Calcium Phosphate Regulation

Calcium supports neuromuscular transmission, myocardial contraction, coagulation, and bone mineralization. Phosphate is needed for ATP production, cell membranes, acid-base buffering, and bone. The body must keep both available for immediate cellular work without allowing excess calcium-phosphate deposition in tissues. Bone is the major storage site for both minerals. It can release calcium and phosphate into the circulation when extracellular calcium falls, but chronic mobilization weakens bone. The kidneys, intestines, parathyroid glands, and bone continuously coordinate this balance. Parathyroid hormone (PTH) rises when ionized calcium is low. It raises serum calcium by increasing bone resorption, increasing renal calcium reabsorption, stimulating calcitriol production, and increasing urinary phosphate excretion. PTH therefore raises calcium while lowering phosphate. Calcitriol, the active form of vitamin D, increases intestinal absorption of both calcium and phosphate. This supports bone mineralization when the minerals are available in appropriate amounts. Fibroblast growth factor-23 (FGF-23), produced largely by bone cells, rises as phosphate retention develops. It promotes phosphate excretion and suppresses calcitriol production. In early kidney disease, this response can temporarily maintain a normal phosphate value. The cost is less calcitriol,...