In the mammalian body, this element plays many important roles. Try to identify this element with the fewest number of clues. This element is required to insure the integrity and permeability of cell membranes, to regulate nerve and muscle excitability, to help maintain normal muscular contraction, and to assure cardiac rhythmicity. It also plays a essential role in several of the enzymatic steps involved in blood coagulation and is the most important element of bone salt. Name this element.