Of the total serum osmolality, sodium, chloride, and bicarbonate ions normally contribute approximately what percent?

Of the total serum osmolality, sodium, chloride, and bicarbonate ions normally contribute approximately what percent?

• A. 8
• B. 45
• C. 75
• D. 92

Answer: (D)

Most of serum osmolality comes from sodium salts. In the blood, the major osmotic particles are sodium with its accompanying anions, mainly chloride and bicarbonate. A handy way to see this is the approximate formula for osmolality: about 2 × [Na+] + [glucose]/18 + [BUN]/2.8. With normal sodium around 140 mEq/L, 2 × [Na+] is about 280 mOsm/kg, which is the large bulk of the total osmolality (roughly 275–295 mOsm/kg). The additional contributions from glucose and urea add only a small fraction, so the combined contribution of sodium, chloride, and bicarbonate is about 92% of the total. That’s why the correct choice is the highest percent.

Most of serum osmolality comes from sodium salts. In the blood, the major osmotic particles are sodium with its accompanying anions, mainly chloride and bicarbonate. A handy way to see this is the approximate formula for osmolality: about 2 × [Na+] + [glucose]/18 + [BUN]/2.8. With normal sodium around 140 mEq/L, 2 × [Na+] is about 280 mOsm/kg, which is the large bulk of the total osmolality (roughly 275–295 mOsm/kg). The additional contributions from glucose and urea add only a small fraction, so the combined contribution of sodium, chloride, and bicarbonate is about 92% of the total. That’s why the correct choice is the highest percent.