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Overview of Disorders of Phosphate Concentration

By James L. Lewis, III, MD

Phosphorus is one of the most abundant elements in the human body. Most phosphorus in the body is complexed with O2 as phosphate (PO4). About 85% of the about 500 to 700 g of PO4 in the body is contained in bone, where it is an important constituent of crystalline hydroxyapatite. In soft tissues, PO4 is mainly found in the intracellular compartment as an integral component of several organic compounds, including nucleic acids and cell membrane phospholipids. PO4 is also involved in aerobic and anaerobic energy metabolism. RBC 2,3-diphosphoglycerate (2,3-DPG) plays a crucial role in O2 delivery to tissue. Adenosine diphosphate (ADP) and ATP contain PO4 and use chemical bonds between PO4 groups to store energy. Inorganic PO4 is a major intracellular anion but is also present in plasma. The normal serum inorganic PO4 concentration in adults ranges from 2.5 to 4.5 mg/dL (0.81 to 1.45 mmol/L). PO4 concentration is 50% higher in infants and 30% higher in children, possibly because of the important roles these PO4-dependent processes play in growth.

The typical American diet contains about 800 to 1500 mg of PO4. The amount in stool varies depending on the amount of PO4 binding compounds (mainly Ca) in the diet. Also, like Ca, GI PO4 absorption is enhanced by vitamin D. Renal PO4 excretion roughly equals GI absorption to maintain PO4 balance. PO4 depletion can occur in various disorders and normally results in conservation of PO4 by the kidneys. Bone PO4 serves as a reservoir, which can buffer changes in plasma and intracellular PO4.

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