All of the following statements regarding calcium are true except:
Calcium is the major extracellular divalent cation, which serves as a second messenger for the activity of many hormones. In the plasma, 45% of calcium is ionized, and the one that is available for exerting physiological effects and when this fraction is reduced it produces hypocalcemic symptoms. Regulation of the extra-cellular calcium is under tight endocrine control and its intestinal absorption efficiency is inversely proportional to calcium intake so that a diet low in calcium leads to a compensatory increase in fractional absorption, due to activation of vitamin D. More than 98% of filtered calcium is reabsorbed in the tubules, which is regulated by parathyroid hormone.
It serves as a second messenger for the actions of many hormones.
Intestinal calcium absorption efficiency is directly to calcium intake.
Tubular reabsorption of calcium is regulated by parathyroid hormone.
Ionized calcium in the plasma exerts physiological effects.