Humans have an internal skeleton. The main purpose of this skeleton is to offer support for the body. The bones are much like the frame of a house to which the outer coverings are attached. For example, the clavicle and scapula (together called the pectoral girdle) support the arm, and the pelvic bones (called the pelvic girdle) support the legs. There are also several other functions provided by the skeletal system. These include:
- Movement: The skeleton itself does not move under its own power; rather, it provides a place of attachment for the muscles. Bone markings called processes are where muscles connect to bone. The bones act like levers as the muscles receive the signals they need to contract and relax. When the muscles contract, the bones move in a particular direction.
- Protection: Bones are strategically located within the body where essential body organs are found. The thoracic cage (also called the rib cage) surrounds and protects the heart and lungs, the bones of the skull are fused to protect the brain, and the vertebral column surrounds and protects the nervous tissue of the spinal cord.
- Hematopoiesis: This is the formation of red blood cells in the marrow of bones such as the bones of the hips and the sternum.
- Storage: Bones store many minerals and nutrients. Bones are the main sites where calcium phosphate, the main mineral that makes bones hard, is found. The bones regulate the amounts of different minerals in the blood by releasing more when levels get low and absorbing excess when levels increase. This helps to maintain blood homeostasis, or constant internal conditions.