Distribution and Transport
Approximately 2% of the adult human body is calcium, and 1% is phosphorus (about 1200 g and 670 g in a 70-kg man, respectively); 99% of the calcium and 80% of the phosphorus in our bodies are found as constituents of bone and teeth, giving them strength and rigidity. Because of this common role and their many interactions in the body, calcium and phosphorus are considered together in most discussions. Yet it is important to recognize that their roles outside the skeleton are distinct and no less essential than their function in the bone.
In an adult, approximately 10 g of calcium are found in the extracellular fluids and soft tissues. In body fluids calcium exists in three forms: as ionized calcium (CaH), which is the physiologically active form; as a complex with organic or inorganic acids, such as calcium citrate, calcium phosphate, and calcium sulfate; and as proteinbound calcium. The first two forms are known as dijjUsible or ultra filterable fractions because they can move from one fluid compartment to another and are filtered by the renal glomeruli, in contrast to the nondiffusible protein-bound fraction.
A relatively large proportion of phosphorus outside the bone is present in the soft tissues as organic phosphates. It is an important component of membrane lipoproteins, cellular nucleic. adds, high-energy compounds, and other substances important in cellular metabolism. Soluble phosphate ions also contribute a sizable fraction of the intracellular anions.
About half of plasma phosphorus circulates as filterable phosphate ions (HP04 = and H2P04 -) and about one-third occurs as complexes with such cations as sodium, calcium, and magnesium; the remaining phosphorus is protein-bound.
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