 calcium metabolism or calcium homeostasis is the mechanism by which the body maintains adequate calcium levels. derangements of this mechanism lead to hypercalcemia or hypocalcemia, both of which can have important consequences for health. in humans, when blood calcium level rises above a set point, the thyroid gland releases calcitonin, causing the blood calcium level to fall. when blood calcium level falls below a set point, the parathyroid gland releases parathyroid hormones (pth), causing the blood calcium level to rise. ==calcium location and quantity== calcium is the most abundant mineral in the human body. the average adult body contains in total approximately 1&nbsp;kg, 99% in the skeleton in the form of calcium phosphate salts. the extracellular fluid (ecf) contains approximately 22.5&nbsp;mmol, of which about 9&nbsp;mmol is in the plasma. approximately 500&nbsp;mmol of calcium is exchanged between bone and the ecf over a period of twenty-four hours. ==biological functions== calcium has several main functions in the body. calcium acts structurally as supporting material in bones as calcium phosphate. calcium is also involved in cellular signalling pathways. intracellular calcium functions as a second messenger in the secretion of many hormones and neurotransmitters. for instance, the influx of calcium into the neuron causes the release of acetylcholine from pre-synaptic terminals into the neural synapse. calcium also acts as an intracellular permeation regulator and mediator of muscle contraction. calcium acts in the contraction of muscles by removing the triosephosphate isomerase (tpi) subunit from myosin heads, which has atpase activity. calcium also acts as an enzyme cofactor for some clotting factors (enzymes) in the coagulation cascade and several proteins need calcium to proceed their function (ex: calmodulin), exocytosis of insulin. ==normal ranges== the plasma level of calcium is closely regulated with a normal total calcium of 2.2-2.6&nbsp;mmol/l (9-10.5&nbsp;mg/dl) and a normal ionized calcium of 1.1-1.4&nbsp;mmol/l (4.5-5.6&nbsp;mg/dl). the amount of total calcium varies with the level of serum albumin, a protein to which calcium is bound. the biologic effect of calcium is determined by the amount of ionized calcium, rather than the 
Calcium Endocrine_system Human_homeostasis Physiology