What is the osmotic activity of plasma and what contributes to it?

Osmotic Activity of Plasma

Plasma osmolality ranges from 275-295 mOsm/kg and is primarily determined by sodium salts (chloride and bicarbonate), glucose, and urea, with sodium being the dominant contributor. 1, 2

Primary Contributors to Plasma Osmolality

The osmotic activity of plasma is generated by dissolved solutes, with sodium and its accompanying anions (chloride and bicarbonate) accounting for the majority of osmotic pressure. 1, 3

• Sodium salts contribute approximately 90% of plasma osmolality, making sodium the single most important determinant 1, 4
• Glucose and blood urea nitrogen (BUN) contribute the remainder of measurable osmolality 1, 3
• Potassium, calcium, and magnesium are present but contribute minimally to total osmolality due to their low concentrations 1

Calculation of Plasma Osmolality

The most accurate and simplest formula for calculating plasma osmolality is:

2 × Na (mEq/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 1, 3

• This formula accounts for all major osmotically active particles in plasma 3
• The factor of 2 for sodium accounts for both the cation and its accompanying anion 3
• Normal calculated osmolality should yield values between 275-295 mOsm/kg 2, 4

Osmolality vs. Tonicity: A Critical Distinction

Osmolality measures all solutes, but tonicity (effective osmolality) only measures solutes that cannot freely cross cell membranes. 1, 2

• Urea contributes to osmolality but NOT to tonicity because it freely crosses cell membranes and does not cause water shifts 1, 2
• Effective osmolality (tonicity) is calculated as: 2 × Na + glucose/18, deliberately excluding urea 2, 4
• Tonicity determines actual fluid shifts between intracellular and extracellular compartments, making it more clinically relevant for assessing cellular hydration status 1, 2

Clinical Significance of Plasma Osmolality

Plasma osmolality >300 mOsm/kg indicates dehydration and is associated with increased mortality and doubled risk of 4-year disability. 2, 4

• Values >320 mOsm/kg define hyperosmolar hyperglycemic state (HHS) in diabetic emergencies 4
• Values <275 mOsm/kg indicate hyposmolality, suggesting overhydration or syndrome of inappropriate antidiuresis (SIAD) 2, 4
• Direct measurement of osmolality is superior to calculated values and carries a Grade B recommendation with 94% consensus from ESPEN guidelines 4, 5

Osmotic Pressure Relationship

Osmotic pressure in mmHg equals 19.3 times the osmolarity, assuming an ideal solution 3

• This relationship explains why small changes in osmolality can generate significant osmotic forces driving water movement across semipermeable membranes 3
• One mole of nonionized impermeant solute equals one osmole 3