Your body’s supporting framework, bones also provide protection for your internal organs and house the system for making your blood cells, in the bone marrow. About 99% of the calcium your body needs is held in your bones.
Your bones aren’t solid; instead, a hard outer layer of compact bone cells surrounds a spongy honeycomb middle, through which run channels allowing room for blood and lymph vessels, and nerves, and a cavity for your bone marrow. Bones grow continuously, but not by cell division; instead your different types of bone cells generate bone matrix, break it down and maintain it. Your body can remodel and strengthen bone in areas where they are under greatest stress, or to heal fractures. This process also helps to keep up the calcium levels in your bodily fluid. This process of breaking down and remodelling bone also causes bones that aren’t being used to atrophy, as in an immobilised limb for example.
In early childhood your skeleton is made up mostly of pliable cartilage rather than solid bone, which means that the risk of fractures and breakages is much lower. Bone fractures heal naturally, although it does help the healing process to restrict movement in the affected area – by encasing a limb in a plaster cast, for example. There are two different types of fracture; a sudden fracture, caused by an accident for example, or a stress fracture, caused over time. As your body ages, you become more vulnerable to fractures, because your bone mass decreases.