In calcium metabolism, which is prioritized to maintain physiologic serum levels?

• A. Serum calcium is prioritized over bone calcium
• B. Bone calcium is prioritized over serum calcium
• C. Vitamin D status determines the priority
• D. Phosphate balance determines the priority

Answer: (A)

The essential idea is that maintaining the extracellular serum calcium concentration within a very narrow range is crucial for normal physiology, because calcium powers nerve impulses, muscle contraction, blood clotting, and many enzymatic processes. Because of this, the body treats serum calcium as the priority and uses bone as a reservoir to keep it within range. When serum calcium falls, parathyroid hormone drives bone resorption, increases renal calcium reabsorption, and, through vitamin D, boosts intestinal absorption, all to raise serum calcium. When calcium is high, calcitonin can dampen bone resorption and promote deposition of calcium into bone. Vitamin D status influences how readily calcium is absorbed from the gut, aiding the maintenance of serum levels, but the hierarchy remains: serum calcium is kept within its normal range, even if it means mobilizing calcium from bone.