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Overview of Disorders of Phosphate Concentration

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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 O 2 as phosphate (PO 4 ). About 85% of the about 500 to 700 g of PO 4 in the body is contained in bone, where it is an important constituent of crystalline hydroxyapatite. In soft tissues, PO 4 is mainly found in the intracellular compartment as an integral component of several organic compounds, including nucleic acids and cell membrane phospholipids. PO 4 is also involved in aerobic and anaerobic energy metabolism. RBC 2,3-diphosphoglycerate (2,3-DPG) plays a crucial role in O 2 delivery to tissue. Adenosine diphosphate (ADP) and ATP contain PO 4 and use chemical bonds between PO 4 groups to store energy. Inorganic PO 4 is a major intracellular anion but is also present in plasma. The normal serum inorganic PO 4 concentration in adults ranges from 2.5 to 4.5 mg/dL (0.81 to 1.45 mmol/L). PO 4 concentration is 50% higher in infants and 30% higher in children, possibly because of the important roles these PO 4 -dependent processes play in growth.

The typical American diet contains about 800 to 1500 mg of PO 4 . The amount in stool varies depending on the amount of PO 4 binding compounds (mainly Ca) in the diet. Also, like Ca, GI PO 4 absorption is enhanced by vitamin D. Renal PO 4 excretion roughly equals GI absorption to maintain PO 4 balance. PO 4 depletion can occur in various disorders and normally results in conservation of PO 4 by the kidneys. Bone PO 4 serves as a reservoir, which can buffer changes in plasma and intracellular PO 4 .

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