ABSTRACT
Calcium homeostasis is quite a complex process, but it is harmoniously regulated by hormonal and nonhormonal components of the blood and tissues. The main body organs involved in calcium metabolism are the parathyroid gland, the thyroid gland, and the kidneys where the parathyroid hormone, calcitonin, and calcitriol are synthesized, respectively. These three hormones mainly regulate the continuous processes of bone calcification and resorption, as well as the absorption of calcium in the intestines and its reabsorption and excretion in the kidneys. Calcium in the body occurs more than 99% in the skeleton of which the majority is nonlabile, while about 1% of it is freely exchangeable with extracellular fluid. The rest of the body calcium is found in the blood, in extracellular fluid, and in soft tissues. Metabolic balance and radioactive calcium in vivo studies are the major techniques that lead to some understanding of the various factors that affect the complex process of calcium absorption.
