The phosphate anion originates from multiple sources, and is responsible for some of the high anion gap metabolic acidosis associated with uraemia. Intracellularly, phosphate is the most numerous anion, available predominantly in the form of inorganic complexes (eg. in phospholipids, phosphoproteins and as a part of ATP). But this is all discussed elsewhere. Suffice to say that phosphate prefers to remain inside cells, and extracellular phosphate is merely in transit, on its way to a cell somewhere.