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All of the bones, cartilage, muscles, joints, tendons and ligaments in a person's body compose what is known as the musculoskeletal system. The bones provide the body with a framework, giving it shape and support; they also serve as protection for internal organs such as the lungs and liver. Muscles are fibers that help to make deliberate movement of a body part or involuntary movement within an internal organ possible. Some people view this system as two separate systems that work very closely together, with one being the muscular system and the other being the skeletal system.
The bones of the musculoskeletal system are categorized according to their appearance or shape — short, long, flat and irregular. For example, the humerus, or bone of the upper arm, and the femur, or thigh bone, are long. The vertebrae, which protect the spinal cord, are irregularly shaped.
Bones store salts and metabolic materials and serve as a site for the body's production of erythrocytes, or red blood cells. Although many people think of them as hard structures that don't really present life-threatening situations when fractured or broken, bones are living tissues that, if fractured, can cause the loss of enough blood to bring on hypoperfusion, also known as shock. A broken femur often causes a blood loss of 2 pints (1,000 cc), and a pelvic fracture can cause a person to lose as much as 4 pints (4,000 cc) of blood.
There are three categories of muscles: voluntary, involuntary and cardiac. Voluntary muscles are fibers that allow for conscious movement of body parts; when a person walks, voluntary muscles are at work. Involuntary muscles are located within certain internal organs such as the esophagus, the tube that leads to the stomach, in which muscles contract to help move food downward. Cardiac muscles are found within the heart, an organ whose movement is a constant pumping action.
Joints, as well as voluntary muscle, are involved in a person's ability to move a body part deliberately. They are the places where bones articulate, such as the elbow or knee. Cartilage is a strong and tough tissue that covers joint ends, and it helps to form some parts of the body, such as the outer ear. Tendons are pieces of tissue that connect muscles to bones, and ligaments connect one bone end to another. This connective part of the musculoskeletal system not only allows for the power of movement but also permits a person to have a specific range of motion.