Calcium homeostasis refers to the regulation of the concentration of calcium ions in the extracellular fluid [Ca++]ECF. This parameter is tightly controlled because the calcium ions have a stabilizing effect on voltage-gated ion channels. For instance, when [Ca++]ECF is too low (hypocalcemia), voltage-gated ion channels start opening spontaneously, causing nerve and muscle cells to become hyperactive. The syndrome of involuntary muscle spasms due to low [Ca++]ECF is called hypocalcemic tetany. Conversely, when [Ca++]ECF is too high (hypercalcemia), voltage-gated ion channels don't open as easily, and there is depressed nervous system function. Another problem of hypercalcemia is that calcium can combine with phosphate ions, forming deposits of calcium phosphate (stones) in blood vessels and in the kidneys.