Our bones are the largest reservoirs of calcium, harboring most of the calcium in our bodies (about 99%) in the form of calcium hydroxyapatite. Calcium is also found in teeth, the circulatory system, extracellular fluid, muscles and other tissues. It plays crucial roles such as regulating the tone of the blood vessels, production of hormones, intracellular signaling (communication of information within body cells), muscle contraction, nerve communication, among other functions.
Because of this critical role played by calcium, its normal blood concentration is tightly controlled by the body to a level of about 2.15 to 2.55mmol/L. This control is achieved via two major biochemicals - vitamin D3 and parathyroid hormone (PTH) which control the absorption of calcium from the intestine, bones and kidneys as well as its excretion by the kidneys and the gastrointestinal tract. There is also calcitonin, a hormone which inhibits calcium release from bones, however to a lesser extent. It is this control that maintains the body’s normal calcium concentration via the balance of input and output.
However, disorders of calcium homeostasis may sometimes arise from a range of causes such as calcium deficiency in diet, malabsorption of calcium and vitamin D3, insufficiency or excess of PTH and vitamin D3, kidney damage, among other causes. These could result in diseases like osteomalacia, osteoporosis, hypocalcaemia, and hypercalcaemia, kidney stones, hypotension, heart dysfunction, tetany, seizures, and other diseases of calcium metabolism.
Depending on the underlying abnormality, various treatments exist which are targeted at different levels of calcium homeostasis (equilibrium).