Calcium-Phosphate Regulation

Calcium and phosphate are linked because both are needed to mineralize bone, but their immediate priorities differ. The body protects ionized calcium tightly because it is required for cardiac conduction, skeletal muscle contraction, nerve transmission, and coagulation. Phosphate is essential for ATP, cell membranes, and bone, yet excess phosphate is difficult to handle when renal function declines. Parathyroid hormone (PTH) is released when ionized calcium falls. It raises serum calcium through three coordinated actions: - It increases calcium reabsorption in the distal nephron, reducing urinary calcium loss. - It decreases phosphate reabsorption in the proximal tubule, increasing phosphate excretion. - It promotes formation of active vitamin D in functioning kidneys and increases bone resorption through osteoblast-mediated activation of osteoclasts. The phosphate-wasting effect prevents calcium and phosphate from precipitating together while PTH restores circulating calcium. In healthy kidneys, this is an effective short-term correction. Vitamin D from diet or sunlight is inactive. The liver converts it to 25-hydroxyvitamin D, the form used to assess body stores. The kidneys then convert it to 1,25-dihydroxyvitamin D, or calcitriol, the active hormone. Calcitriol increases...