Define Fraction in which Calcium in plasma is Present?

Calcium in plasma is present in three major fractions:

• Ionized calcium
• Protein bound calcium
• Complexed calcium

The ionized calcium that constitutes 50% of the total plasma calcium is the only biologically active fraction. Of the remaining half 40% of the plasma calcium is protein bound, 80% being bound to albumin and 20% to globulin. Although the protein bound calcium, does not participate in the biological actions, it serves as a ready reservoir of ionized calcium. When plasma calcium levels fall, the first line of defense against hypocalcemia is the dissociation of the calcium from the protein complexes to make the cation Ca⁺⁺ available.

About 8% of plasma calcium is present as complexes with organic phosphate. This has little importance as a reservoir of ionized calcium. The regulation of plasma calcium is finely controlled by three sets of hormones and these are crucial for maintaining plasma level constant in the long term. The normal range for total serum calcium is very narrow i.e. 2.0-2.5 millimoles per litre (9-10 mg/dl) when plasma protein concentrate are normal, total plasma calcium less than 2.2 millimoles/litre reflects hypercalcemia. Let us read a little further about the regulation of extra and intracellular levels of calcium. We will first talk about plasma calcium concentration.