The body regulates calcium homeostasis with two pathways; one is signaled to turn on when blood calcium levels drop below normal and one is the pathway that is signaled to turn on when blood calcium levels are elevated.
Calcium metabolism is the movement and regulation of calcium ions (Ca2+) in (via the gut) and out (via the gut and kidneys) of the body, and between body compartments: the blood plasma, the extracellular and intracellular fluids, and bone. Bone acts as a calcium storage center for deposits and withdrawals as needed by the blood via continual bone remodeling.[1]: 276–277
An important aspect of calcium metabolism is plasma calcium homeostasis, the regulation of calcium ions in the blood plasma within narrow limits.[2] The level of the calcium in plasma is regulated by the hormones parathyroid hormone (PTH) and calcitonin. PTH is released by the chief cells of the parathyroid glands when the plasma calcium level falls below the normal range in order to raise it; calcitonin is released by the parafollicular cells of the thyroid gland when the plasma level of calcium is above the normal range in order to lower it.
Body compartment content[edit]Calcium is the most abundant mineral in the human body.[3] The erage adult body contains in total approximately 1 kg, 99% in the skeleton in the form of calcium phosphate salts.[3] The extracellular fluid (ECF) contains approximately 22 mmol, of which about 9 mmol is in the plasma.[4] Approximately 10 mmol of calcium is exchanged between bone and the ECF over a period of twenty-four hours.[5]
Blood concentration[edit]The concentration of calcium ions inside cells (in the intracellular fluid) is more than 7,000 times lower than in the blood plasma (i.e. at