The function of compact bone is structural support, both for overall body structure and the protection of cancellous bone, which contains marrow. Mature compact bone is layered and very dense, with the mineral calcium phosphate embedded in collagen proteins, with tiny spaces for the living, bone-producing cells.
The bone-producing cells exude proteins, which are then mineralized to form solid bone. They also release the hormones onto the proteins that encourage mineralization. There are also cells in bone that break down bone materials, which are then replaced. Bone is constantly being renewed by these two types of cells. The reabsorption of bone is also important for maintaining a proper balance of calcium in the blood and other tissues.
Compact bone is a component of all bones. A majority of the bone in long bones, such as most of the bones in the limbs, is compact bone. Short bones, such as those in the wrist, have only a thin layer of compact bone over cancellous bone. Flat bones, such as those of the skull, have cancellous bone sandwiched between two layers of compact bone. Besides their structural roles, these bones also protect internal organs and provide attachment points for muscles, allowing efficient movement.