Calcium is essential for muscle contraction, blood clotting, plus brain and nerve function, as well as being a key ingredient in the bone demineralization process. If you don't get enough calcium, your body is in negative calcium balance. To correct the imbalance, the body's hormones will release calcium from the skeleton into the bloodstream to make calcium available for nerves and muscles. If there is a long term deficiency, the skeletal storage depot can become depleted of its calcium, leaving porous, brittle, breakable bone the condition known as osteoporosis. Structurally, there are basically two different types of bone cortical and trabecular.
Cortical bone is very dense and solid.
The long, hard bones of your arms and legs are mostly of this type. Trabecular bone, on the other hand, is much more porous, honeycombed with many minute spaces. Every bone has both types, with the porous, trabecular bone on the inside, and solid, cortical bone on the outside, though in different proportions depending on which bone it is. Spinal vertebrae, for example, are mostly porous, trabecular bone, with a thin, cortical shell. Since osteoporosis affects trabecular bone more than cortical bone, the spine is one of the first areas to be affected by osteoporosis.