Cedars-Sinai Medical Center, Los Angeles, California. Body Distribution Calcium Calcium is the most abundant mineral in the body.
Serum calcium exists in three separate fractions which are in dynamic equilibrium.
Calcium is also found complexed to a number of anions, such as citrate, phosphate, bicarbonate, and sulfate. It is this form which represents the physiologically active form of calcium. The serum concentration of calcium varies significantly during the immediate neonatal period. In general, the serum calcium concentration decreases over the first days of life, followed by a gradual increase to adult concentrations by the second or third week of life. The plasma concentrations of inorganic phosphate in the neonatal period are maintained at concentrations greater than those of the adult.