Phosphorus is an essential element for all living cells. It has a critical role as a high-energy phosphate bond in adenosine triphosphate (ATP) and other high-energy compounds.  These energy sources maintain many physiological functions such as muscle contractility, neurological function, and electrolyte transport. Biological membranes are made from a phospholipid matrix and proteins.

An average adult human contains a little less than 1 kg of phosphorus, about 85% of which is present in bones and teeth in the form of apatite, and the remainder inside cells in soft tissues. A well-fed adult consumes and excretes about 1-3 g of phosphorus per day, with consumption in the form of inorganic phosphate and phosphorus-containing biomolecules such as nucleic acids and phospholipids; and excretion almost exclusively in the form of urine phosphate ion. Only about 0.1% of body phosphate circulates in the blood, but this amount reflects the amount of phosphate available to soft tissue cells.

In medicine, low phosphate (hypophosphatemia) is caused by malnutrition, by failure to absorb phosphate, and by metabolic syndromes which draw phosphate from the blood or pass too much of it into the urine. Too much phosphate can lead to diarrhoea and calcification (hardening) of organs and soft tissue, and can interfere with the body's ability to use iron, calcium, magnesium, and zinc.

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