What fraction of total calcium is nondiffusible protein-bound?

• A. 80-90%
• B. 51% - 60%
• C. 40% - 50%
• D. 10% - 30%

Answer: (C)

The main concept is how calcium in plasma is partitioned into forms, with protein-bound calcium being the nondiffusible portion. Calcium binds to albumin and other proteins, which keeps this fraction from diffusing across capillary membranes; it is not readily available to tissues. In plasma, about half of total calcium is ionized (diffusible and physiologically active), a substantial portion is protein-bound, and the remainder is complexed with anions. Therefore, the nondiffusible protein-bound fraction is typically around 40-50% of total calcium. This is why the correct choice is the 40-50% range. Changes in albumin affect total calcium measurements without necessarily altering the ionized (free) calcium level.