Plays key role in many fundamental biological processes. Normal blood clotting, maintains membrane potential (with na+ & k+), excitability of nervous tissues, muscle contraction and release of hormones & neurotransmitters. 50% of calcium in plasma is bound to albumin, other 50% is free. 99% of calcium in body is found in bones, which act as calcium reservoirs. Concentration is maintained by three key hormones and their different actions: parathyroid hormone, calcitonin & vitamin d. Calcitonin: antiparathyroid hormone: decreases circulating concentration of calcium. Is a peptide produced by c-cells in thyroid gland. Raw form absorbed in intestine, first processed by liver, then by kidney to make active form. Aims to maintain circulating calcium level at 10mg/100ml. Body will sacrifice bone structure to maintain circulating concentrations. First absorbed from intestines, mostly duodenum & upper jujunum. Pth & vitamin d promote absorption of calcium into plasma. Can be deposited into bone & other tissue. Resorption of calcium from bone to plasma when needed.