Bones are impressive structures that are even more amazing than many people realize. They provide attachment sites for muscles and enable us to move. Some, such as the skull and ribs, protect vital organs. They also make our blood cells, store minerals such as calcium and release them when necessary, and store lipids, which are an energy reserve. One very important function of bone is to send calcium into the bloodstream when the body needs it. Calcium is a vital chemical in our bodies. It's necessary for muscle contraction, blood clotting, nerve conduction, and other functions. It also provides strength to bones and teeth. Specialized cells called osteoclasts break down bone to free the calcium. Cells known as osteoblasts deposit calcium into bone, remaking it. The process of replacing old bone with new bone is known as remodeling.