Define Mobilization of bone calcium and phosphorous?

It is now firmly established that vitamin D₃ is metabloized first in the liver to 25-hydroxyvitamin D (25 OH- D) (calcicliol) and subsequently in the kidneys to I. 25 dihydroxycholecalciferol  or 1.25 (OH)2 D (calcitriol to produce a biological active hormone. The functions of vitamin D are mediated by this vital vitamin D hormone by a homeostatic mechanism which involves the hormone acting on the intestines kidney and bone to increase serum calcium and phosphorus levels. 1.25 (OH)2D stimulates intestinal absorption of calcium and  phosphate and mobilized calcium and phosphate by stimulating bone resorption. The functions serve the common purpose of rstnring blood levels of calcium and phosphate to normal when concentrations of the two ions are low. This helps to achieve a normal blood calcium concentration and maintenance of calcium homeostasis.

In calcium homeostasis, 1, 25-(OH) D works in conjunction with pa athyroid hormone (PTH) to produce its beneficial effects on the plasma levels of ionized calcium and phosphate. The physiologic loop starts with the calcium receptor of the parathyroid gland. When the level of ionized calcium in plasma falls, PTH is secreted by the parathyroid gland and stimulates the tightly regulated renal enzyme 25-OH-D-I-α-hydroxylnse to make more 1, 25-(OI-I)₂D from the' large circulating  ,pool of 25-OH-D. The resulting increase in 1, 25-(OH)2D (with the rise in PTH)  causes an increase in calcium transport within the intestine, bone and kidney. All these events raise plasma calcium levels back to normal, which in turn is sensed by the calcium receptor of the parathyroid gland. The further secretion of PTH is turned off not only by the feedback action of calcium, but also by a short feedback loop involving 1, 25-(OH)₂D directly suppressing PTH synthesis in the parathyroid  gland.