Calcium is the most abundant mineral in our bodies and it makes up 1.9% of the body by weight. [1,2]
The body gets calcium in two ways, either by eating calcium-rich foods or supplements or by drawing calcium from the body or bones specifically. 
The borrowed calcium from bones is usually replaced at a later stages when the nutrient levels go up, but it might not happen always. 
Calcium salts provides rigidity to the skeleton whereas calcium ions help in the metabolic process. 
Different hormones work with the kidney to control the calcium reserves in the body as well as excretion. 
In case of calcium deficiency, the body will remove calcium from the bones.