Almost all your body's calcium is stored in bone. But the tiny amount that circulates in your bloodstream is disproportionately vital to normal physiology. About half of this circulating calcium is "ionized," which means it carries electrical charges. Ionized calcium participates in the firing of muscle and nerve cells, promotes blood clotting, and prevents the depletion of bone mass. (See References 1)
Ionized Calcium and Cell Function
The most important role of calcium in blood is to circulate and be available to tissues. Every cell of the body uses calcium, but certain "excitable" cells such as heart cells, muscle cells and neurons are particularly dependent on calcium for their function. These excitable cells require calcium to contract or send impulses.
These cells work because of the huge difference in calcium concentration between the outside and inside of the cell, and also between different compartments within the cell. Muscle, nerve and heart cells use this "calcium voltage" for contraction and neural transmission. When ionized calcium deviates outside a narrow range, severe neuromuscular and cardiac symptoms develop, including spasm, weakness, confusion, seizures and cardiac rhythm disturbances.
Ionized Calcium and the Coagulation Cascade
Blood coagulation is a complex biochemical process involving platelets and dozens of proteins. Activation of platelets triggers a multistep "cascade" that produces a "thrombus" or clot. Calcium is involved at several steps in this cascade, including the activation of the platelets themselves. Coagulation is so dependent on calcium that blood banks routinely add citric acid to banked blood to bind ionized calcium and prevent the product from clotting before it can be used.
Ionized Calcium and Bone Mass
Ionized calcium in the blood is so vital that the body cannot permit it to fluctuate. Precise calcium balance is maintained by dietary intake, intestinal absorption, excretion in the urine and by shuttling calcium to and from the skeleton.
The skeleton is the body's reservoir of calcium, containing 99 percent of body stores. This means that if blood calcium begins to drop, the body can correct the deficit by reabsorbing a tiny fraction of bone tissue. In fact, this kind of "bone remodeling" is going on all the time, and in healthy individuals the net flow of calcium to and from bone is balanced. But if the body has trouble maintaining normal ionized calcium levels for any reason, such as dietary calcium or vitamin D deficiency, hormonal imbalance, or disease of the kidney or gut, constant withdrawals from this bone bank will eventually lead to decreased bone mass, or osteoporosis.
The Complexity of Calcium
Maintaining calcium balance, or "homeostasis," involves a complex interplay between dietary intake, excretion in the kidneys, absorption from the gut, bone remodeling, vitamin D and several hormones. Moreover, calcium metabolism is closely intertwined with metabolism of phosphorous and magnesium. The system is complex, but you can support it simply with a healthy lifestyle, including a balanced diet and regular exercise.