Calcium transport along the nephron.
Figure 1. An overview of calcium homeostasis.
Calcium and phosphate homeostasis
I. David Weiner, M.D.
Calcium and phosphate play a central role in maintenance of the skeletal, cell signaling and
function and provision of energy, as ATP, to cells. Excessive blood calcium levels leads to nausea,
leads to precipitation with calcium in skeletal tissues, including the cardiac conduction system and
arteries, leading to failure of normal electrical conduction and vascular disease, respectively. Lack
of phosphate leads to ATP deficiency, due to an inability to form ATP from ADP, and complications
such as muscular weakness, respiratory failure and skeletal muscle death.
Normal calcium homeostasis reflects a balance between oral calcium intake, intestinal
excretion and urinary excretion. Figure 1 summarizes the major features of whole
Dietary calcium intake averages 1000 mg/d
and ~800 mg/d is excreted in the stool.
calcium absorption is regulated primarily by 1,25
, also known as calcitriol. Calcitriol
is formed through enzymatic hydroxylation of 25
by renal tubular cells; this process is
stimulated by parathyroid hormone (PTH).
Plasma calcium is ~35
40% protein bound,
and other anions,
and only ~50% is
“free” in solution,
and thus filterable at the glomerulus.
The primary long
term regulation of calcium balance is
through renal calcium excretion.
Figure 2 summarizes the
relative transport of calcium along the nephron.
The proximal tubule reabsorbs the majority of filtered
driven by the lumen positive voltage that develops as a result of
paracellular chloride reabsorption. A small amount of calcium
absorption occurs via transcellular routes.
Once again, the loop of Henle is a major site for ion