The typical human skeleton consists of 206 bones in adults. More bones are present at birth, which gradually fuse together as the body matures. The skeleton is divided into two parts. The axial skeleton includes the bones of the skull, face and spine along with the ribs and breastbone. The appendicular skeleton includes the bones of the arms, hands, legs, feet and pelvis as well as the clavicles and shoulder blades. The skeleton serves several vital functions.
The skeleton gives the body its shape, which changes with growth. In addition to determining characteristics such as height and the size of the hands and feet, stable body shape enables essential functions. For example, a stable rib cage and spine enable the lungs to fully inflate when breathing. Conditions such as osteoporosis of the spine and broken ribs can alter the shape of the chest and impair this vital body function.
Along with the muscular system, the skeleton provides support to the body and keeps the internal organs in their proper place. The strong bones of the spine, pelvis and legs enable people to stand upright, supporting the weight of the entire body. Body cavities -- hollow spaces framed by the skeleton -- hold the internal organs. For example, the skull holds the brain, the chest cavity houses the heart and lungs, and the abdominal cavity encases the organs of the digestive, urinary and internal reproductive systems.
The skeletal bones are held together by ligaments, and tendons attach the muscles to the bones of the skeleton. The muscular and skeletal systems work together as the musculoskeletal system, which enables body movement and stability. When muscles contract, they pull on bones of the skeleton to produce movement or hold the bones in a stable position. The shape of the bones and how they fit together at the joints allows for different types of movement. For example, the leg bones come together at the knee to form a hinge joint that enables the knee to bend back and forth. The joining portions of the bones of the hip and shoulder have a much different shape and form ball-and-socket joints that allow movement in multiple directions.
The skeleton protects the internal organs from damage by surrounding them with bone. Bone is living tissue that is hard and strong, yet slightly flexible to resist breaking. The strength of bone comes from its mineral content, which is primarily calcium and phosphorus. The flexibility is due to a substance called collagen. The combination of strength and flexibility gives the skeleton the capacity to absorb the impact of blows to the body without breaking. Examples of important protecting bones of the skeleton include the skull, spinal column and rib cage, which protect the brain, spinal cord, and heart and lungs.
Blood Cell Production
Larger bones contain bone marrow, a spongy tissue inside the bones. There are two main types of marrow, red and yellow. Red marrow is responsible for production of all of the body's red blood cells and many of its white blood cells. Red blood cells are produced at an average rate of approximately 200 million per day. These cells carry life-sustaining oxygen to the body tissues. In adults, red marrow is found primarily in the breastbone, hips, ribs, skull, spinal bones and at the end of long bones of the arms and legs. Several types of white blood cells, which protect the body from infections, are also produced in red bone marrow. Yellow bone marrow contains primary fat cells but can transform into red marrow if the body needs to increase blood cell production, such as if anemia develops.