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Calcium regulation

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Calcium is transported through the bloodstream as dissolved divalent cations (Ca2+) or bound to proteins (eg, serum albumin). Calcium levels are tightly regulated. Parathyroid hormone, secreted by the parathyroid gland, regulates (1) the resorption of calcium from bone (the major calcium storage site in the body); (2) reabsorption in the kidney back into circulation; and (3) increases in the activation of vitamin D3 to calcitriol. Calcitriol, the active form of vitamin D3, promotes absorption of calcium from the intestines and the mobilization of calcium ions from bone matrix. Calcitonin, which is secreted from the parafollicular cells of the thyroid gland, also affects calcium levels by opposing parathyroid hormone, but its physiological significance in humans is limited. (Image by Mikael Häggström (2009). (Courtesy of Wikimedia Commons. Public domain.)