. . . . . . "CSP: divalent cation essential in cellular signal transduction and the normal functioning of nerves and muscles; plays a role in blood coagulation and in many enzymatic processes.,NCI: The metabolically-active portion of calcium, not bound to proteins, circulating in the blood.,NCI: Calcium ion requiring two electrons to return to its elemental state." . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .