Bones, muscles and joints play an important role in the human body. They, along with tendons, ligaments and cartilage, form the musculoskeletal system and enable us to perform physical activity. The musculoskeletal system protects and supports internal organs, allows movement, gives the body its shape, produces blood cells, stores calcium and phosphorus, and produces heat.
The human skeleton consists of 206 bones. Bones are dry, dense tissues composed of calcium-phosphorus minerals, organic matter and water. A living membrane called periosteum covers the bones. Bone contains three types of cells: osteoblasts, which repairs damage; osteocytes, which form new bone; and osteoclasts, which break down, sculpt and shape the bone. Although very light, bones are strong enough to support the body's weight.
Joints are areas where two or more bones meet. Joints allow movement and flexibility within the body. Some joints open and close, similar to a hinge. Others allow for more complex movements. For example, shoulder and hip joints allow for backward, forward, sideways and rotating movement.
The human body consists of more than 600 muscles. These muscles move body parts by contracting and relaxing. There are three different types of muscle in the human body: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones. They are also known as voluntary muscles because their movement is controllable. Smooth, or involuntary, muscles are controlled automatically by the nervous system. Cardiac muscle, also involuntary, forces blood out of the heart through rhythmic contractions.