Using spectacular graphics based on the latest science and stories of remarkable people around the world, Michael Mosley examines the human body.
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You're a member of the most fascinating species on this planet.
And the secret lies under your skin.
This is a voyage through the most extraordinary organism on Earth.
Your eye is a massive construction project.
And much of the development only begins after you are born.
The iris, which controls the amount of light entering your eye is complete,
but the muscles around the lens in the middle are still learning how to focus on the world.
At the back of your eye
lies the vast red plane of your retina.
Light from above shines down, casting images of the outside world across its surface.
Underneath, these rays enter a forest of 125 million light sensitive cells.
Each cell senses just a tiny part of the image but together
they send their information to the brain, which makes sense of it all.
Most of these are rod cells which can only see in shades of blue.
They help you see in the dark.
To see clearly in daylight you need an entirely different set of cells.
These are known as cone cells.
Four and a half million are spread over your retina.
But in one location, they are much denser.
Here, they begin to group together and the retina's surface begins to bulge.
Over the first four years of your life, the cones raise a volcano-like mound at the back of your eye.
Then, around 20,000 cone cells burst through at the summit.
This is your fovea.
The part of your eye where your vision is crystal clear.
It can sense over a million different colours.
Muscles in your eye work to focus light onto the retina.
Muscles in the iris respond to light levels, opening up
the iris in low light, and narrowing it when things get too bright.
On the remote Thai island of Ko Surin,
there are a group of people whose brains have learnt to overrule the muscles in their eyes.
Goong and his friends belong to the Moken tribe get much of their food from the sea.
But finding food in water is not easy.
That's because your eyes have adapted to see clearly in air,
so underwater they lose more than two-thirds of their power to see.
To compensate, Goong's brain does something remarkable.
As he descends, light levels drop quickly.
Normally, your eye would react
by opening the iris, making the pupil larger and allowing more light in.
The image may appear brighter, but it comes at a cost.
Because underwater, a wider iris makes everything appear more blurred.
Over time, Goong's brain has learned
to overrule this reflex of the eye...with an astonishing adaptation
best seen with the help of an infra-red camera.
Rather than opening his pupils, he closes them -
some of the muscles of the iris contract to their limit.
Which constricts the pupil as far as it will go.
Goong's view of the salty underwater world becomes much sharper.
It's so effective that Goong can see fine details twice as well as you can.
Your brain has an incredible ability to adapt your eyes to suit its needs.
And, for Goong, that means his dinner.
It is the incredible flexibility of the human brain
which enables you to respond to almost anything that happens to you.
But what you think you're seeing is really what your brain is interpreting.
It's an image that your brain constructs from the nerve impulses
it gets when light hits receptors in the retina.
Your brain makes sense of those signals and assembles a picture of the world around you.
The actual picture on your retina is upside down,
but your brain is clever enough to turn this the right way up.
One man's brain has done something even more spectacular.
I started going blind about ten years old.
Erik Weihenmayer's brain is 40 years old.
It lost contact with the visual world 25 years ago.
Every week I would wake up with different levels of vision.
Because my retinas were splitting away from my eyes.
And over a matter of four or five years I was totally blind.
Ever since, his brain has got used to being in the dark.
Today, Erik and his co-climber, Greg Childs, are in Utah.
They're about to attempt the formidable Castleton Rock.
It's a hard technical climb.
It's Erik's first attempt to climb here
and his brain is about to regain a sense of sight...
with a new piece of technology.
This is the brain port device.
This is the camera...
on these sunglasses here.
The camera sends a feed to a computer on Erik's hip...
..Which translates the images into a low-resolution picture of the world.
This blocky image is then sent to one of the most sensitive parts of Erik's body...his tongue.
Via a device he puts in his mouth.
On the surface are hundreds of tiny electrical stimulators.
When the camera sees an outline, a corresponding line of stimulators buzz away, tickling Erik's tongue.
I can feel each dot and together they create lines and shapes
and ultimately images that my brain then reinterprets as
the space around me.
Decades after he lost his sight, the visual part of Erik's brain is reawakening.
Erik's brain has found a way to connect his mind's eye and his tongue.
Instead of receiving nerve impulses from his dead retina,
his brain's learning to build up a picture of the world based on nerve impulses from his tongue.
Is that sight?
Well, kind of, you know, because I think seeing is more in your brain than in your eyes.
In rock climbing, most of the risk is taken by the lead climber.
Erik is feeling so confident with the brainport device,
that he makes a dramatic decision to lead the final push.
You pop over this lip and it's completely flat and the wind just
gusts in your face and you're up there on this flat tower 1,000 feet above the desert floor.
That was good, thanks.
This is totally beautiful up here.
Erik is living proof of the brain's astonishing ability to remould itself...
and respond to any challenge you throw at it.
Deep inside your head is a remarkably beautiful structure.
A maze of tunnels and caverns submerged in fluid.
These are the semicircular canals
and the cochlea, which are part of your inner ear...
they are crucial for both your balance and hearing.
When sound hits your ear, it sets off a wonderful chain of events.
It enters as pressure waves, which push and pull your ear drum, making it vibrate.
On the other side of the ear drum, slowing time allows us to see
how these vibrations set a series of bones jiggling.
They end with the smallest bone in your entire body,
called the stirrup. It is smaller than a grain of rice.
These bones transmit the vibrations to a sensitive area called the oval window.
They also protect your ear.
If a sound is too loud...
they pull the stirrup away from the most sensitive parts.
Temporarily at least, you go a bit deaf, but the rest of your ear is protected.
Beyond the stirrup is a liquid-filled cavern called your cochlea.
The sound waves enter the water, tickling clumps of tiny hair-like sensors on the floor,
which begin to dance to the sound of the world outside.
Thousands of clusters of hair cells each pick out a different part of the sound.
Each sends a tiny piece of information to your brain,
where it's interpreted so you can make sense of the sounds around you.
The ear has evolved to be most sensitive to the sounds of another voice,
allowing your brain to tune in to the words of another human.
But your ear doesn't just allow you to hear.
It also plays an essential role in allowing you to walk.
For you to take just one step, your brain has to coordinate the precise
movement of over 100 different muscles, bones and tendons.
And there is a place where people learn to walk sooner than anywhere else on Earth.
This is Koarmba.
She is mother to a baby girl called Kossini.
They live in Rhumsiki, a tiny village in the remote northern highlands of Cameroon.
Here, most mothers believe in actively teaching their babies
to stand and walk, to get them off their backs as soon as possible.
And it works. These people have trained their brains to find their feet much earlier than you.
Ever since Kossini was a month old, her mother has repeated the ancient ritual of katete,
which means "to make jump".
Every day, she takes hold under the arms and bounces her.
This daily encouragement helps the gradual development into mature walking.
But before you can stand up for any length of time,
your brain must learn to understand the orientation of your body...
..and that's why your inner ear is so important.
The semi-circular canals that form three twisting tunnels inside your ear
are all orientated in a different direction.
In each lies a saddle-topped fleshy mountain, known as your crista.
The mountain's slopes are covered in a thick forest of tiny hair cells.
For the moment, they lie still, waiting...
But this inner sea never remains calm for long.
Because every now and then...
..there's a tsunami.
A shockwave races through the tunnel and pummels the mountain.
On its flanks, the hair cells are thrown about in the turbulent waters.
The pressure builds until...
creating a powerful electrical current.
Every time your head moves...
the hairs cells are thrown about inside one or more of the canals.
And in a fraction of a second, electrical impulses are fired straight to your brain.
The feeling that emerges is your sense of balance.
To walk, your brain
has to learn to sense when you are over-balancing to one side...
and then instruct your leg to bring your weight back to the centre.
All of this happens within a fraction of a second.
To begin with, it's a real struggle.
But Kossini's half brother
is already a master of bipedalism.
And he is just ten months old.
From now on, and for the rest of his life, walking will be automatic.
The amazing construction of your ear allows you to both sense and explore the planet you inhabit.
For nine months, you were enveloped in the warm, comfortable world of your mother's womb.
Your every need was taken care of.
The placenta supplied you with oxygen from your mother's blood,
so your own lungs were hardly used.
At this stage of your life, your heart had completely different plumbing...
a hole through its centre diverted blood away from your lungs almost entirely.
And then, suddenly, your tranquillity was shattered.
As you were born,
you went through the most dramatic minute of your life as your body took over from your mother's.
Once out into the world, you were bombarded with new stimuli.
Bright glaring lights...
Cold air on your skin...
Scientists now believe it's the shock of these stimuli that triggers your first critical breath.
But before you can absorb life-giving oxygen, your circulation must be rapidly re-plumbed.
As you draw your first breath...
The airways of your lungs open, causing blood to rush into them to pick up oxygen...
That oxygen-rich blood then flows out of the lungs and into the heart.
As it does so, pressure builds up, closing a flap over the hole.
This hole will, in time, seal completely.
Your circulation is now complete.
Oxygen-rich blood can flow from your heart to the rest of your body...
deoxygenated blood flows back to your heart and to the lungs, where you get new oxygen.
So now you can take another breath...
for the rest of your life...
Your heart's will to beat, to keep going, is incredibly strong.
If the heart fails, so does everything else...
Because it's your heart's job to deliver to every cell, to every nook and cranny of your body,
the substance which keeps it alive - oxygen.
But how does the body do this?
The hard graft of carrying oxygen is done by some of the smallest
and most peculiar cells in your body.
These red blood cells are well suited to the job of carrying oxygen around your body.
Each red blood cell contains millions of haemoglobin proteins.
They have a structure that oxygen likes to bind to.
Haemoglobin is bulky, so there's not much space for anything else
inside the red blood cells, which have to squeeze into tiny blood vessels.
It means that red blood cells are unique amongst all your cells,
because to carry oxygen, they don't have a nucleus.
25 trillion red blood cells are pumped around the body, completing a circuit within a minute.
Each cell makes a tortuous journey...
through wide arteries that surge like a river in full flood.
Then branching off into smaller streams called arterioles.
It's perfectly shaped to squeeze through tiny, slow-moving capillaries.
And without a fat nucleus, it just about gets through.
Here, in the smallest, narrowest vessels, our cell does the job it's built for...
it releases its payload of oxygen into the tissues.
The reason that your body goes to such lengths to ensure a steady supply of oxygen
is because oxygen is an essential ingredient in unlocking the energy you need to survive.
Oxygen from the red blood cells passes into every other cell of your body.
Here, there are hundreds of little powerhouses called mitochondria.
They burn oxygen to release all the energy you need to live your life.
But to do this, they need another ingredient
which also comes via the bloodstream...
Food is mainly absorbed in the small intestine, which is covered in
finger-like projections called villi and microvilli.
They increase the surface area of your gut to that of a tennis court,
so you can absorb as many nutrients as possible.
If you eat a packet of crisps, for example, it is here that the crisps will be broken down into smaller
and smaller particles, until they are reduced to glucose...
which is small enough to be absorbed into the bloodstream,
ready to be transported to the mitochondria in your cells.
Over your lifetime, you will eat more than 50 tons of food, and take over 800 million breaths, which you
will convert into enough energy to power a house for five years.
And every mouthful and every breath has finished here, with your
mitochondria using the energy released to get you through the day.
Your heart is an exquisitely engineered pump made almost entirely of muscle.
And you can see the extraordinary engineering in action if we slow your heart to a single beat.
Inside the cavernous chambers, the muscles work together in perfect harmony.
These muscles never get tired and never stop working until you die.
As your heart expands, blood flows from your body into its chambers.
First the atria,
then the ventricles.
The left ventricle has to work particularly hard, because each of its contractions must have
enough power to push blood all the way through your body's vast network of blood vessels.
And that's a long way. If they were strung together, these vessels would circle the Earth at least twice.
Like the plumbing in your house, your heart needs valves
between its chambers and arteries to stop the blood flowing backwards.
As the valves slam shut, they make the familiar "lub-dub, lub-dub" sound
of the heartbeat in your chest.
Every single minute, your heart does this around 70 times.
And it's all regulated by some little cells at its core...
At the start of your life,
when you were just a three-week-old embryo, something happened
inside your body which was nothing short of a miracle...
These tireless cells -
called pacemakers, which control the beat of your heart - came into being...
They spontaneously beat out a rhythm...
sending synchronised signals through your heart.
Which speed up or slow down, according to what your body's doing.
And these pacemaker cells will stay with you always...
..faithfully responding to every demand of your life.
Katlyn Hagan will be relying on her pacemaker cells to do something extraordinary.
She needs to have major heart surgery...
her heart will be stopped for anything up to an hour.
There is a risk of death with heart surgery. I hate saying it, it's not zero.
I have a great team and we'll take great care of you tomorrow, I promise you.
We'll do everything we can for you.
I'm very scared.
I want to make sure I'm still living after my operation
so I can be there for my daughters growing up
and just live a normal life.
The operation will be tricky.
It's the very same pacemaker cells that keep Katlyn alive which are causing her problem.
But before they can operate, they must transfer the job of
pumping blood around Katlyn's body to a machine...
..and then stop her heart.
See it gradually slowing down.
As the fluid goes in, the heart gets a little whiter,
cos there's no blood going into it.
Without a heartbeat,
Katlyn is in a hinterland between life and death.
Now they can begin to remove the faulty pacemaker cells.
To do this, the surgeons use a cryoprobe,
which freezes and destroys them.
They have to be careful
to remove only the cells which are malfunctioning.
The main procedure is finished.
It's time to get Katlyn off the bypass machine
and reconnect her heart.
By now, her heart has been stopped for nearly an hour...
The heart's starting to get blood right now.
So we're inflating the lungs.
As blood flows back into Katlyn's heart,
its warmth and nutrients
are enough to re-start the pacemaker cells.
All right. Come on.
-And the rhythm returns.
It looks good.
The operation has been a success.
Katlyn's heart is now beating correctly.
Driven by a group of pacemaker cells
created within weeks of her conception.
These cells will remain with her
until her last heartbeat.
We are all born with a shield
which protects us from the dangers of the outside world.
It's our first line of defence -
Your skin is amazing.
The largest organ in your body -
up to two meters squared.
Each centimetre of skin is built from ten million individual cells.
This tiny square bristles with over 100 hairs.
And packs 100 sensors
that can detect the lightest of touches.
But it must also act as an impenetrable barrier.
Because your skin
is covered in millions of bacteria.
If they get inside your body,
your warm, moist tissues
will provide the perfect environment
for them to take over.
What prevents them getting in
is the clever way your skin is constructed.
Skin cells lock together like armour plates.
And it's not just passive protection,
your skin is constantly pushing outwards.
New layers grow underneath the old
and push the surface layers away.
This constant shedding
prevents most microbes
from getting a permanent hold.
However, it also means you lose 30,000 skin cells every day.
This time next month,
you'll have replaced all the skin on your body.
But your skin doesn't just protect you from living organisms.
It also needs to keep your internal organs safe from getting dehydrated.
At the base of every hair on your body is a tiny gland,
known as a sebaceous gland.
It protects you by squeezing an oily substance call Sebum
onto the surface of your skin.
Sebum is what makes your hair greasy.
And what gives you spots
and makes your skin waterproof.
This oil helps to prevent fluids inside your body
from evaporating into the air,
which can cause dehydration.
Your skin is constantly working in two directions.
Firstly, to stop bacteria from getting into your body.
And secondly, to protect the organs inside your body from drying out.
It's a perfectly-engineered protective layer.
Every minute of your life
your body is silently performing a host of small miracles
to keep you alive.
But all of them would stop, and you with them,
if one crucial factor in your body
were to change dramatically.
And that's your temperature.
Your body is designed to function at 37 degrees centigrade.
When your body overheats,
it stimulates sweat glands deep within your skin.
They produce beads of sweat.
Which work their way out of your body
and onto your skin's surface.
And it's here that sweat does it's work.
It cools your body by evaporating into the air...
keeping you alive when things hot up.
Extreme heat can be deadly.
For these elite firefighters in Texas,
their body's ability to keep their temperature constant
is a matter of life and death.
Their triple-lined fire suits
do much to protect them from the flames.
But there's something else -
their ability to sweat.
Firefighter Mario Rodriguez
is getting weighed to see how much sweat he looses
when fighting a fire.
To measure any temperature change inside his body,
Rodriguez takes an electronic pill.
To keep his heart and brain safe,
his core temperature must remain close to 37 degrees centigrade.
Before he goes in, his core temperature is just over 37 degrees.
If his core temperature rises by just four degrees
he will become confused and fall unconscious.
A rise of six degrees could cause death.
After 45 seconds in the 1,200 degree fire...
he's poaching in his own juices.
'It was real hot,
'my bones and all my joints were burning.'
The heat... Just got to get out of there and get some cool air.
His core temperature has risen by a very minimal one degree centigrade.
You're at 207 now.
So that looks like you lost three pounds of body weight.
Three pounds equals over a litre of sweat
lost in just one minute's exposure to the fire.
Rodriguez walked into a 1,200 degree fire
and walked out with his body temperature almost exactly the same.
This is the story of your creation.
It began with a sperm,
the smallest cell in the human body,
fusing with the biggest cell...
Everyday, the human male
produces hundreds of millions of DNA Torpedoes...
otherwise known as sperm.
Each ejaculation can contain 250 million of them.
That's enough, at least in theory,
to cover every inch of Manhattan with people.
Fortunately, the human female has other ideas.
For her, it's quality, not quantity.
Her ovaries usually produce just one egg every month.
The process of reproduction is so complex
it's a wonder it happens at all.
Most of the time, the sperm die in a pool,
trapped inside the vagina.
Because the entrance into the womb, through the cervix,
is out of reach, blocked shut.
For the sperm to have a chance of getting in,
timing is everything.
There are just a few days in any month
when the woman's body offers them an opportunity.
Hormones soften the blockage
at the entrance to the cervix.
Transforming it from a barrier into a life line.
But the sperm have to be strong
to make the 15cm journey.
It's a long way
for the smallest cell in the body.
The straighter and faster they can swim,
the better chance they have of making it.
The deformed, lazy and the dead
are left behind.
And, of course,
the more fit sperm you have,
the greater the chance of success.
If 250 million sperm began the journey,
only 1%, that's two and a half million,
will make it through to the cervix.
As they swim out of the vagina,
they're just at the beginning of a long and perilous journey.
The woman's body is about to launch an attack.
They have entered the cervix -
a labyrinth of dead ends.
perfect for an ambush.
The sperm have triggered the body's defence system.
White blood cells have recognised the sperm as a foreign invader
and, just as they would if they were fighting an infection,
they've been mobilised to kill.
They attack the sperm
as they swim through the cervix and into the uterus.
By the time the surviving sperm reach the fallopian tube,
where they are safe from the white blood cells
there could be as few as 20 left.
They will be 20 of the very best on offer.
Here, scientists have recently discovered
that the woman's body
has come up with an amazing trick.
She takes control of the sperm by holding them, one by one,
on the walls of her fallopian tube.
She then powers them down.
They're alive, safe, but fast asleep.
The woman now has up to five days
to release the precious egg growing inside her ovary.
As soon as it's ripe, the egg is released.
And it's wafted into the opening of the fallopian tube.
Once the egg is ready and waiting,
it's time to wake up the sleeping sperm...
Sending out a powerful chemical beacon,
the egg guides the sperm in.
It's the precursor of every new life.
The sperm - some male, some female -
compete to reach the egg.
But there can be only one winner.
The competing sperm break off the surrounding cloud of cells.
Until one finally pushes through
the soft shell underneath.
The egg is now in danger.
If a second sperm gets in,
the egg will die.
It must protect itself - and quickly.
Under the shell,
tiny granules detonate in a chain reaction.
Firing out chemicals,
hardening the shell,
making the egg impenetrable.
Fertilised, the egg is now safe.
This was how we all began.
You truly are an amazing creation.
We're made up of around a hundred trillion cells,
all coming from just one single fertilised egg.
Within hours of fertilisation,
this new cell, called a zygote,
divided into two identical cells.
Then into four...
16, and so on...
before implanting in the lining of the womb
and becoming an embryo.
In some cases, on rare occasions,
a single embryo creates two bodies.
one in 250 early embryos split.
If they do,
they must divide completely
within the first two weeks,
or they probably never will.
Once split, the embryos create near-replicas of themselves.
This quirk of nature
has given synchronised divers, Helen and Carol Galashan
a distinct advantage.
Being an identical twin
definitely helps with synchronised diving.
We don't really have to try with the synchronised part,
that part comes quite naturally to us.
We actually think we're mirror-image twins.
-We fold our arms opposite ways. Our hair parts the opposite way.
-Even when we're diving,
The first foot I put on is my right foot, Carol's is her left foot.
Identical twins actually come from one egg...
-That splits into two, and non-identical twins come from two eggs.
So the way we see it is that we were one person that split in two...
One person in two bodies.
This woman's pregnancy is even more unlikely than identical twins.
She's beaten odds of 4,500 to 1.
Diane is carrying non-identical triplets.
Remarkably, Diane's body
naturally produced not one, but three eggs in a single go.
It's an incredibly rare type of pregnancy.
Not only did Diane produce multiple eggs,
but all of them were fertilised.
Entirely independently of each other.
Apparently, I released three eggs,
and Mike, he had three separate sperm
that fertilised all three eggs.
They've all got their own placentas
and they're all in separate sacks.
So they've all got have their own little bedroom.
Effectively, Diane got pregnant three times in one go.
But there's a downside to having triplets.
31 weeks - nine weeks early -
Diane has gone into labour.
She is going to need an emergency caesarean.
I'm just going to bring him round to show you, quickly.
Here we go, you two. Here he is. He's beautiful.
So he's a little breach baby.
There he is.
-Isn't he lovely?
This is the skinny Minnie.
All the babies have now been delivered.
-Last but not least.
And against the odds,
they are all alive and well.
For your body,
this world is a dangerous place.
Threats lurk around every corner.
And it's not just the obvious dangers that threaten you.
Even now, as you watch this film,
there are pathogens waiting to get inside you.
A pathogen is a foreign invader that causes disease.
They spread in all sorts of ways,
commonly through sneezing.
A simple sneeze
is often all it takes
for viruses to jump from one person to another.
Sneezing is one of the most powerful forces your body produces.
You expel air at 100 miles an hour,
ejecting anything that isn't bolted down.
Once a pathogen invader, like the flu virus,
gets inside your body,
you have to respond quickly.
Time to turn on your immune system
to destroy them.
Your first response to infection is fever.
Raising your temperature by just a few degrees
is enough to slow them down.
Meanwhile, deep within your tissues,
an internal army is on the march.
On the frontline are phagocytes,
a form of white blood cell.
They flood the infection site to fight the viruses.
But in this case, the viruses are too strong.
Instead, the soldiers themselves become infected.
Now the only way these cells can kill the viruses
is to self-destruct.
As their bodies pile up,
they form the sticky basis of your snot.
But your immune system hasn't given up.
A second wave of attack is released
as another kind of white blood cell is unleashed...
the Killer T-cells.
Instead of attacking the viruses directly,
they take aim at your own infected cells.
They give a kiss of death,
they make the infected cell implode,
then self destruct,
destroying the viruses inside.
It's one of the reasons you'll get a sore throat.
Despite these two assaults,
the viruses haven't yet been defeated.
But your immune system has another trick.
Yet more white bloods cells,
this time called B-cells,
are able to recognise the specific invading pathogen
and produce a specialised weapon.
The Y-shaped antibody.
These can be produced at a rate of 2,000 per cell per second.
They coat the viruses...
slowing them down and making them stick together.
Now the viruses are easily swept up.
And you begin to feel better,
the fever drops and your energy starts to return.
The cells of your immune system
have won the day.
One of the most remarkable defence mechanisms your body has
is its ability to repair itself.
Johnny Greaves is 33 years old.
He is a professional boxer.
For Johnny, boxing isn't just a sport, it's a livelihood.
I'm here to pay my bills and keep my kids OK,
so obviously, that's the first thought in my mind,
bringing home the bacon and paying the kids' bills.
Johnny is in a dangerous game.
If he picks up the slightest injury,
boxing regulations mean he'll have to cancel his next fight.
If he show's up with a bruise he'll be disqualified.
His body is going to do everything in its power to avoid this.
All of Johnny's biological defence mechanisms
are clicking into action.
He ducks and dives
to avoid the punches.
But as Johnny tires, his defences begin to fail.
The full extent of Johnny's injuries have yet to be revealed.
A black eye is about to form.
His next fight is in two weeks,
if he is to collect that pay cheque
he has to heal.
Now its time for Johnny's body to really earn his living.
The delicate blood vessels in the tissue under Johnny's eye
were destroyed by a single punch.
As the vessels burst,
blood cells rush out.
But despite the catastrophic damage,
a repair crew is on the way.
Flowing out with the blood are cell fragments.
they are built to stop bleeding.
They gradually form a lattice
that catches the leaking blood cells like a net.
Extra support comes from a stringy protein - fibrin.
Together, they form the clot,
which plugs the hole
and the bleeding stops.
Five hours after the fight,
the effects of the punch
are beginning to show on Johnny's face.
His eye is beginning to swell.
Fluid is flowing into the tissue around the eye.
It's coming from tiny holes in the walls of the blood vessel.
It is a form of defence.
The force of the flow stops any infection in the wound
from travelling into the bloodstream,
trapping it in the tissue instead.
The results of this inflammation are dramatic.
Johnny's eye has now turned a striking shade of purple.
The colour is the product of decaying blood cells
trapped outside the circulatory system,
where they can't survive.
Now Johnny's body starts to clear up the mess.
Macrophages - giant white blood cells -
sweep through the tissue and absorb the dying cells.
Inside the macrophage,
haemoglobin, the chemical that gives red blood cells their unique redness, breaks down.
It's this that gives the bruise
it's familiar cocktail of colours.
As it breaks apart,
haemoglobin transforms into different-coloured chemicals.
Over time, the colour shifts
from green to yellow and finally to brown.
As the Macrophages leave,
they draw the coloured chemicals away from the skin.
Healing is now complete.
Johnny's body has repaired the damage,
just in time for another fight.
Subtitles by Red Bee Media Ltd
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