The human body is composed of more than 200 bones of different shapes and sizes. Bones consist of living cells (osteocytes called osteoclasts and osteoblasts) that are interconnected by a hard material, such as calcium. This makes the bone strong and rigid. The bones are hollow and filled with a spongy material called bone marrow that produces blood cells.
Joints of bones are covered in cartilage - a resistant material, flexible, like cartilage. As the cartilage is more elastic than bone it allows the bones to move freely at the joints. It also cushions the bones in joints to stop them rubbing against each other.
The bones have several important functions. The skeleton gives the body a rigid and the joints act as levers that the body can move. Bones also protect the organs in the body: for example, the ribcage protects the heart and lungs. They also remember some of the essential minerals in the body, especially calcium.
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