Total body calcium content is 20 to 25 g/kg lean body weight, which accounts for ~1.4% to 1.6% of total body weight. The normal range of serum [Ca2+] is 2.25 to 2.62 mmol/L (9-10.5 mg/dL). Slight interlaboratory variations may exist.
In the body, 98% of calcium is located in bones. The remaining 1% to 2% can undergo rapid mobilization; half of this is ionized (biologically active) calcium, and the rest is protein-bound (mainly albumin-bound). Alkalosis increases binding of calcium to proteins, thus decreasing the levels of ionized calcium; acidosis has the opposite effect.
Daily dietary calcium intake is ~1 g; ~30% of this is absorbed. The gastrointestinal absorption of calcium is increased by 1,25(OH)2D3 and parathyroid hormone—mediated by 1,25(OH)2D3—and decreased by oxalate, phosphate, and fatty acids as a result of binding in the intestinal lumen. From 98% to 99% of calcium filtrated in the glomeruli undergoes reabsorption in the renal tubules. Daily urinary calcium excretion is 3 to 5 mmol.
Intracellular and extracellular calcium are important in many enzymatic reactions and play a regulatory role in vital body functions (eg, blood coagulation, signal transmission in the nervous system, muscle contraction).