What are the three major electrolytes for osmolality (distinct from osmolarity)?

• A. Na, HCO3, Cl
• B. Na, K, Cl
• C. Na, Ca, Mg
• D. HCO3, Cl, PO4

Answer: (A)

Osmolality reflects the number of osmotically active particles per kilogram of water. In extracellular fluid, the dominant osmotic players are the sodium ion and its accompanying anions. Sodium sets the main extracellular osmotic pressure, while chloride and bicarbonate are the primary anions that balance charge and contribute substantially to the total osmolality. Together they account for most of the electrolyte-related osmotic effect. Potassium, calcium, and magnesium exist too, but at much lower extracellular levels, so their impact on osmolality is smaller. Glucose and urea also affect osmolality as non-electrolytes, but since the focus is on electrolytes, the three major contributors are sodium, chloride, and bicarbonate. Therefore, the trio Na, Cl, and HCO3 are the major electrolytes determining osmolality.