What regulates calcium concentration?

Calcium concentrations are regulated by the parathyroid hormone, which is produced by the parathyroid glands. Regulating calcium concentrations is crucial for efficient generation of nerve impulses and muscle contractions.

When calcium concentrations drop below a certain level, the parathyroid glands release parathyroid hormone, which increases calcium to optimum levels minimizing the risk of convulsions and spasms. When calcium concentrations are too high, the parathyroid glands stop releasing parathyroid hormone to bring calcium levels back down to normal. 

Different tissues contain calcium in different concentrations. Within a typical cell, the intracellular concentration of ionized calcium is roughly 100 nM, but is subject to increases of 10– to 100-fold during various cellular functions.