Calcium (A), phosphate (B), and magnesium (C) balance is a complex process involving bone, intestinal absorption of dietary calcium, phosphate, and magnesium, and renal excretion of calcium, phosphate, and magnesium.

The kidneys play a central role in the homeostasis of these ions.

When body stores of these ions decline significantly, gastrointestinal absorption, bone resorption, and renal tubular reabsorption increase to normalize their levels.

Under physiologic conditions, the whole body balance of calcium, phosphate, and magnesium is maintained by fine adjustments of urinary excretion to equal the net intake.

This review discusses how calcium, phosphate, and magnesium are handled by the kidneys.

The kidney plays a critical role in regulating serum levels of these ions.

Regulation of calcium, phosphate, and magnesium occurs in different parts of the nephron and involves a number of different channels, transporters, and pathways.

Below we describe the mechanisms governing renal control of these ions.

Calcium The total amount of calcium in the human body ranges from 1000 to 1200 g.

Approximately 99 of body calcium resides in the skeleton; the other 1 is present in the extracellular and intracellular spaces.

Although 99 of the total body calcium is located in bone, calcium is a critical cation in both the extracellular and intracellular spaces.

Approximately 1 of the calcium in the skeleton is freely exchangeable with calcium in the extracellular fluid compartment.

Serum calcium concentration is held in a very narrow range in both spaces.

Calcium serves a vital role in nerve impulse transmission, muscular contraction, blood coagulation, hormone secretion, and intercellular adhesion ( 1, 2 ).

Gastrointestinal Absorption of Calcium Calcium balance is tightly regulated by the concerted action of calcium absorption in the intestine, reabsorption in the kidney, and exchange from bone, which are all under the control of the calciotropic hormones that are released upon a demand for calcium ( Figure 1A ).

In healthy adults, approximately 8001000 mg of calcium should be ingested daily.

This amount will vary depending on the amount of dairy product consumed.

When 1 g of calcium is ingested in the diet, approximately 800 mg is excreted in the feces and 200 mg in the urine.

Approximately 400 mg of the usual 1000 mg dietary calcium intake is absorbed by the intestine, and calcium loss by way of intestinal secretions is approximately 200 mgd.

Therefore, a net absorption of calcium is approximately 200 mgd (20) ( 3 ).

Although serum calcium levels can be maintained in the normal range by bone resorption, dietary intake is the only source by which the body can replenish stores of calcium in bone.

Calcium is absorbed almost exclusively within the duodenum, jejunum, and ileum.

Each of these intestinal segments has a high absorptive capacity for calcium, with their relative calcium absorption being dependent on the length of each respective intestinal segment and the transit time of the food bolus ( 3 ).

Calcium, phosphate, and magnesium flux between body compartments.

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