The total calcium content in normal human serum is quite constant, at 2.25-2.75 mmol/L, and is higher in children. Calcium in plasma and body fluids exists mainly in two forms: bound calcium and free calcium. The former is mainly bound to albumin, and a small amount is bound to organic acids, such as calcium citrate, calcium lactate, and calcium phosphate. Free calcium and bound calcium are constantly exchanged in a dynamic equilibrium, which is mainly affected by pH. In acidemia, free calcium (Ca2+) increases, while in alkalemia, the opposite is true. In addition, a certain product is maintained between blood calcium and blood phosphorus concentrations, that is, [Ca]×[P]=350-400 mg/L. Only free calcium truly has the physiological function of calcium. People with serum calcium below 2.2 mmol/L are called hypocalcemia.