Growth and repair

Bones lengthen during childhood and adolescence from growth regions near their ends, where new bone tissue is continuously laid down until the skeleton reaches its adult size. But growth is only the start of a bone’s activity — throughout adult life, bone tissue keeps remodelling itself, with old bone broken down and new bone rebuilt in a slow, continuous cycle, so that a large portion of the adult skeleton is effectively replaced over a period of years.

This same rebuilding process is what allows a broken bone to heal. Blood clots at the break, a soft repair tissue bridges the gap, and that soft tissue is gradually replaced by new bone until the fracture is fully knitted back together — often ending up just as strong as before. It is one of the clearest examples in the whole body of a tissue actively repairing serious physical damage on its own.

Together, this unit has covered a system built for support (the frame that holds you up), protection (the cage around vital organs), movement (joints turning muscle pull into motion), and mineral storage and blood-cell production happening quietly inside the bone itself. In the next unit, you’ll meet the muscular system — the tissue that actually pulls on this frame to create movement.