The human body is made up of more than 200 bones of different shapes and sizes. They are made of collagen – a type of protein – calcium salts and other minerals. These make bones strong and rigid.
Long bones, for example in the legs and arms, are made up of a hard outer shell of compact bone. Each end of the bone is filled with cancellous or spongy bone. The middle of the bone is filled with bone marrow, which makes blood cells.
- provide support for our bodies, and the joints act as levers so that we can move around
- protect various parts of our bodies from injury – for example, the ribs protect the heart and lungs
- store important minerals, such as calcium, that are used by our bodies
- provide a place for bone marrow which is where blood cells are made.
Our bones are constantly being broken down and replaced with new bone. This helps maintain their strength and shape. There are two main types of cells that break down and replace bone:
- Osteoclasts break down and remove old bone.
- Osteoblasts make new bone.
The joints at the end of bones are covered in cartilage. This is a tough, flexible material, like gristle. Because cartilage is more elastic than bone, it allows the bones to move freely at the joints. It also cushions the bones at the joints, to stop them rubbing against each other.