In a healthy adult with a normal total serum calcium of 9.5 mg/dL, approximately what percentage of the circulating calcium exists as free, biologically active ionized Ca²⁺?
- A About 20 percent
- B About 50 percent ✓
- C About 35 percent
- D About 80 percent
Correct answer: B. About 50 percent
Explanation
Total plasma calcium divides into roughly three fractions: about 50 percent ionized (the physiologically active form sensed by the CaSR), about 40 percent bound to plasma proteins mainly albumin, and about 10 percent complexed to anions such as citrate and phosphate. Option D overestimates because a large share remains protein-bound, and option C underestimates the ionized fraction.
Reference: Ganong's Review of Medical Physiology, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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