0:00:03 > 0:00:09You're a member of the most fascinating species on this planet.
0:00:09 > 0:00:11And the secret lies under your skin.
0:00:13 > 0:00:18This is a voyage through the most extraordinary organism on Earth.
0:00:21 > 0:00:23You.
0:00:35 > 0:00:39Your eye is a massive construction project.
0:00:39 > 0:00:43And much of the development only begins after you are born.
0:00:46 > 0:00:52The iris, which controls the amount of light entering your eye is complete,
0:00:52 > 0:00:58but the muscles around the lens in the middle are still learning how to focus on the world.
0:01:00 > 0:01:03At the back of your eye
0:01:03 > 0:01:10lies the vast red plane of your retina.
0:01:10 > 0:01:16Light from above shines down, casting images of the outside world across its surface.
0:01:20 > 0:01:28Underneath, these rays enter a forest of 125 million light sensitive cells.
0:01:30 > 0:01:35Each cell senses just a tiny part of the image but together
0:01:35 > 0:01:40they send their information to the brain, which makes sense of it all.
0:01:40 > 0:01:45Most of these are rod cells which can only see in shades of blue.
0:01:46 > 0:01:49They help you see in the dark.
0:01:53 > 0:01:58To see clearly in daylight you need an entirely different set of cells.
0:02:02 > 0:02:05These are known as cone cells.
0:02:05 > 0:02:09Four and a half million are spread over your retina.
0:02:09 > 0:02:12But in one location, they are much denser.
0:02:14 > 0:02:20Here, they begin to group together and the retina's surface begins to bulge.
0:02:24 > 0:02:31Over the first four years of your life, the cones raise a volcano-like mound at the back of your eye.
0:02:33 > 0:02:38Then, around 20,000 cone cells burst through at the summit.
0:02:41 > 0:02:44This is your fovea.
0:02:44 > 0:02:48The part of your eye where your vision is crystal clear.
0:02:48 > 0:02:51It can sense over a million different colours.
0:02:56 > 0:03:01Muscles in your eye work to focus light onto the retina.
0:03:03 > 0:03:07Muscles in the iris respond to light levels, opening up
0:03:07 > 0:03:12the iris in low light, and narrowing it when things get too bright.
0:03:18 > 0:03:21On the remote Thai island of Ko Surin,
0:03:21 > 0:03:27there are a group of people whose brains have learnt to overrule the muscles in their eyes.
0:03:29 > 0:03:36Goong and his friends belong to the Moken tribe get much of their food from the sea.
0:03:36 > 0:03:40But finding food in water is not easy.
0:03:40 > 0:03:44That's because your eyes have adapted to see clearly in air,
0:03:48 > 0:03:53so underwater they lose more than two-thirds of their power to see.
0:03:56 > 0:04:01To compensate, Goong's brain does something remarkable.
0:04:01 > 0:04:06As he descends, light levels drop quickly.
0:04:08 > 0:04:10Normally, your eye would react
0:04:10 > 0:04:18by opening the iris, making the pupil larger and allowing more light in.
0:04:18 > 0:04:23The image may appear brighter, but it comes at a cost.
0:04:23 > 0:04:28Because underwater, a wider iris makes everything appear more blurred.
0:04:30 > 0:04:34Over time, Goong's brain has learned
0:04:34 > 0:04:40to overrule this reflex of the eye...with an astonishing adaptation
0:04:40 > 0:04:44best seen with the help of an infra-red camera.
0:04:44 > 0:04:48Rather than opening his pupils, he closes them -
0:04:48 > 0:04:52some of the muscles of the iris contract to their limit.
0:04:52 > 0:04:56Which constricts the pupil as far as it will go.
0:05:01 > 0:05:06Goong's view of the salty underwater world becomes much sharper.
0:05:12 > 0:05:20It's so effective that Goong can see fine details twice as well as you can.
0:05:20 > 0:05:26Your brain has an incredible ability to adapt your eyes to suit its needs.
0:05:26 > 0:05:30And, for Goong, that means his dinner.
0:05:42 > 0:05:45It is the incredible flexibility of the human brain
0:05:45 > 0:05:52which enables you to respond to almost anything that happens to you.
0:05:52 > 0:05:58But what you think you're seeing is really what your brain is interpreting.
0:05:58 > 0:06:03It's an image that your brain constructs from the nerve impulses
0:06:03 > 0:06:06it gets when light hits receptors in the retina.
0:06:08 > 0:06:14Your brain makes sense of those signals and assembles a picture of the world around you.
0:06:16 > 0:06:20The actual picture on your retina is upside down,
0:06:20 > 0:06:25but your brain is clever enough to turn this the right way up.
0:06:25 > 0:06:30One man's brain has done something even more spectacular.
0:06:30 > 0:06:34I started going blind about ten years old.
0:06:37 > 0:06:41Erik Weihenmayer's brain is 40 years old.
0:06:43 > 0:06:47It lost contact with the visual world 25 years ago.
0:06:50 > 0:06:54Every week I would wake up with different levels of vision.
0:06:54 > 0:06:58Because my retinas were splitting away from my eyes.
0:06:58 > 0:07:03And over a matter of four or five years I was totally blind.
0:07:05 > 0:07:10Ever since, his brain has got used to being in the dark.
0:07:14 > 0:07:20Today, Erik and his co-climber, Greg Childs, are in Utah.
0:07:20 > 0:07:25They're about to attempt the formidable Castleton Rock.
0:07:32 > 0:07:34It's a hard technical climb.
0:07:36 > 0:07:39It's Erik's first attempt to climb here
0:07:39 > 0:07:43and his brain is about to regain a sense of sight...
0:07:43 > 0:07:46with a new piece of technology.
0:07:46 > 0:07:49This is the brain port device.
0:07:49 > 0:07:53This is the camera...
0:07:53 > 0:07:55on these sunglasses here.
0:07:55 > 0:07:59The camera sends a feed to a computer on Erik's hip...
0:08:02 > 0:08:06..Which translates the images into a low-resolution picture of the world.
0:08:11 > 0:08:19This blocky image is then sent to one of the most sensitive parts of Erik's body...his tongue.
0:08:19 > 0:08:22Via a device he puts in his mouth.
0:08:22 > 0:08:26On the surface are hundreds of tiny electrical stimulators.
0:08:26 > 0:08:36When the camera sees an outline, a corresponding line of stimulators buzz away, tickling Erik's tongue.
0:08:36 > 0:08:42I can feel each dot and together they create lines and shapes
0:08:42 > 0:08:48and ultimately images that my brain then reinterprets as
0:08:48 > 0:08:50the space around me.
0:08:50 > 0:08:57Decades after he lost his sight, the visual part of Erik's brain is reawakening.
0:09:00 > 0:09:05Erik's brain has found a way to connect his mind's eye and his tongue.
0:09:10 > 0:09:13Instead of receiving nerve impulses from his dead retina,
0:09:13 > 0:09:20his brain's learning to build up a picture of the world based on nerve impulses from his tongue.
0:09:20 > 0:09:22Is that sight?
0:09:22 > 0:09:29Well, kind of, you know, because I think seeing is more in your brain than in your eyes.
0:09:35 > 0:09:40In rock climbing, most of the risk is taken by the lead climber.
0:09:45 > 0:09:48Erik is feeling so confident with the brainport device,
0:09:48 > 0:09:53that he makes a dramatic decision to lead the final push.
0:10:11 > 0:10:16You pop over this lip and it's completely flat and the wind just
0:10:16 > 0:10:22gusts in your face and you're up there on this flat tower 1,000 feet above the desert floor.
0:10:28 > 0:10:30That was good, thanks.
0:10:30 > 0:10:33This is totally beautiful up here.
0:10:33 > 0:10:39Erik is living proof of the brain's astonishing ability to remould itself...
0:10:39 > 0:10:43and respond to any challenge you throw at it.
0:10:53 > 0:10:59Deep inside your head is a remarkably beautiful structure.
0:10:59 > 0:11:04A maze of tunnels and caverns submerged in fluid.
0:11:04 > 0:11:08These are the semicircular canals
0:11:08 > 0:11:11and the cochlea, which are part of your inner ear...
0:11:11 > 0:11:16they are crucial for both your balance and hearing.
0:11:19 > 0:11:24When sound hits your ear, it sets off a wonderful chain of events.
0:11:29 > 0:11:36It enters as pressure waves, which push and pull your ear drum, making it vibrate.
0:11:39 > 0:11:45On the other side of the ear drum, slowing time allows us to see
0:11:45 > 0:11:48how these vibrations set a series of bones jiggling.
0:11:50 > 0:11:53They end with the smallest bone in your entire body,
0:11:53 > 0:11:57called the stirrup. It is smaller than a grain of rice.
0:11:58 > 0:12:04These bones transmit the vibrations to a sensitive area called the oval window.
0:12:04 > 0:12:08They also protect your ear.
0:12:08 > 0:12:09If a sound is too loud...
0:12:09 > 0:12:14they pull the stirrup away from the most sensitive parts.
0:12:14 > 0:12:20Temporarily at least, you go a bit deaf, but the rest of your ear is protected.
0:12:22 > 0:12:28Beyond the stirrup is a liquid-filled cavern called your cochlea.
0:12:28 > 0:12:34The sound waves enter the water, tickling clumps of tiny hair-like sensors on the floor,
0:12:34 > 0:12:38which begin to dance to the sound of the world outside.
0:12:38 > 0:12:44Thousands of clusters of hair cells each pick out a different part of the sound.
0:12:44 > 0:12:48Each sends a tiny piece of information to your brain,
0:12:48 > 0:12:53where it's interpreted so you can make sense of the sounds around you.
0:12:58 > 0:13:04The ear has evolved to be most sensitive to the sounds of another voice,
0:13:04 > 0:13:09allowing your brain to tune in to the words of another human.
0:13:13 > 0:13:17But your ear doesn't just allow you to hear.
0:13:17 > 0:13:23It also plays an essential role in allowing you to walk.
0:13:23 > 0:13:28For you to take just one step, your brain has to coordinate the precise
0:13:28 > 0:13:33movement of over 100 different muscles, bones and tendons.
0:13:35 > 0:13:42And there is a place where people learn to walk sooner than anywhere else on Earth.
0:13:46 > 0:13:48This is Koarmba.
0:13:48 > 0:13:52She is mother to a baby girl called Kossini.
0:13:52 > 0:13:59They live in Rhumsiki, a tiny village in the remote northern highlands of Cameroon.
0:13:59 > 0:14:02Here, most mothers believe in actively teaching their babies
0:14:02 > 0:14:07to stand and walk, to get them off their backs as soon as possible.
0:14:09 > 0:14:17And it works. These people have trained their brains to find their feet much earlier than you.
0:14:19 > 0:14:26Ever since Kossini was a month old, her mother has repeated the ancient ritual of katete,
0:14:26 > 0:14:29which means "to make jump".
0:14:31 > 0:14:35Every day, she takes hold under the arms and bounces her.
0:14:35 > 0:14:40This daily encouragement helps the gradual development into mature walking.
0:14:40 > 0:14:47But before you can stand up for any length of time,
0:14:47 > 0:14:52your brain must learn to understand the orientation of your body...
0:14:54 > 0:14:58..and that's why your inner ear is so important.
0:15:03 > 0:15:09The semi-circular canals that form three twisting tunnels inside your ear
0:15:09 > 0:15:12are all orientated in a different direction.
0:15:14 > 0:15:20In each lies a saddle-topped fleshy mountain, known as your crista.
0:15:22 > 0:15:28The mountain's slopes are covered in a thick forest of tiny hair cells.
0:15:28 > 0:15:32For the moment, they lie still, waiting...
0:15:35 > 0:15:40But this inner sea never remains calm for long.
0:15:40 > 0:15:42Because every now and then...
0:15:48 > 0:15:49..there's a tsunami.
0:15:58 > 0:16:03A shockwave races through the tunnel and pummels the mountain.
0:16:05 > 0:16:10On its flanks, the hair cells are thrown about in the turbulent waters.
0:16:12 > 0:16:15The pressure builds until...
0:16:15 > 0:16:17electricity flows...
0:16:21 > 0:16:23creating a powerful electrical current.
0:16:28 > 0:16:30Every time your head moves...
0:16:30 > 0:16:35the hairs cells are thrown about inside one or more of the canals.
0:16:35 > 0:16:41And in a fraction of a second, electrical impulses are fired straight to your brain.
0:16:43 > 0:16:47The feeling that emerges is your sense of balance.
0:16:50 > 0:16:52To walk, your brain
0:16:52 > 0:16:57has to learn to sense when you are over-balancing to one side...
0:16:58 > 0:17:05and then instruct your leg to bring your weight back to the centre.
0:17:05 > 0:17:10All of this happens within a fraction of a second.
0:17:13 > 0:17:17To begin with, it's a real struggle.
0:17:17 > 0:17:20But Kossini's half brother
0:17:20 > 0:17:24is already a master of bipedalism.
0:17:24 > 0:17:27And he is just ten months old.
0:17:27 > 0:17:32From now on, and for the rest of his life, walking will be automatic.
0:17:35 > 0:17:42The amazing construction of your ear allows you to both sense and explore the planet you inhabit.
0:17:50 > 0:17:57For nine months, you were enveloped in the warm, comfortable world of your mother's womb.
0:17:59 > 0:18:02Your every need was taken care of.
0:18:02 > 0:18:07The placenta supplied you with oxygen from your mother's blood,
0:18:07 > 0:18:09so your own lungs were hardly used.
0:18:09 > 0:18:15At this stage of your life, your heart had completely different plumbing...
0:18:15 > 0:18:23a hole through its centre diverted blood away from your lungs almost entirely.
0:18:23 > 0:18:27And then, suddenly, your tranquillity was shattered.
0:18:31 > 0:18:33Relax...
0:18:36 > 0:18:38Relax.
0:18:43 > 0:18:45As you were born,
0:18:45 > 0:18:52you went through the most dramatic minute of your life as your body took over from your mother's.
0:18:52 > 0:18:58Once out into the world, you were bombarded with new stimuli.
0:18:58 > 0:19:00Bright glaring lights...
0:19:00 > 0:19:02Cold air on your skin...
0:19:04 > 0:19:06Strange sounds.
0:19:08 > 0:19:15Scientists now believe it's the shock of these stimuli that triggers your first critical breath.
0:19:20 > 0:19:28But before you can absorb life-giving oxygen, your circulation must be rapidly re-plumbed.
0:19:32 > 0:19:35As you draw your first breath...
0:19:35 > 0:19:42The airways of your lungs open, causing blood to rush into them to pick up oxygen...
0:19:43 > 0:19:48That oxygen-rich blood then flows out of the lungs and into the heart.
0:19:52 > 0:19:57As it does so, pressure builds up, closing a flap over the hole.
0:19:59 > 0:20:02This hole will, in time, seal completely.
0:20:02 > 0:20:05Your circulation is now complete.
0:20:06 > 0:20:12Oxygen-rich blood can flow from your heart to the rest of your body...
0:20:12 > 0:20:18deoxygenated blood flows back to your heart and to the lungs, where you get new oxygen.
0:20:21 > 0:20:25So now you can take another breath...
0:20:25 > 0:20:27and another...
0:20:27 > 0:20:28for the rest of your life...
0:20:38 > 0:20:43Your heart's will to beat, to keep going, is incredibly strong.
0:20:43 > 0:20:47If the heart fails, so does everything else...
0:20:47 > 0:20:54Because it's your heart's job to deliver to every cell, to every nook and cranny of your body,
0:20:54 > 0:20:59the substance which keeps it alive - oxygen.
0:20:59 > 0:21:02But how does the body do this?
0:21:05 > 0:21:10The hard graft of carrying oxygen is done by some of the smallest
0:21:10 > 0:21:13and most peculiar cells in your body.
0:21:13 > 0:21:21These red blood cells are well suited to the job of carrying oxygen around your body.
0:21:21 > 0:21:26Each red blood cell contains millions of haemoglobin proteins.
0:21:26 > 0:21:30They have a structure that oxygen likes to bind to.
0:21:33 > 0:21:36Haemoglobin is bulky, so there's not much space for anything else
0:21:36 > 0:21:43inside the red blood cells, which have to squeeze into tiny blood vessels.
0:21:43 > 0:21:47It means that red blood cells are unique amongst all your cells,
0:21:47 > 0:21:50because to carry oxygen, they don't have a nucleus.
0:21:53 > 0:22:0125 trillion red blood cells are pumped around the body, completing a circuit within a minute.
0:22:03 > 0:22:07Each cell makes a tortuous journey...
0:22:07 > 0:22:12through wide arteries that surge like a river in full flood.
0:22:12 > 0:22:16Then branching off into smaller streams called arterioles.
0:22:22 > 0:22:29It's perfectly shaped to squeeze through tiny, slow-moving capillaries.
0:22:29 > 0:22:32And without a fat nucleus, it just about gets through.
0:22:34 > 0:22:40Here, in the smallest, narrowest vessels, our cell does the job it's built for...
0:22:40 > 0:22:44it releases its payload of oxygen into the tissues.
0:22:49 > 0:22:55The reason that your body goes to such lengths to ensure a steady supply of oxygen
0:22:55 > 0:23:02is because oxygen is an essential ingredient in unlocking the energy you need to survive.
0:23:10 > 0:23:15Oxygen from the red blood cells passes into every other cell of your body.
0:23:21 > 0:23:26Here, there are hundreds of little powerhouses called mitochondria.
0:23:27 > 0:23:33They burn oxygen to release all the energy you need to live your life.
0:23:41 > 0:23:45But to do this, they need another ingredient
0:23:45 > 0:23:50which also comes via the bloodstream...
0:23:50 > 0:23:51food.
0:23:55 > 0:24:00Food is mainly absorbed in the small intestine, which is covered in
0:24:00 > 0:24:05finger-like projections called villi and microvilli.
0:24:05 > 0:24:11They increase the surface area of your gut to that of a tennis court,
0:24:11 > 0:24:14so you can absorb as many nutrients as possible.
0:24:17 > 0:24:24If you eat a packet of crisps, for example, it is here that the crisps will be broken down into smaller
0:24:24 > 0:24:28and smaller particles, until they are reduced to glucose...
0:24:28 > 0:24:32which is small enough to be absorbed into the bloodstream,
0:24:32 > 0:24:37ready to be transported to the mitochondria in your cells.
0:24:39 > 0:24:47Over your lifetime, you will eat more than 50 tons of food, and take over 800 million breaths, which you
0:24:47 > 0:24:51will convert into enough energy to power a house for five years.
0:24:54 > 0:24:58And every mouthful and every breath has finished here, with your
0:24:58 > 0:25:03mitochondria using the energy released to get you through the day.
0:25:11 > 0:25:18Your heart is an exquisitely engineered pump made almost entirely of muscle.
0:25:18 > 0:25:24And you can see the extraordinary engineering in action if we slow your heart to a single beat.
0:25:30 > 0:25:35Inside the cavernous chambers, the muscles work together in perfect harmony.
0:25:42 > 0:25:47These muscles never get tired and never stop working until you die.
0:25:50 > 0:25:55As your heart expands, blood flows from your body into its chambers.
0:25:57 > 0:25:59First the atria,
0:25:59 > 0:26:01then the ventricles.
0:26:04 > 0:26:10The left ventricle has to work particularly hard, because each of its contractions must have
0:26:10 > 0:26:16enough power to push blood all the way through your body's vast network of blood vessels.
0:26:18 > 0:26:26And that's a long way. If they were strung together, these vessels would circle the Earth at least twice.
0:26:28 > 0:26:31Like the plumbing in your house, your heart needs valves
0:26:31 > 0:26:36between its chambers and arteries to stop the blood flowing backwards.
0:26:39 > 0:26:45As the valves slam shut, they make the familiar "lub-dub, lub-dub" sound
0:26:45 > 0:26:47of the heartbeat in your chest.
0:26:49 > 0:26:55Every single minute, your heart does this around 70 times.
0:26:56 > 0:27:00And it's all regulated by some little cells at its core...
0:27:04 > 0:27:06At the start of your life,
0:27:06 > 0:27:11when you were just a three-week-old embryo, something happened
0:27:11 > 0:27:14inside your body which was nothing short of a miracle...
0:27:16 > 0:27:18These tireless cells -
0:27:18 > 0:27:22called pacemakers, which control the beat of your heart - came into being...
0:27:24 > 0:27:27They spontaneously beat out a rhythm...
0:27:27 > 0:27:30sending synchronised signals through your heart.
0:27:30 > 0:27:35Which speed up or slow down, according to what your body's doing.
0:27:37 > 0:27:40And these pacemaker cells will stay with you always...
0:27:42 > 0:27:47..faithfully responding to every demand of your life.
0:27:50 > 0:27:57Katlyn Hagan will be relying on her pacemaker cells to do something extraordinary.
0:27:57 > 0:28:00She needs to have major heart surgery...
0:28:00 > 0:28:04her heart will be stopped for anything up to an hour.
0:28:04 > 0:28:07There is a risk of death with heart surgery. I hate saying it, it's not zero.
0:28:07 > 0:28:11I have a great team and we'll take great care of you tomorrow, I promise you.
0:28:11 > 0:28:12We'll do everything we can for you.
0:28:19 > 0:28:20I'm very scared.
0:28:21 > 0:28:25I want to make sure I'm still living after my operation
0:28:25 > 0:28:30so I can be there for my daughters growing up
0:28:31 > 0:28:34and just live a normal life.
0:28:39 > 0:28:43The operation will be tricky.
0:28:43 > 0:28:48It's the very same pacemaker cells that keep Katlyn alive which are causing her problem.
0:28:55 > 0:28:59But before they can operate, they must transfer the job of
0:28:59 > 0:29:02pumping blood around Katlyn's body to a machine...
0:29:05 > 0:29:08..and then stop her heart.
0:29:08 > 0:29:10See it gradually slowing down.
0:29:10 > 0:29:14As the fluid goes in, the heart gets a little whiter,
0:29:14 > 0:29:17cos there's no blood going into it.
0:29:21 > 0:29:22Without a heartbeat,
0:29:22 > 0:29:27Katlyn is in a hinterland between life and death.
0:29:29 > 0:29:33Now they can begin to remove the faulty pacemaker cells.
0:29:33 > 0:29:36To do this, the surgeons use a cryoprobe,
0:29:36 > 0:29:39which freezes and destroys them.
0:29:41 > 0:29:42They have to be careful
0:29:42 > 0:29:45to remove only the cells which are malfunctioning.
0:29:52 > 0:29:55The main procedure is finished.
0:29:55 > 0:29:58It's time to get Katlyn off the bypass machine
0:29:58 > 0:29:59and reconnect her heart.
0:30:04 > 0:30:09By now, her heart has been stopped for nearly an hour...
0:30:09 > 0:30:14The heart's starting to get blood right now.
0:30:14 > 0:30:17So we're inflating the lungs.
0:30:17 > 0:30:19As blood flows back into Katlyn's heart,
0:30:19 > 0:30:21its warmth and nutrients
0:30:21 > 0:30:25are enough to re-start the pacemaker cells.
0:30:25 > 0:30:27All right. Come on.
0:30:29 > 0:30:32- MACHINE BEEPS - And the rhythm returns.
0:30:32 > 0:30:34It looks good.
0:30:39 > 0:30:42The operation has been a success.
0:30:47 > 0:30:52Katlyn's heart is now beating correctly.
0:30:52 > 0:30:55Driven by a group of pacemaker cells
0:30:55 > 0:30:58created within weeks of her conception.
0:30:58 > 0:31:01These cells will remain with her
0:31:01 > 0:31:03until her last heartbeat.
0:31:08 > 0:31:11We are all born with a shield
0:31:11 > 0:31:15which protects us from the dangers of the outside world.
0:31:15 > 0:31:17It's our first line of defence -
0:31:17 > 0:31:19our skin.
0:31:21 > 0:31:24Your skin is amazing.
0:31:24 > 0:31:26The largest organ in your body -
0:31:26 > 0:31:29up to two meters squared.
0:31:32 > 0:31:37Each centimetre of skin is built from ten million individual cells.
0:31:40 > 0:31:44This tiny square bristles with over 100 hairs.
0:31:46 > 0:31:48And packs 100 sensors
0:31:48 > 0:31:51that can detect the lightest of touches.
0:31:51 > 0:31:54But it must also act as an impenetrable barrier.
0:31:56 > 0:31:58Because your skin
0:31:58 > 0:32:01is covered in millions of bacteria.
0:32:01 > 0:32:03If they get inside your body,
0:32:03 > 0:32:05your warm, moist tissues
0:32:05 > 0:32:08will provide the perfect environment
0:32:08 > 0:32:09for them to take over.
0:32:09 > 0:32:12What prevents them getting in
0:32:12 > 0:32:15is the clever way your skin is constructed.
0:32:15 > 0:32:18Skin cells lock together like armour plates.
0:32:22 > 0:32:24And it's not just passive protection,
0:32:24 > 0:32:27your skin is constantly pushing outwards.
0:32:29 > 0:32:32New layers grow underneath the old
0:32:32 > 0:32:35and push the surface layers away.
0:32:35 > 0:32:36This constant shedding
0:32:36 > 0:32:38prevents most microbes
0:32:38 > 0:32:40from getting a permanent hold.
0:32:42 > 0:32:47However, it also means you lose 30,000 skin cells every day.
0:32:49 > 0:32:51This time next month,
0:32:51 > 0:32:54you'll have replaced all the skin on your body.
0:32:59 > 0:33:03But your skin doesn't just protect you from living organisms.
0:33:05 > 0:33:10It also needs to keep your internal organs safe from getting dehydrated.
0:33:14 > 0:33:19At the base of every hair on your body is a tiny gland,
0:33:19 > 0:33:22known as a sebaceous gland.
0:33:23 > 0:33:28It protects you by squeezing an oily substance call Sebum
0:33:28 > 0:33:31onto the surface of your skin.
0:33:31 > 0:33:34Sebum is what makes your hair greasy.
0:33:34 > 0:33:36And what gives you spots
0:33:36 > 0:33:39and makes your skin waterproof.
0:33:40 > 0:33:44This oil helps to prevent fluids inside your body
0:33:44 > 0:33:47from evaporating into the air,
0:33:47 > 0:33:49which can cause dehydration.
0:33:51 > 0:33:54Your skin is constantly working in two directions.
0:33:56 > 0:34:01Firstly, to stop bacteria from getting into your body.
0:34:01 > 0:34:06And secondly, to protect the organs inside your body from drying out.
0:34:06 > 0:34:10It's a perfectly-engineered protective layer.
0:34:18 > 0:34:20Every minute of your life
0:34:20 > 0:34:24your body is silently performing a host of small miracles
0:34:24 > 0:34:26to keep you alive.
0:34:26 > 0:34:30But all of them would stop, and you with them,
0:34:30 > 0:34:32if one crucial factor in your body
0:34:32 > 0:34:35were to change dramatically.
0:34:35 > 0:34:37And that's your temperature.
0:34:37 > 0:34:43Your body is designed to function at 37 degrees centigrade.
0:34:48 > 0:34:50When your body overheats,
0:34:50 > 0:34:55it stimulates sweat glands deep within your skin.
0:34:55 > 0:34:58They produce beads of sweat.
0:34:58 > 0:35:01Which work their way out of your body
0:35:01 > 0:35:03and onto your skin's surface.
0:35:06 > 0:35:09And it's here that sweat does it's work.
0:35:13 > 0:35:18It cools your body by evaporating into the air...
0:35:20 > 0:35:24keeping you alive when things hot up.
0:35:28 > 0:35:31Extreme heat can be deadly.
0:35:37 > 0:35:40For these elite firefighters in Texas,
0:35:40 > 0:35:43their body's ability to keep their temperature constant
0:35:43 > 0:35:45is a matter of life and death.
0:35:45 > 0:35:48Their triple-lined fire suits
0:35:48 > 0:35:50do much to protect them from the flames.
0:35:52 > 0:35:55But there's something else -
0:35:55 > 0:35:57their ability to sweat.
0:36:02 > 0:36:05Firefighter Mario Rodriguez
0:36:05 > 0:36:08is getting weighed to see how much sweat he looses
0:36:08 > 0:36:09when fighting a fire.
0:36:11 > 0:36:14To measure any temperature change inside his body,
0:36:14 > 0:36:17Rodriguez takes an electronic pill.
0:36:17 > 0:36:19To keep his heart and brain safe,
0:36:19 > 0:36:24his core temperature must remain close to 37 degrees centigrade.
0:36:32 > 0:36:37Before he goes in, his core temperature is just over 37 degrees.
0:36:50 > 0:36:54If his core temperature rises by just four degrees
0:36:54 > 0:36:58he will become confused and fall unconscious.
0:36:58 > 0:37:00A rise of six degrees could cause death.
0:37:04 > 0:37:10After 45 seconds in the 1,200 degree fire...
0:37:10 > 0:37:14he's poaching in his own juices.
0:37:29 > 0:37:30'It was real hot,
0:37:30 > 0:37:34'my bones and all my joints were burning.'
0:37:34 > 0:37:37The heat... Just got to get out of there and get some cool air.
0:37:37 > 0:37:43His core temperature has risen by a very minimal one degree centigrade.
0:37:46 > 0:37:48You're at 207 now.
0:37:48 > 0:37:51So that looks like you lost three pounds of body weight.
0:37:51 > 0:37:54Three pounds equals over a litre of sweat
0:37:54 > 0:37:58lost in just one minute's exposure to the fire.
0:38:00 > 0:38:04Rodriguez walked into a 1,200 degree fire
0:38:04 > 0:38:09and walked out with his body temperature almost exactly the same.
0:38:17 > 0:38:21This is the story of your creation.
0:38:27 > 0:38:29It began with a sperm,
0:38:29 > 0:38:32the smallest cell in the human body,
0:38:32 > 0:38:34fusing with the biggest cell...
0:38:34 > 0:38:35an egg.
0:38:40 > 0:38:41Everyday, the human male
0:38:41 > 0:38:48produces hundreds of millions of DNA Torpedoes...
0:38:48 > 0:38:50otherwise known as sperm.
0:38:51 > 0:38:56Each ejaculation can contain 250 million of them.
0:38:58 > 0:39:01That's enough, at least in theory,
0:39:01 > 0:39:05to cover every inch of Manhattan with people.
0:39:08 > 0:39:12Fortunately, the human female has other ideas.
0:39:12 > 0:39:16For her, it's quality, not quantity.
0:39:18 > 0:39:23Her ovaries usually produce just one egg every month.
0:39:27 > 0:39:31The process of reproduction is so complex
0:39:31 > 0:39:34it's a wonder it happens at all.
0:39:35 > 0:39:39Most of the time, the sperm die in a pool,
0:39:39 > 0:39:41trapped inside the vagina.
0:39:42 > 0:39:47Because the entrance into the womb, through the cervix,
0:39:47 > 0:39:50is out of reach, blocked shut.
0:39:50 > 0:39:54For the sperm to have a chance of getting in,
0:39:54 > 0:39:55timing is everything.
0:40:03 > 0:40:06There are just a few days in any month
0:40:06 > 0:40:09when the woman's body offers them an opportunity.
0:40:09 > 0:40:11Hormones soften the blockage
0:40:11 > 0:40:15at the entrance to the cervix.
0:40:15 > 0:40:18Transforming it from a barrier into a life line.
0:40:23 > 0:40:25But the sperm have to be strong
0:40:25 > 0:40:27to make the 15cm journey.
0:40:27 > 0:40:29It's a long way
0:40:29 > 0:40:32for the smallest cell in the body.
0:40:32 > 0:40:35The straighter and faster they can swim,
0:40:35 > 0:40:37the better chance they have of making it.
0:40:37 > 0:40:40The deformed, lazy and the dead
0:40:40 > 0:40:42are left behind.
0:40:44 > 0:40:45And, of course,
0:40:45 > 0:40:47the more fit sperm you have,
0:40:47 > 0:40:49the greater the chance of success.
0:40:53 > 0:40:57If 250 million sperm began the journey,
0:40:57 > 0:41:00only 1%, that's two and a half million,
0:41:00 > 0:41:03will make it through to the cervix.
0:41:05 > 0:41:07As they swim out of the vagina,
0:41:07 > 0:41:11they're just at the beginning of a long and perilous journey.
0:41:17 > 0:41:22The woman's body is about to launch an attack.
0:41:22 > 0:41:26They have entered the cervix -
0:41:26 > 0:41:28a labyrinth of dead ends.
0:41:28 > 0:41:31perfect for an ambush.
0:41:33 > 0:41:36The sperm have triggered the body's defence system.
0:41:38 > 0:41:42White blood cells have recognised the sperm as a foreign invader
0:41:42 > 0:41:45and, just as they would if they were fighting an infection,
0:41:45 > 0:41:48they've been mobilised to kill.
0:41:50 > 0:41:52They attack the sperm
0:41:52 > 0:41:55as they swim through the cervix and into the uterus.
0:42:01 > 0:42:04By the time the surviving sperm reach the fallopian tube,
0:42:04 > 0:42:08where they are safe from the white blood cells
0:42:08 > 0:42:10there could be as few as 20 left.
0:42:12 > 0:42:15They will be 20 of the very best on offer.
0:42:20 > 0:42:23Here, scientists have recently discovered
0:42:23 > 0:42:24that the woman's body
0:42:24 > 0:42:26has come up with an amazing trick.
0:42:29 > 0:42:32She takes control of the sperm by holding them, one by one,
0:42:32 > 0:42:36on the walls of her fallopian tube.
0:42:38 > 0:42:40She then powers them down.
0:42:42 > 0:42:45They're alive, safe, but fast asleep.
0:42:52 > 0:42:54The woman now has up to five days
0:42:54 > 0:42:58to release the precious egg growing inside her ovary.
0:43:07 > 0:43:10As soon as it's ripe, the egg is released.
0:43:16 > 0:43:20And it's wafted into the opening of the fallopian tube.
0:43:31 > 0:43:33Once the egg is ready and waiting,
0:43:33 > 0:43:36it's time to wake up the sleeping sperm...
0:43:43 > 0:43:45Sending out a powerful chemical beacon,
0:43:45 > 0:43:48the egg guides the sperm in.
0:43:54 > 0:43:57It's the precursor of every new life.
0:44:00 > 0:44:03The sperm - some male, some female -
0:44:03 > 0:44:05compete to reach the egg.
0:44:11 > 0:44:14But there can be only one winner.
0:44:19 > 0:44:24The competing sperm break off the surrounding cloud of cells.
0:44:27 > 0:44:29Until one finally pushes through
0:44:29 > 0:44:31the soft shell underneath.
0:44:39 > 0:44:41The egg is now in danger.
0:44:44 > 0:44:46If a second sperm gets in,
0:44:46 > 0:44:49the egg will die.
0:44:52 > 0:44:55It must protect itself - and quickly.
0:44:58 > 0:44:59Under the shell,
0:44:59 > 0:45:03tiny granules detonate in a chain reaction.
0:45:03 > 0:45:05Firing out chemicals,
0:45:05 > 0:45:07hardening the shell,
0:45:07 > 0:45:09making the egg impenetrable.
0:45:11 > 0:45:14Fertilised, the egg is now safe.
0:45:16 > 0:45:19This was how we all began.
0:45:19 > 0:45:22You truly are an amazing creation.
0:45:29 > 0:45:33We're made up of around a hundred trillion cells,
0:45:33 > 0:45:37all coming from just one single fertilised egg.
0:45:39 > 0:45:42Within hours of fertilisation,
0:45:42 > 0:45:44this new cell, called a zygote,
0:45:44 > 0:45:47divided into two identical cells.
0:45:47 > 0:45:49Then into four...
0:45:49 > 0:45:50then eight...
0:45:50 > 0:45:5316, and so on...
0:45:54 > 0:45:57before implanting in the lining of the womb
0:45:57 > 0:45:59and becoming an embryo.
0:46:05 > 0:46:09In some cases, on rare occasions,
0:46:09 > 0:46:12a single embryo creates two bodies.
0:46:16 > 0:46:21one in 250 early embryos split.
0:46:21 > 0:46:22If they do,
0:46:22 > 0:46:24they must divide completely
0:46:24 > 0:46:26within the first two weeks,
0:46:26 > 0:46:28or they probably never will.
0:46:31 > 0:46:35Once split, the embryos create near-replicas of themselves.
0:46:35 > 0:46:38Identical twins.
0:46:40 > 0:46:41This quirk of nature
0:46:41 > 0:46:45has given synchronised divers, Helen and Carol Galashan
0:46:45 > 0:46:47a distinct advantage.
0:46:47 > 0:46:49Being an identical twin
0:46:49 > 0:46:51definitely helps with synchronised diving.
0:46:51 > 0:46:54We don't really have to try with the synchronised part,
0:46:54 > 0:46:56that part comes quite naturally to us.
0:46:56 > 0:46:58We actually think we're mirror-image twins.
0:46:58 > 0:47:02- We fold our arms opposite ways. Our hair parts the opposite way. - Even when we're diving,
0:47:02 > 0:47:06The first foot I put on is my right foot, Carol's is her left foot.
0:47:07 > 0:47:11Identical twins actually come from one egg...
0:47:11 > 0:47:15- That splits.- That splits into two, and non-identical twins come from two eggs.
0:47:15 > 0:47:18So the way we see it is that we were one person that split in two...
0:47:18 > 0:47:20One person in two bodies.
0:47:23 > 0:47:25- Right...- Ooh.
0:47:27 > 0:47:31This woman's pregnancy is even more unlikely than identical twins.
0:47:33 > 0:47:37She's beaten odds of 4,500 to 1.
0:47:37 > 0:47:41Diane is carrying non-identical triplets.
0:47:41 > 0:47:43Remarkably, Diane's body
0:47:43 > 0:47:48naturally produced not one, but three eggs in a single go.
0:47:49 > 0:47:52It's an incredibly rare type of pregnancy.
0:47:52 > 0:47:55Not only did Diane produce multiple eggs,
0:47:55 > 0:47:57but all of them were fertilised.
0:47:57 > 0:48:00Entirely independently of each other.
0:48:01 > 0:48:04Apparently, I released three eggs,
0:48:04 > 0:48:06and Mike, he had three separate sperm
0:48:06 > 0:48:08that fertilised all three eggs.
0:48:08 > 0:48:11They've all got their own placentas
0:48:11 > 0:48:13and they're all in separate sacks.
0:48:13 > 0:48:17So they've all got have their own little bedroom.
0:48:18 > 0:48:22Effectively, Diane got pregnant three times in one go.
0:48:28 > 0:48:32But there's a downside to having triplets.
0:48:32 > 0:48:3431 weeks - nine weeks early -
0:48:34 > 0:48:37Diane has gone into labour.
0:48:37 > 0:48:40She is going to need an emergency caesarean.
0:48:46 > 0:48:50I'm just going to bring him round to show you, quickly.
0:48:50 > 0:48:54Here we go, you two. Here he is. He's beautiful.
0:48:56 > 0:48:59So he's a little breach baby.
0:48:59 > 0:49:01There he is.
0:49:01 > 0:49:03- Hello!- Isn't he lovely?
0:49:07 > 0:49:10This is the skinny Minnie.
0:49:10 > 0:49:12All the babies have now been delivered.
0:49:12 > 0:49:15- She's beautiful.- Last but not least.
0:49:15 > 0:49:17And against the odds,
0:49:17 > 0:49:19they are all alive and well.
0:49:28 > 0:49:30For your body,
0:49:30 > 0:49:32this world is a dangerous place.
0:49:35 > 0:49:38Threats lurk around every corner.
0:49:38 > 0:49:42And it's not just the obvious dangers that threaten you.
0:49:44 > 0:49:46Even now, as you watch this film,
0:49:46 > 0:49:49there are pathogens waiting to get inside you.
0:49:49 > 0:49:53A pathogen is a foreign invader that causes disease.
0:49:55 > 0:49:59They spread in all sorts of ways,
0:49:59 > 0:50:02commonly through sneezing.
0:50:02 > 0:50:04A simple sneeze
0:50:04 > 0:50:06is often all it takes
0:50:06 > 0:50:09for viruses to jump from one person to another.
0:50:09 > 0:50:13Sneezing is one of the most powerful forces your body produces.
0:50:13 > 0:50:16You expel air at 100 miles an hour,
0:50:16 > 0:50:19ejecting anything that isn't bolted down.
0:50:21 > 0:50:25Once a pathogen invader, like the flu virus,
0:50:25 > 0:50:26gets inside your body,
0:50:26 > 0:50:28you have to respond quickly.
0:50:30 > 0:50:33Time to turn on your immune system
0:50:33 > 0:50:34to destroy them.
0:50:39 > 0:50:43Your first response to infection is fever.
0:50:43 > 0:50:46Raising your temperature by just a few degrees
0:50:46 > 0:50:48is enough to slow them down.
0:50:54 > 0:50:57Meanwhile, deep within your tissues,
0:50:57 > 0:51:00an internal army is on the march.
0:51:02 > 0:51:04On the frontline are phagocytes,
0:51:04 > 0:51:07a form of white blood cell.
0:51:07 > 0:51:10They flood the infection site to fight the viruses.
0:51:12 > 0:51:16But in this case, the viruses are too strong.
0:51:16 > 0:51:20Instead, the soldiers themselves become infected.
0:51:20 > 0:51:23Now the only way these cells can kill the viruses
0:51:23 > 0:51:25is to self-destruct.
0:51:29 > 0:51:33As their bodies pile up,
0:51:33 > 0:51:36they form the sticky basis of your snot.
0:51:38 > 0:51:40But your immune system hasn't given up.
0:51:43 > 0:51:45A second wave of attack is released
0:51:45 > 0:51:49as another kind of white blood cell is unleashed...
0:51:49 > 0:51:52the Killer T-cells.
0:51:52 > 0:51:56Instead of attacking the viruses directly,
0:51:56 > 0:51:59they take aim at your own infected cells.
0:51:59 > 0:52:02They give a kiss of death,
0:52:02 > 0:52:06they make the infected cell implode,
0:52:06 > 0:52:08then self destruct,
0:52:08 > 0:52:12destroying the viruses inside.
0:52:12 > 0:52:14It's one of the reasons you'll get a sore throat.
0:52:21 > 0:52:23Despite these two assaults,
0:52:23 > 0:52:26the viruses haven't yet been defeated.
0:52:26 > 0:52:30But your immune system has another trick.
0:52:30 > 0:52:31Yet more white bloods cells,
0:52:31 > 0:52:33this time called B-cells,
0:52:33 > 0:52:36are able to recognise the specific invading pathogen
0:52:36 > 0:52:39and produce a specialised weapon.
0:52:42 > 0:52:44The Y-shaped antibody.
0:52:46 > 0:52:53These can be produced at a rate of 2,000 per cell per second.
0:52:53 > 0:52:55They coat the viruses...
0:52:56 > 0:53:00slowing them down and making them stick together.
0:53:00 > 0:53:03Now the viruses are easily swept up.
0:53:08 > 0:53:11And you begin to feel better,
0:53:11 > 0:53:14the fever drops and your energy starts to return.
0:53:16 > 0:53:19The cells of your immune system
0:53:19 > 0:53:20have won the day.
0:53:29 > 0:53:33One of the most remarkable defence mechanisms your body has
0:53:33 > 0:53:35is its ability to repair itself.
0:53:39 > 0:53:42Johnny Greaves is 33 years old.
0:53:42 > 0:53:44He is a professional boxer.
0:53:45 > 0:53:48For Johnny, boxing isn't just a sport, it's a livelihood.
0:53:51 > 0:53:53I'm here to pay my bills and keep my kids OK,
0:53:53 > 0:53:56so obviously, that's the first thought in my mind,
0:53:56 > 0:53:59bringing home the bacon and paying the kids' bills.
0:54:01 > 0:54:03Johnny is in a dangerous game.
0:54:03 > 0:54:06If he picks up the slightest injury,
0:54:06 > 0:54:10boxing regulations mean he'll have to cancel his next fight.
0:54:13 > 0:54:16If he show's up with a bruise he'll be disqualified.
0:54:20 > 0:54:24His body is going to do everything in its power to avoid this.
0:54:26 > 0:54:29All of Johnny's biological defence mechanisms
0:54:29 > 0:54:30are clicking into action.
0:54:30 > 0:54:32He ducks and dives
0:54:32 > 0:54:34to avoid the punches.
0:54:41 > 0:54:44But as Johnny tires, his defences begin to fail.
0:54:58 > 0:55:03The full extent of Johnny's injuries have yet to be revealed.
0:55:03 > 0:55:05A black eye is about to form.
0:55:07 > 0:55:09His next fight is in two weeks,
0:55:09 > 0:55:11if he is to collect that pay cheque
0:55:11 > 0:55:14he has to heal.
0:55:14 > 0:55:17Now its time for Johnny's body to really earn his living.
0:55:24 > 0:55:27The delicate blood vessels in the tissue under Johnny's eye
0:55:27 > 0:55:29were destroyed by a single punch.
0:55:36 > 0:55:38As the vessels burst,
0:55:38 > 0:55:39blood cells rush out.
0:55:41 > 0:55:44But despite the catastrophic damage,
0:55:44 > 0:55:45a repair crew is on the way.
0:55:49 > 0:55:52Flowing out with the blood are cell fragments.
0:55:53 > 0:55:55Called Platelets,
0:55:55 > 0:55:57they are built to stop bleeding.
0:55:57 > 0:55:59They gradually form a lattice
0:55:59 > 0:56:03that catches the leaking blood cells like a net.
0:56:04 > 0:56:10Extra support comes from a stringy protein - fibrin.
0:56:10 > 0:56:12Together, they form the clot,
0:56:12 > 0:56:13which plugs the hole
0:56:13 > 0:56:15and the bleeding stops.
0:56:18 > 0:56:20Five hours after the fight,
0:56:20 > 0:56:21the effects of the punch
0:56:21 > 0:56:24are beginning to show on Johnny's face.
0:56:27 > 0:56:29His eye is beginning to swell.
0:56:33 > 0:56:38Fluid is flowing into the tissue around the eye.
0:56:38 > 0:56:42It's coming from tiny holes in the walls of the blood vessel.
0:56:45 > 0:56:46It is a form of defence.
0:56:50 > 0:56:54The force of the flow stops any infection in the wound
0:56:54 > 0:56:56from travelling into the bloodstream,
0:56:56 > 0:56:59trapping it in the tissue instead.
0:56:59 > 0:57:03The results of this inflammation are dramatic.
0:57:05 > 0:57:09Johnny's eye has now turned a striking shade of purple.
0:57:18 > 0:57:22The colour is the product of decaying blood cells
0:57:22 > 0:57:24trapped outside the circulatory system,
0:57:24 > 0:57:27where they can't survive.
0:57:31 > 0:57:35Now Johnny's body starts to clear up the mess.
0:57:38 > 0:57:41Macrophages - giant white blood cells -
0:57:41 > 0:57:44sweep through the tissue and absorb the dying cells.
0:57:46 > 0:57:47Inside the macrophage,
0:57:47 > 0:57:53haemoglobin, the chemical that gives red blood cells their unique redness, breaks down.
0:57:53 > 0:57:56It's this that gives the bruise
0:57:56 > 0:57:59it's familiar cocktail of colours.
0:57:59 > 0:58:00As it breaks apart,
0:58:00 > 0:58:05haemoglobin transforms into different-coloured chemicals.
0:58:06 > 0:58:09Over time, the colour shifts
0:58:09 > 0:58:12from green to yellow and finally to brown.
0:58:23 > 0:58:25As the Macrophages leave,
0:58:25 > 0:58:29they draw the coloured chemicals away from the skin.
0:58:29 > 0:58:32Healing is now complete.
0:58:35 > 0:58:38Johnny's body has repaired the damage,
0:58:38 > 0:58:40just in time for another fight.
0:58:42 > 0:58:46Subtitles by Red Bee Media Ltd
0:58:46 > 0:58:49E-mail subtitling@bbc.co.uk