What bones do

The skeletal system, with its cartilage, ligaments and tendons, makes up about 20 percent of your body weight.

Bones are alive

A skeleton in a classroom looks dry and dead, but the bones inside your body are living tissue. They use oxygen and give off waste like other organs, they need a blood supply, and they change shape over time in response to the forces put on them. The skeletal system includes the bones together with cartilage, ligaments and tendons, and altogether it accounts for about 20 percent of body weight.

Support and protection

The first job of the skeleton is to hold you up. It supports the body against the pull of gravity, and the large bones of the legs carry the weight of the trunk when you stand. Without a rigid frame, muscles would have nothing to pull against and soft organs would sag.

The second job is protection. The fused bones of the cranium surround the brain and make it less likely to be injured. The vertebrae form a bony tunnel around the spinal cord. The rib cage curves around the heart and lungs. In the 3D model, select the skull, the thoracic vertebrae and the ribs one after another and notice how each one wraps around something soft.

Movement

Bones work with muscles as simple lever systems. A muscle is attached to bones by tendons, and when it shortens it pulls one bone toward another across a joint. Long bones such as those of the arms and legs act as the levers. That is how a small shortening of a muscle near the elbow can swing your whole hand through a wide arc.

A store of minerals and a factory for blood

Bones contain more calcium than any other organ. Most of it is held in the hard material between bone cells as calcium salts, mainly calcium phosphate. The body uses bone as a bank: when the calcium level in the blood drops below normal, calcium is released from the bones, and when there is extra, it is stored in the bone again. This exchange goes on almost all the time.

Bones also make blood. The formation of blood cells mostly takes place in the red marrow inside bones. In infants, red marrow fills the bone cavities. With age much of it is replaced by yellow marrow, which stores fat, and in adults red marrow is found mainly in the spongy bone of the skull, ribs, sternum, collarbones, vertebrae and pelvis. Red marrow makes red blood cells, white blood cells and platelets.

Four shapes of bone

Bones are grouped by shape. Long bones are longer than they are wide, with a shaft and two bulky ends; the thigh, leg, arm and forearm bones are long bones. Short bones are roughly cube shaped, like the bones of the wrist and ankle. Flat bones are thin and usually curved, like most bones of the cranium. Irregular bones fit none of these, such as the vertebrae. Most bones have a dense outer layer called compact bone and lighter, honeycomb-like spongy bone inside.

Try the model with these jobs in mind. Select the femur and the tibia: they are the load-carrying columns of the legs. Select the ribs and the sternum: together they form a cage that can still expand when you breathe because cartilage joins the ribs to the sternum. Select the pelvis: it carries the weight of the upper body down to the legs and also cradles organs in the lower belly.