Bone is constantly being turned over. Specific cells within bone are constantly breaking down bone components such as proteins and mineral complexes. Meanwhile, other cells are constantly building bone. Although this may seem counterproductive its merit lies in the ability of bone to adapt or be remodeled according to the demands placed upon it. For example, one of the benefits of weightlifting is an increased stress placed on bone, which causes the bone to adapt by increasing its density. In this case, the efforts of cells that build bone will exceed the efforts of cells that will break down bone components. On the contrary, prolonged exposure to zero gravity (weightlessness) in outer space will decrease the stress placed upon bone resulting in a loss of bone density. In this situation, the efforts of cells that break down bone will exceed those efforts of cells that build bone components.
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