Bones are composed of a very active constantly changing tissue that responds to 2 factors.The first being calcium levels in the blood, the second being the force of gravity and the force exerted by the muscles pulling on them.

When calcium levels in the blood drop below normal levels the parathyroid gland releases a hormone into the blood stream called the parathyroid hormone, or PHT for short. The PHT the turns on bone destroying cells located inside of each bone, to bone to break down the bone itself. The calcium from the breakdown is then released into the bloodstream

The opposite occurs if blood calcium is too high. the excess is then stored in the bone as hard calcium salts, which is what gives bones there hardness.

As the body grows in size and weight, or gravitational resistance (resistance training), is increased the bones must also increase their strength and thickness to match. This is the reason that bones become thicker around areas where stronger muscles are attached. Bones of people that are physically inactive lose mass and weaken because there is no stimulation to do otherwise.