The Human Skeleton

Your body's living frame

Stand up straight, jump in the air, or wave your hand. None of that would be possible without your skeleton — the strong, inner frame of bones that holds you up and lets you move. Without a skeleton, your body would collapse like a tent with no poles, just a soft heap on the floor.

An adult human skeleton is made of 206 bones. That might sound like a lot, but every single one has a job to do. In this lesson we'll explore what bones are made of, the four important jobs the skeleton does, and how bones, joints and muscles work together to get you moving.

What is a bone actually made of?

It's easy to imagine that bones are dry and dead, like the ones in a museum. In a living body, that's completely wrong — bones are alive. Each bone is a living organ with its own blood supply and nerves.

If you could look inside a bone, you would find different layers:

• A hard outer shell called compact bone, which is dense and strong. This gives the bone its strength.
• A lighter, honeycomb layer called spongy bone, full of tiny spaces. This makes the bone strong but not too heavy.
• In the centre of many bones, a soft jelly called bone marrow.

This clever design is the reason bones are both light and tough. Bone is one of the strongest natural materials we know — gram for gram, it is stronger than steel, yet light enough to let you run and jump. Bones are made mostly of a mineral called calcium phosphate, plus a stretchy protein called collagen that stops them being too brittle. That's why we are told to eat foods rich in calcium, like milk and leafy greens — they keep our bones strong.

The four jobs of the skeleton

Why do we have a skeleton at all? It does four crucial jobs.

1. Support. The skeleton is the framework that gives your body its shape and holds you upright. Your spine (backbone) is a column of 33 small bones called vertebrae stacked on top of each other. It supports your head and body while still bending so you can twist and lean.

2. Protection. Bones act like armour around your softest, most important organs. Your skull is a hard helmet protecting your brain. Your ribcage forms a curved cage around your heart and lungs. Your spine wraps around the delicate spinal cord that carries messages to and from your brain.

3. Movement. Bones cannot move by themselves — they need muscles. Muscles are attached to bones by tough cords called tendons. When a muscle contracts (shortens), it pulls on the bone and creates movement at a joint. We'll look at this more in a moment.

4. Making blood cells. This is the surprising one. Inside the red bone marrow of certain bones, your body makes new blood cells — millions of them every second. Red blood cells carry oxygen, and white blood cells help fight infection. So your bones are also a busy factory keeping your blood healthy.

Joints: where bones meet

A skeleton of solid, fixed bone would be useless — you couldn't move at all. That's why the skeleton has joints, the places where two or more bones meet.

Different joints allow different movements:

• A hinge joint, like your elbow or knee, works like a door hinge — it bends one way and straightens.
• A ball-and-socket joint, like your shoulder or hip, lets the bone rotate in almost any direction.
• Some joints, like those in your skull, are fixed and don't move at all, because their job is protection, not movement.

At a moving joint, the ends of the bones are covered in smooth, slippery cartilage so they don't grind against each other. Tough bands called ligaments hold the bones together and keep the joint stable.

Muscles and bones working together

Muscles work in pairs to move your bones, because a muscle can only pull, not push. Think about bending your arm. The biceps muscle on the front of your upper arm contracts and pulls your forearm up. To straighten your arm again, the biceps relaxes and the triceps muscle at the back contracts and pulls it down. This push-pull teamwork is called an antagonistic pair.

The skeleton is just one of the body's amazing systems. To see how it fits alongside the heart, lungs and brain, explore the systems of the human body.

How bones heal

Because bones are alive, they can repair themselves. When a bone breaks, special cells rush to the spot. First they form a soft, rubbery patch, then they slowly fill it in with new bone, knitting the two ends back together. This is why a doctor puts a broken arm in a plaster cast — to hold the bone still and straight while it heals itself, usually over six to eight weeks. Bones are also constantly rebuilt throughout your whole life, replacing old bone with new.

Activity: the rubbery chicken bone

Here is a safe experiment that shows what gives bones their hardness.

1. Save a clean chicken bone after a meal (a leg or wing bone works well). Wash it and let it dry.
2. Bend it gently — notice that it is hard and stiff and will not bend.
3. Now place the bone in a jar and cover it completely with vinegar. Put a lid on.
4. Leave it for about three days, then take it out and rinse it.
5. Try to bend it again. This time it will be rubbery and floppy!

Why? The vinegar is an acid that dissolves the hard calcium minerals in the bone, leaving behind only the stretchy collagen protein. This proves that the calcium is what makes our bones strong — and it shows why a calcium-rich diet matters so much for growing bodies.

The next time you run, climb or even just sit and read, remember the 206 silent, living bones holding you together and keeping you safe.