Movement: skeleton and muscles · System
What the skeleton does
A broken finger is an inconvenience. A broken rib makes every breath hurt. Same material — so what makes the difference?
Start here
Two broken bones, two completely different disasters.
Break a finger and you tape it to the next one and carry on. Break a femur and you are in hospital, off your feet for months, and at real risk from a blood clot. Both are bone. Both heal.
What actually decides how serious a broken bone is?
Not the size, and not the pain. What matters is the job that bone was doing, and how many other things were relying on it. The skeleton is doing four jobs at once, and they fail in four different ways.
The way to find out what a part does is not to label it. It is to switch it off and follow what breaks next.
Your turn · four parts, switched off
Take one part away and follow the damage
0 of 4 parts switched off
The skull
Eight curved plates, fused into a single box around the brain, with holes only where nerves and blood vessels have to pass through.
Switch it off. What is the first thing that goes wrong?
Skull switched off
Nerve cells are torn or crushed by an impact that the skin above them survives easily.
Unlike skin, nerve tissue does not replace what it loses. The damaged patch stops carrying signals for good.
Whatever that region of the brain controlled — a hand, speech, balance, breathing — is weakened or gone.
A bump that would have been a bruise is now permanent, and it can be fatal.
Protection is the only job on this list where nothing happens at all until something goes wrong — and then everything does.
The ribcage
Twelve pairs of curved bones hinged at the spine, with muscles slung between them. Swinging them up and outwards is how you pull in more air than quiet breathing needs.
Switch it off. What is the first thing that goes wrong?
Ribcage switched off
The diaphragm still pulls down, but with nothing holding the chest wall out it is sucked inwards as you breathe in. The volume of the chest barely changes.
Air only moves into the lungs because that volume increases. Far less arrives with every breath, and there is no way to pull in more.
Too little fresh air reaches the alveoli, so the blood leaving the lungs carries far less oxygen than it should.
Within minutes every cell in the body — brain cells first — is short of the oxygen it needs for respiration.
This one is fast. Minutes, not months. The ribcage is doing protection and movement at the same time, and it is the movement you die without.
The femur and hip
The thigh bone is the longest and strongest bone you have. Standing still, it carries the entire weight of everything above it into the ground.
Switch it off. What is the first thing that goes wrong?
Femur and hip switched off
The big muscles of the leg still contract and shorten. There is nothing rigid for them to pull against.
The leg cannot straighten or take weight, so it cannot hold up the body above it.
No standing, no walking, no carrying. Nothing can be reached, and nothing can be escaped from.
The route to food is gone, and within days the cells that depend on it have nothing to respire.
Support and movement are the same job seen from two sides: a rigid bar to hold you up, and a rigid bar for muscles to pull on.
The marrow inside the bones
The soft tissue in the hollow middle of the big bones. It makes about two million new red blood cells every second, for your whole life.
Switch it off. What is the first thing that goes wrong?
Marrow switched off
No new red blood cells are made anywhere in the body.
The red blood cells you already have wear out and are removed after about four months. Nothing replaces them.
The blood carries less and less oxygen with every week that passes.
Every cell in the body is short of the oxygen it needs to respire, and the whole organism slows down and fails.
Slow, invisible, and it ends up in exactly the same place as the ribcage. This is the job nobody guesses, because it is the only one you cannot see from the outside.
Four parts, four different routes, one destination. The ribcage kills in minutes and the marrow takes months, and both end in exactly the same place: cells with no oxygen to respire with.
Key fact
The skeleton does four jobs at once: support, protection, movement, and making blood cells. Every one of them, when it fails, ends at the same place — cells that can no longer work.
Your turn · six things people notice
Which job is this?
0 of 6 decided
Every one of these is something you could see or measure. Decide which of the four jobs it is evidence for. One of them is evidence for two.
A doctor takes a sample from inside the hip bone of a patient who is short of red blood cells.
Making blood cells. The marrow is the factory, so it is the marrow you sample when the supply has failed.
A 60 kg person stands still for an hour and does not fold up.
Support. Soft tissue cannot hold that load. Rigid bars, stacked in line, carry it into the ground.
A cyclist’s helmet is designed to crush and be thrown away after one impact.
Protection. The helmet is doing the skull’s job, more slowly. Both work by taking the energy so the brain does not.
Your ribs swing up and outwards every time you breathe in.
Movement and protection — both. This is the one that is evidence for two jobs at once. The same cage that shields the heart is also the thing that pulls air in.
A tennis player’s racket arm has denser bone than the other arm.
Support. The bone has been loaded harder, so it has built more material exactly where the force goes. Support is the job it is tuning itself for.
A sprinter’s heel bone has a tendon attached to the back of it, well behind the ankle joint.
Movement. A muscle needs something rigid to pull on, and where it attaches decides what the pull does.
Six words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Bones are dead. They are the hard bits left over — like the frame of a tent.”
The skeleton in the corner of the lab is dead. The one you are sitting on is not. Commit to an answer before you read on.
All four are true, and every one of them is impossible for something dead. Bone is a living tissue: it is made of cells, it has its own blood supply, it repairs itself when it breaks, and it is rebuilt continuously — you replace roughly a tenth of your skeleton every year.
It also responds to what you do with it. Load a bone regularly and it lays down more material exactly where the force goes; stop loading it and it thins. That is why a tennis player's racket arm has measurably denser bone than the other one, and why astronauts come home with weaker skeletons than they left with.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Where are new red blood cells made?
Rung 2 · The one that catches people
Why would damage to the ribcage stop oxygen reaching the cells in your toes?
Rung 3 · Explain
"The skeleton is a frame that holds you up." Explain why that is not good enough, using two jobs a frame does not do.
Rung 4 · Take it somewhere new
Astronauts lose roughly 1 to 2% of their bone mass every month in orbit, even though they eat well and exercise. Explain which of the skeleton’s four jobs is being lost, which are not, and why weightlessness causes it.
Key note
Support, protection, movement, blood cells. Bone is living tissue with its own cells and blood supply, and it rebuilds itself around the forces you put through it. Switch any part of the skeleton off and the failure ends at the cells.
Going further
A femur is hollow. So is every long bone you own, and so is the wing bone of a bird. Filling it in would add weight everywhere and strength almost nowhere, because a bar loaded from the side is barely stressed down its middle — the work happens near the surface. Engineers reached the same answer independently: scaffolding poles, bicycle frames and aircraft spars are all tubes. And the space inside the tube is not wasted. That is where your blood is made.
Before this lesson
Next in this unit
At GCSE this becomes
- Bone as a tissue, osteoporosis, and the transport system that carries what the marrow makes.
Where to next
- Next: Joints
- Previous: Unicellular organisms
Cells and organisation
Ask Mr Badmus AI
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