Renal handling of calcium.

More than 95% of the filtered calcium is reabsorbed. In the proximal convoluted tubule calcium absorption is passive. The pars recta transports calcium actively but little is known about its regulation. Transport of calcium in the thick ascending limb is both active and passive, and function of medullary and cortical segments is heterogeneous. Parathyroid hormone (PTH) stimulates calcium transport in the cortical segment. Calcium absorption in the distal convoluted tubule is active, and both chlorothiazide and PTH enhance it. Function of the granular portion of the collecting tubule resenbles the distal convoluted tubule. The light portion does not transport calcium.