Skeletal System

The skeletal system is the frame of bones inside your body. It holds you up, gives your body its shape, and protects the soft organs inside. Without it, you would flop to the ground like a rag doll. An adult human skeleton has 206 bones, and more than half of them are in the hands and feet.

The skeleton has four main jobs. First, it supports the body, the way wooden beams support a house. Second, it protects organs. Your skull is a hard helmet around your brain. Your ribs form a cage around your heart and lungs. Third, it works with muscles to let you move. Muscles pull on bones, and bones swing around joints like hinges. Fourth, the soft inside of large bones, called bone marrow, makes new blood cells every day. Your body makes about 2 million red blood cells every second, mostly inside your bones.

Bones are not dry or dead, even though that is how they look in a museum. A living bone is full of blood vessels and nerves. The outside is hard and strong. The inside is more like a sponge, with tiny holes that make the bone lighter without making it weak. Pound for pound, bone is stronger than steel.

Where two bones meet, you have a joint. Some joints, like the ones in your skull, do not move at all. Others move a little, like the joints between the bones in your spine. The most exciting ones move a lot. Your shoulder is a ball-and-socket joint that swings in almost any direction. Your elbow and knee are hinge joints that bend one way, like a door. Most joints have a slippery cushion called cartilage that keeps the bones from grinding against each other.

The skeleton is also tied tightly to the muscular system. Muscles attach to bones with tough cords called tendons. Tough strips called ligaments hold bones to other bones. When a muscle pulls, the bone moves, and you walk, jump, or wave.

Babies start out with about 300 bones, but many of them fuse together as a kid grows. The skull of a newborn baby has soft spots where the bones have not joined yet. This is what lets the baby's head squeeze through during birth and lets the brain grow fast in the first year. By the time a person is in their twenties, the skeleton has settled into its final 206 bones, and it will carry that person for the rest of their life.