Disorders of Phosphate: Physiology

Abstract

Phosphate and phosphorus are generally used interchangeably. Phosphate, although not as abundant as Ca²⁺, is an important constituent of the body. It plays a significant role in mitochondrial respiration and oxidative phosphorylation. Phosphate constitutes approximately 1 % of the body weight. A 70-kg man contains about 700 g of phosphate. Of this amount, 85 % is present in bones and teeth, 14 % in soft tissues, and the remaining 1 % in the extracellular fluid. In biologic fluids, phosphate is measured as elemental phosphorus. However, the latter participates in biologic functions as phosphate. For example, it is phosphate that is filtered at the glomerulus or transported across the renal tubules. In plasma, the concentration of phosphorus is expressed as mg/dL, and in transport and other processes it is generally expressed as mEq or mmol/L. In plasma, phosphate exists as organic (70 %) and inorganic (30 %) forms. The inorganic form is physiologically active. Only 10 % of inorganic phosphate is bound to albumin. However, unlike Ca²⁺, phosphate concentration is not influenced by changes in plasma albumin concentration. At a pH of 7.40, inorganic phosphate exists predominantly as divalent phosphate (HPO₄ ²⁻) and monovalent phosphate (H₂PO⁴⁻) in the ratio of 4:1. However, at a pH of 6.8, which is the pK_a of this buffer pair, the ratio falls to 1:1. The concentration of intracellular phosphate is several-fold higher than the plasma concentration. Inside the cell, 75 % of phosphate exists as organic phosphate compounds such as adenosine triphosphate (ATP), creatine phosphate, and adenosine monophosphate. In red blood cells, it occurs predominantly as 2,3-diphosphoglycerate.