The primary function of bone is to be a supporting framework for the rest of the body. It is often com-pared to the steel girders that support buildings. But, unlike steel girders, bone is one of the most metabol-ically active tissue; remaining active throughout life and having the capacity to change shape and density according to mechanical demands. Bone contributes to the shape and positioning of the various structures of the body. Together, some bones protect important organs. The heart and lungs, for example, lie securely in the bony thoracic cage. The brain lies in the pro-tective cranial cavity made up of many bones.
The bones, with their joints, act as levers that are manipulated by the muscles attached to them and po-sitioned across the joints. Bones are the main reser-voir for minerals, such as calcium and phosphorus. Calcium is an important mineral required for con-duction of impulses in nerves, muscle contraction, and clotting of blood. It is vital for the body to main-tain the blood levels of calcium within a narrow range, and bone serves as a reservoir when the blood levels of calcium fluctuate. Bone is also a factory where blood cells are manufactured, and bone may also be considered as one of the sites where fat is stored because yellow bone marrow is primarily adi-pose tissue.