Editing Phosphorus (section)

===Bone and teeth enamel===
{{see also|Calcium metabolism}}
The main component of bone is [[hydroxyapatite]] as well as amorphous forms of calcium phosphate, possibly including carbonate. Hydroxyapatite is the main component of tooth enamel. [[Water fluoridation]] enhances the resistance of teeth to decay by the partial conversion of this mineral to the still harder material [[fluorapatite]]:{{r|Greenwood1997}}
:{{chem2|Ca5(PO4)3OH + F- -> Ca5(PO4)3F + OH-}}
An average adult human contains about {{convert|0.7|kg|lb}} of phosphorus, about 85–90% in bones and teeth in the form of [[apatite]], and the remainder in soft tissues and extracellular fluids. The phosphorus content increases from about 0.5% by mass in infancy to 0.65–1.1% by mass in adults. In comparison, average phosphorus concentration in the blood is about 0.4&nbsp;g/L; about 70% of that is organic and 30% inorganic phosphates.{{r|Bernhardt2008}}