Originally Posted by Nodo
Calcium has a very high concentration gradient. Cytoplasm has almost no calcium. Calcium is either stored in the smooth endoplasmic reticulum or is pumped outside. Cytoplasmic calcium accumulation is pathologic.
Calcium accumulation is the hallmark of irreversible cell injury. Ca2+ pumps require ATP. No ATP=no pump activity=Ca2+ stays in the cytoplasm and causes damage.
Just to add a little more, calcium activates a bunch of enzymes, which may not be good for the cell, if in excess. So it's only natural for the cell to either - sequester it inside a binding protein (like, say calmodulin) or to pump it out. Ionised calcium is mostly extracellular.