Bone is an impressive structural material: it is light yet strong, flexible in joints, and hard where support is needed. It regenerates over time and also contains the bone marrow, a vital part of the body.
The primary function of bone is to support the body and to serve as a mount for muscles, which then use a lever action against the bone to make your body move. Bones also protect important organs (lungs, heart, brain) from damage. The centre of most large bones also houses bone marrow.
Bone is essentially a composite material: composed of hard calcium phosphate (in a form known as hydroxyapatite, formula Ca5(PO4)3OH) and flexible and strong collagen protein. The proportions of the two materials vary depending on the qualities required in the bone: major structural bones such as the femur contain lots of calcium phosphate and relatively little collagen. The structure of the bone also changes, depending on the stress on it: unstressed bone has a more spongy texture, while areas that get a lot of stress become completely solid. As bones become older the proportion of calcium phosphate in the bones also increases, leading to brittle bones.
Bone growth is produced by cells known as osteoblasts, which produce collagen and calcium phosphate, and secrete them outside the cell, until the entire cell, except for gaps for blood vessels is surrounded by a thick layer of bone. As the bone matures, the osteoblasts become osteocytes, which maintain the bone, but do not produce any new growth. The surface of the bone is maintained by cells called osteoclasts, which keep the surface of the bone smooth and under control by destroying excess bone.
The bone marrow is one of the most important parts of the body because it contains haematopoietic cells, a form of stem cells, which produce new red and white blood cells and platelets, vital for the maintenance of the blood.