Bone is a living, growing material. It has a framework of protein. Calcium strengthens the bone framework. The outer layer of bone has nerves and a network of small blood vessels.
Old bone is removed and new bone is added all the time.
In young people, more bone is added than is removed. Our bones get heavier and stronger. After age 30, more bone is removed. Bones become lighter and more brittle. We don't know if these are caused by HIV or by the medications used to treat it.