**'''Fast Phase''' - This phase begins in minutes and progressively increases for hours. PTH acts on existing osteoblasts and osteocytes to increase calcium uptake from the ''bone fluid''. Nearby calcium phosphate crystals replace the calcium which has been removed from the bone fluid. | **'''Fast Phase''' - This phase begins in minutes and progressively increases for hours. PTH acts on existing osteoblasts and osteocytes to increase calcium uptake from the ''bone fluid''. Nearby calcium phosphate crystals replace the calcium which has been removed from the bone fluid. |