Calcium
Calcium, also Ca++, physiologically important divalent alkaline earth metal, is used for the formation of bones (99% of the body-Ca), is present in the blood as ionized Ca++ and bound to protein (90 - 110 mg/l).
Calcium plays an important role in nerve excitation, muscle contraction and regulation of intracellular metabolism. Calcium, for example, is released by stimuli from the endoplasmic reticulum and triggers contraction in the muscle by lifting the tropomyosin barrier. This requires energy, which the body provides in the form of ATP.
Like other ions, it can be transported through membranes against the concentration gradient by special, voltage-dependent and receptor-dependent calcium channels, by opposing sodium influx, by ATP dependent active transport and by coupling to electron transport.