Calcium is the fifth most abundant mineral on Earth and is the top most abundant element in the human body. It is essential in the biosphere for all living beings on Earth. Calcium is one of the body's main electrolyte minerals. Over ninety per cent of the calcium in the body is stored in the bones and teeth, which function as reservoirs which calcium can be withdrawn as required for extra-skeletal functions. When combined with phosphate, a substance called hydroxyapatite is formed. Hydroxyapatite is the mineral portion of bones and teeth, in humans and animals, and some coral.