Phosphorus is one of the most abundant elements in the human body. Most phosphorus in the body is complexed with oxygen as phosphate (PO4). About 85% of the roughly 500 to 700 g of PO4 in the body is contained in bone, where it is an important constituent of the crystal hydroxyapatite. In soft tissues, PO4 is mainly found in the intracellular compartment. It is an integral component of several organic compounds, including nucleic acids and the phospholipids of cell membranes. PO4 is also intimately involved in aerobic and anaerobic energy metabolism.