The Human Skeleton
The human body is supported by an internal skeleton made of about 206 bones. The skeleton gives shape, protects delicate organs (the skull protects the brain, the ribs protect the heart and lungs), and works with muscles to allow movement.
Joints and Muscles
Bones meet at joints. A hinge joint, such as the elbow or knee, allows movement in one plane like a door. A ball-and-socket joint, such as the shoulder or hip, allows movement in many directions. Bones are joined by ligaments, and muscles are joined to bones by tendons.
Key idea
Ligament joins bone to bone; tendon joins muscle to bone. At the elbow, the biceps and triceps are an antagonistic pair: the biceps contracts to bend the arm, the triceps contracts to straighten it.
How the Arm Bends
- To bend (flex) the arm: the biceps contracts while the triceps relaxes.
- To straighten (extend) the arm: the triceps contracts while the biceps relaxes.
Human Growth
Growth is the permanent increase in size and mass of the body as cells divide and develop. Human growth is not even; it is fastest during two periods: infancy and the adolescent growth spurt during puberty. Growth is controlled by hormones such as growth hormone from the pituitary gland, and depends on a balanced diet with enough protein, calcium and vitamins.
Example
During puberty, a teenager may grow several centimetres taller in a single year. Drinking milk and eating protein-rich foods such as ikan and eggs supplies the calcium and protein needed for strong bones and muscles.
Movable joints are cushioned by smooth cartilage at the ends of the bones and lubricated by synovial fluid. Together these reduce friction and allow the bones to move easily against one another without wearing away, even after years of use.
Remember
Hinge joint = one direction (elbow, knee). Ball-and-socket = many directions (shoulder, hip). Biceps bends the arm; triceps straightens it.