Calcium is integrated in the bone matrix in the form of hydroxyapatite. This mineral is in some way bounded to a collagen matrix. The calcium intake is therefore associated to the bone formation process which is taken in charge by osteoblasts. The bone formation is crucial to maintain balance between bone formation and bone resorption, and preserve maximum bone capital.
Osteoporosis is the result of an imbalance between these two processes. Either the bone formation is not processed fast enough or the bone resorption is too fast, or the two processes are at fault.
If the body does not obtain enough calcium to maintain maximum bone formation, it will slowdown bone formation. Since the resorption is independent, it continues at the same speed, therefore causing rapid net bone loss.