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 indirect ly to Na absorption and is a passive process through the paracellular pathw ay. In the distal convoluted tubule, Ca absorption is regulated independent ly of Na absorption; this is the principal site of action of parathyroid ho rmone, calcitonin, and 1,25-(OH)(2)D-3. Models for transport in proximal tu bule, 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 a s Bartter's and Gitelman's syndromes, and diuretic action.