It also serves as an anchor for skeletal muscles. Bones act as a hard shell to protect internal organs such as the brain, the skull is a hard barrier for most objects to pass through. Another function is movement: the system serves as an attachment point for the body's skeletal muscles. Joints provide movement to bones, and each skeletal muscle functions by moving two or more bones closer together or further away from the body. The skeletal system also helps maintain hematopoiesis in the bones. This is where the red bone marrow produces red and white blood cells in a process known as hematopoiesis. The red marrow is found in the empty space inside the bone cavity. Red bone marrow is mostly found in children because when puberty is reached the red bone marrow changes to yellow bone marrow. Finally, bones provide storage for many different types of essential substances to facilitate the body's growth and repair. Calcium banks are found in bones and release calcium into the bloodstream when needed to reach other parts of the body. Bone cells also release osteocalcin, a hormone that helps regulate blood sugar and fat deposition. The yellow bone marrow inside the long hollow bones of the legs stores energy in the form of lipids. The skeletal system does amazing things that most people wouldn't even think about and protects the human body