Renal handling of calcium.

Ca absorption is mediated by both active and passive processes. Absorption in both proximal tubule and thick ascending limb is mainly coupled indirectly to Na absorption and is a passive process through the paracellular pathway. In the distal convoluted tubule, Ca absorption is regulated independently of Na absorption; this is the principal site of action of parathyroid hormone, calcitonin, and 1,25-(OH)2D3. Models for transport in proximal tubule, thick ascending limb, and distal convoluted tubule afford attractive explanations for the influence on Ca transport of effective arterial blood volume, parathyroid hormone, acid-base balance, genetic disturbances such as Bartter's and Gitelman's syndromes, and diuretic action.