Renal Calcium Reabsorption

Renal calcium reabsorption is the process by which the kidneys return filtered calcium from the tubular fluid to the bloodstream. Plasma calcium that is bound to proteins is not readily filtered, whereas ionized and small-molecule-complexed calcium can pass through the glomerular filtration barrier. After filtration, the nephron reabsorbs most of this calcium, so normally only about 1% to 2% of the filtered amount is excreted in urine .¹

Reabsorption occurs at several points along the nephron. About 70% of filtered calcium is reabsorbed in the proximal tubule and roughly 20% in the thick ascending limb of the loop of Henle. In these regions, calcium moves mainly through the spaces between neighboring tubular cells, a route called paracellular transport. This movement is largely passive and is linked to the transport of sodium, water, and other ions, which establish conditions that favor calcium reabsorption .¹

A smaller fraction is reabsorbed in the distal nephron. Here calcium primarily takes a transcellular route, entering tubular cells through calcium channels such as TRPV5 and then leaving across the opposite membrane toward the blood. Although this region handles less calcium overall, it is important for fine control of how much calcium ultimately appears in urine .^1,2

Parathyroid hormone (PTH) is an important regulator of renal calcium handling. When ionized calcium falls, PTH secretion increases and acts at several nephron segments. Its effects vary by location, but overall PTH promotes calcium conservation, particularly by increasing calcium reabsorption in the thick ascending limb and distal nephron. Renal calcium reabsorption is distinct from intestinal calcium absorption: intestinal absorption transfers calcium from the digestive tract into the body, whereas renal reabsorption conserves calcium that has already entered the circulation and been filtered by the kidneys .^1,2