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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 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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