmotic gradient
- Blood Urea Nitrogen (BUN) does not affect tonicity, but does influence osmolality
- BUN crosses easily between compartments and does not generate an osmotic gradient
- Extracellular Fluid Tonicity is based on only a few effective osmoles (of which Sodium is key)
- Sodium concentration (primary)
- Glucose (severe Hyperglycemia in Diabetes Mellitus)
- Mannitol or Sorbitol (used to decrease Intracranial Pressure in closed Head Trauma)
III. Interpretation: Normal
- Serum Osmolality: 280-300 mOsm/(kg water)
IV. Calculation: Calculated Serum Osmolality
- Serum Osms = 2*Na + Glu/18 + BUN/2.8 + Et/4.6 + Man/18
- Annotation
- Where Na is Serum Sodium
- Where Glu is Serum Glucose
- Where BUN is Blood Urea Nitrogen
- Where Et is Blood Alcohol Level
- Where Man is Mannitol
V. Causes: Increased Serum Osmolality
VI. Causes: Decreased Serum Osmolality
- SIADH
- Hyponatremia
- Overhydration