[Calcium and phosphorus homeostasis in the developmental population. Part I: homeostatic mechanisms in health].

Calcium and phosphorus form the major inorganic constituents of bone and play a vital role in cell function and cell metabolism. About 80% of total body P and 98% of total body Ca are in bone. Bone minerals exist in two physical forms: amorphous and cristalline. Serum Ca is found in three forms: approximately 40% is bound predominantly to albumin, approximately 10% is complexed with serum anions, and approximately 50% is ionized. The total P in serum can be divided into organic ester phosphate and inorganic phosphate. In the cell, phosphate is mostly in the form of organic phosphate. Parathyroid hormone is synthesized in the parathyroid gland. In physiological terms, PTH is the most important regulator of extracellular Ca concentration. Parathyroid hormone increases bone mobilization of Ca, increases renal Ca reabsorption, and possibly increases Ca absorption in the intestine. Increased serum Ca suppresses PTH production. Calcitonin is secreted from thyroid C cells. Calcitonin decreases Ca mobilization from bone and increases Ca excretion in the kidney. Ca stimulates the production of calcitonin. Vitamin D3 is synthesized in the skin from precursors. Vitamin D is catalyzed to 25(OH)D and it is metabolized to most active metabolite 1.25(OH)2D. Vitamin D through 1.25(OH)2D acts on the intestine to increase Ca absorption, mobilizes Ca from bone, and reabsorbs Ca in the kidney. The feedback loop might be completed by inhibition of PTH production by Ca, which in turn results in decreased production of 1.25(OH)2D.