Distribution of Calcium in the Body

Bone accounts for 98% of the calcium content in the body. Calcium in bone is present largely in the form of hydroxy-apatite crystals, which are relatively insoluble. One percent of the total body calcium is in a soluble form in extracellular and intracellular fluid compartments, and another 1% is freely exchangeable within extracellular fluid.8

Calcium in serum is present in three distinct fractions in equilibrium. Figure 45-1 graphically displays the approximate distribution of calcium in serum. The ionized and com-plexed calcium together make up the ultrafiltrable fraction. Ultrafiltrable calcium represents about 50% of the total serum calcium. Ionized calcium accounts for 90% of the ultrafiltrable calcium and about 45% of the total serum calcium.

Complexed calcium is that fraction of the ultrafiltrable component that is ionically bound to citrate, phosphate, and carbonate, which represents approximately 10% of the ultrafiltrable calcium.9 Protein-bound calcium dependent on pH and temperature represents about 50% of the total serum calcium. Eighty percent of the protein-bound calcium is bound to albumin and 20% is bound to globulin; therefore, hypoalbuminemia can significantly lower the total serum calcium. The total serum calcium is reduced 0.8 to 1.0 mg/dL for each l-g/100 mL reduction in the serum albumin.10

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