The skeletal system gives the body its basic framework, providing structure, protection, and movement. The 206 bones in the body also produce blood cells, store important minerals, and release hormones necessary for bodily functions. Unlike other living organs, bones are firm and strong, but they have their own blood, lymphatic vessels, and nerves.
There are two distinct tissue types inside all bones:
- Compact bone: This hard and dense tissue makes up the outer layer of most bones and the main shaft of long bones, such as those in the arms and legs. Nerves and blood vessels live inside this tissue.
- Spongy bone: This tissue is made up of smaller plates filled with red bone marrow. It is found at the ends of long bones, like the head of the femur, and at the center of other bones.
Red bone marrow forms most of the blood cells in the body and helps destroy old blood cells. Another type of marrow, yellow bone marrow, resides in the central cavities of long bones. It is mostly made up of fat. However, if the body suffers large amounts of blood loss, it can convert yellow marrow to red in order to make more red blood cells.
The skull consists of 22 separate bones that combine to form the cranium, which is the housing for the brain. Twenty-one of those bones are fused together by sutures, or nearly rigid fibrous joints. The lower-most bone of the skull is the mandible, or jawbone.
The spine, or vertebral column, is a series of irregularly shaped bones in the back that connects to the skull. At birth, humans have 33 or 34 of these bones. But bones fuse as we age, and the result is 26 separate bones in the spines of adults.
The rib cage is made up of 12 pairs of bones that encase vital organs in the chest. The bones curve from the back at the vertebral column to the front of the body. The upper seven pairs meet with the sternum, or chest bone. The remaining five pairs are attached to each other via cartilage or do not connect.
The muscles of the shoulders and arms include the clavicle (collarbone), scapula (shoulder blade), humerus, radius, ulna, and the bones of the wrist and hand.
The bones of the pelvis are created by the fusion of three bones—ilium, ischium, and pubis—that fuse together as we grow older. These form the majority of the pelvis at the base of the spine as well as the socket of the hip joint. The sacrum—five fused bones at the bottom of the spine—and the coccyx, or tailbone, make up the rest of the bones in the pelvic region.
The head of the femur, the largest and longest bone in the body, creates the other half of the hip joint and extends down to form part of the knee. It begins the bones of the leg. The other bones of the leg include the tibia, fibula, and the bones of the ankle and foot.
The most common condition that affects bones is fracture, which occurs when a bone endures such a great impact that it breaks.
Other common conditions that affect the skeletal system include:
- Osteoporosis: This is a disease in which the bones become fragile and prone to fracture.
- Leukemia: This is a cancer of the white blood cells.
- Osteopenia, osteitis deformans, and osteomalacia: Similar to osteoporosis, these are other types of bone loss.
- Scoliosis, kyphosis, and lordosis: These are abnormalities of the spinal curve.