What is calcium homeostasis?
Calcium homeostasis refers to the regulation of the calcium concentration in the extracellular fluid. Normal serum calcium concentration varies between laboratories, but is usually 8.5 to 10.5 mg/dL (2.1 to 2.6 mmol/L) and it represents the sum of the three circulating fractions: 45% protein bound (albumin ∼80%, globulins ∼20%), 15% complexed to anions (citrate, bicarbonate, lactate, phosphate), and 40% free, or ionized. The ionized calcium is the physiologically active form, which is recognized by the calcium-sensing receptor (CaSR). The main hormonal regulators of ionized calcium are parathyroid hormone (PTH) and 1,25 dihydroxyvitamin D (1,25D).