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The incredible journey of life, from birth through infancy, childhood, puberty, adulthood,

and a slow maturity to old age.

Using the latest medical imaging, state of the art modeling, and 3D computer graphics,

we embark on an extraordinary voyage through the body.

This is not just the story of one life, it is the story of all our lives.

Told from a unique perspective, deep inside the living body.

40 weeks have passed since a sperm fertilized an egg, and a new life was created.

A single cell has developed into a fully formed baby, but the warmth and tranquility of the

womb is about to be shattered.

We are about to be born.

It's impossible to know exactly what it's like to be born, even though we've all been

through it.

It's certainly surprising, it's probably painful.

What scientists do know is that at the moment of birth, we experience a surge of adrenaline

even more intense than during a heart attack.

This surge of adrenaline helps us live.

It kick-starts our lungs into drawing their first breath.

Our lungs have never breathed before, they are still filled with amniotic fluid.

We're in danger of drowning.

On top of the kidneys, the adrenal glands flood the bloodstream with adrenaline.

Muscles we need to breathe suddenly go into spasm.

Our lungs splatter into life, and we take our first breath.

It's the most important breath of our life, the first of 700 million.

Water rushes down the trachea, through thousands of branching tubes into nearly 30 million

tiny air sacs, the alveoli.

These absorb oxygen into our blood, and draw out the carbon dioxide we exhale with every

breath.

Now the umbilical cord, the physical link between mother and baby, is cut.

We are on our own.

A baby's organs must adapt to life outside the womb.

It's a challenging and risky time.

Our heart, no bigger than a walnut, has already been pumping for eight months, but in the

first days of life, there's a problem with it.

It has two holes, one in the aorta, and one in the heart.

In fact, these holes are not defects, they are a remnant of our life before birth, when

blood circulated through a system of blood vessels to the placenta.

The holes used to divert most of the blood away from our inactive lungs, but now the

lungs are fully functioning, they seal up forever.

Now the heart is working normally, it pumps blood through tens of thousands of miles of

blood vessels.

Our other organs are also coming online too.

The liver performs over 500 varied jobs, from generating body heat to processing toxins.

The kidneys function is to maintain the balance of water in the body.

Our digestive tract must clear itself out, ready to take its first meal.

Our bowels are full of digested amniotic fluid and dead cells, a sticky green black tarry

material called meconium.

It's corrosive stuff.

In some babies, meconium can end up in the lungs and attack the delicate lining, though

here in the gut, meconium is harmless and is flushed out within hours.

The first gulps of breast milk accelerate this process.

The trauma of birth might now be over, but the business of adapting to life outside the

womb has only just begun.

A newborn baby in the first months of life.

This is the time when we start to make sense of the world around us and begin to explore

it.

Infancy is a period of rapid growth, both outside and in.

The human body is a miracle of microscopic design, tiny, perfectly formed organs, each

made up of billions of perfectly functioning cells.

These cells are the building blocks of the body.

They make us what we are, a hundred thousand billion cells, all working in perfect harmony.

Inside every cell is the same extraordinary engine, a machine that tells each cell how

to grow and what functions it must perform.

It's DNA and it's unique to every person.

DNA is a chemical blueprint for who we are, the instructions that create each new person

and sends them on the journey of life.

DNA not only determines our looks and character, but also sets out the timeline of our lives.

It decides when we grow, when we develop, when we age and even when we die.

But our life is also shaped by the environment around us and the people we share it with.

Our life outside the womb is still a challenge.

Even though our mothers keep us warm, we can't regulate our body temperature.

It was 38 degrees in the womb.

Here at home, 18 degree room temperature is a shock to the system.

The problem is, the region of the brain we use for temperature control, the hypothalamus

is immature.

Our infant brain is under pressure.

It is already making 100 trillion simultaneous calculations every second.

Our hypothalamus just can't cope with the additional task of keeping us warm as well.

Right now, we are in danger of hypothermia.

An infrared camera shows the struggle to keep warm.

We are losing precious body heat.

The yellow areas show where we lose the most.

Luckily we are born prepared.

We have a special layer of tissue that keeps us warm.

It's fat.

Not just any old baby fat but brown fat, normally found in hibernating animals.

Much more than just an insulating layer, this fat is packed with special cells which actively

generate heat.

Usually most of this fat melts away as the hypothalamus matures and takes over the job

of temperature control.

Just days old, we know almost nothing about our world.

Everything we do, we do by instinct.

Even breastfeeding.

Our mother's milk gives us all the nutrients we need and we drink half a litre of it every

day.

It's much more than the ultimate superfood.

It also protects us from hidden danger.

Outside the womb, there are bacteria everywhere, invisible and potentially deadly.

Our day old skin is under constant attack.

There are ten times more bacteria than human cells in and on our body.

Our immune system is underdeveloped and we can't fight infections off ourselves.

Finally our mother fights them for us, through her milk.

The close contact between mother and baby means she absorbs the same germs that are

attacking us.

Her mature immune system creates antibodies to kill off these bacteria.

Even in a masterpiece of natural design, she passes those antibodies to her baby in

breast milk.

Until our own immune system develops, she will keep us safe.

Breastfeeding builds a deep bond between mother and child, a bond that will guide us through

infancy and into the outside world.

At this age, even a simple trip to the supermarket is a sensory overload.

It's noisy, bright and smelly.

By up inside the nose, the specialized olfactory nerves dangle in the stream of air we breathe.

They detect chemicals and send an electrical signal to the brain.

Our brain interprets the signals as smells.

Our sense of smell is very sensitive.

We quickly learn to recognize smells and can identify our mother with a nose.

The strange new world is also very noisy.

They're bombarded with hundreds of strange new sounds.

Sound waves make the eardrum vibrate.

On the other side of the eardrum, these tiny bones, the ossicles, vibrate in sympathy.

They're the smallest three bones in our body.

They're so small, they could fit on a fingertip, nor do they get any bigger.

They stay this size all our life.

But without them, we would never hear a thing.

The ossicles are the ears amplifiers.

They amplify the volume 22 times.

The amplified vibrations enter the inner ear, the cochlea.

It's lined with delicate hairs.

When vibrations pass through, the hairs vibrate.

At the top, low-frequency hairs.

The base, fragile hairs for high-frequency sounds.

Each one 200 times thinner than a hair on our head.

Over time, loud noises will damage these hairs.

But at this age, they are perfect.

Our hearing will never be this good again.

For eyesight, the story is different.

The world is blurred and without color.

Our eyes are underdeveloped.

We can't point our eyes where we want to.

And our immature lenses cannot focus.

The lens flips the image it receives.

Our brains correct the image later.

Babies can't yet see in color because the retina, the screen at the back of the eye,

is also immature.

The retina has two types of special cells, rods and cones.

They turn light into electrical signals.

The ones that detect color, the cones, are still developing.

So we see mostly in black and white.

From the retina, the signals travel along two thick nerves to the back of the brain,

where we process visual information.

When the image arrives, the real challenge begins.

Our immature brain hasn't yet learned to interpret the data.

That's changing fast.

By two months, we can distinguish colors and shades.

At four months, we can identify facial patterns.

And by eight months, we have 20-20 vision.

Babies eyes undergo another amazing change.

When we are born, our eyes are blue.

But gradually, cells of the iris begin making pigments.

The iris changes into a startling pattern of color, a pattern that is unique to each

of us.

Over the first three months, we grow by a quarter our original body weight every month.

Fortunately, that growth rate slows.

If it didn't, we would all weigh a frightening 134 tons by our fourth birthday, around the

same as a blue whale.

At eight months, all our senses are working properly.

We're beginning to explore the world.

And the sense we use most is touch.

When we touch something, receptors in our fingertips send electrical impulses through

sensory nerves in our skin, up our arms, along the spinal cord, and into the brain.

The impulses travel fast, 320 kilometers per hour.

Sometimes even this is too slow.

The body reacts to painful sensations, like extreme heat, by taking a shortcut.

The spinal cord intercepts pain messages and immediately sends back a reflex response,

and we move our hand away.

We have sensory receptors all over our skin, but some areas are more sensitive than others.

The hands, face, and mouth.

There are 9,000 sensory receptors on the tongue alone.

This is why babies use their mouths to explore their world.

There's another reason for all that gnawing.

Our baby teeth are coming through.

Milk teeth form deep in the gums while we're still in the womb.

Now one by one, they are bursting through.

It's painful, but it's progress.

At last we can eat solid food.

Digestion starts in the mouth.

The teeth grind up the food.

Next, special glands under the tongue pump out saliva.

It helps break down the food.

The saliva also lubricates the food on its 12-hour, 4-meter journey through the gut.

It'll pass from the stomach into the small intestine, and finally, the large intestine.

Waves of contracting muscle keep our food moving throughout the journey, in a process

called peristalsis.

The contractions are so powerful, we can even eat upside down.

This is a unique view of where the food is heading.

For the first time, a new camera shows a high-definition view of the journey to our

stomach.

Food enters the stomach through a hole at the top.

The stomach is a bag of muscle.

It churns, squashes and squeezes the food into liquid.

At the same time, acid breaks down the food.

The acid is so corrosive, the stomach continually coats its interior walls with mucus for protection.

Without its stomach ulcers would form.

After about an hour, the stomach squeezes the broken-down food through a tiny hole called

the pyloric sphincter.

The food enters the small intestine, a 3.5-meter coil of tube.

This is where we absorb most of the nutrients.

First, the pancreas pumps out a juice that neutralizes the stomach acid.

Then bile from the liver breaks down the fats into tiny droplets.

Smaller droplets are easier for the intestine to absorb.

The interior wall of the small intestine is lined with millions of microscopic projections

called villi.

These increase the surface area of the gut, making it easier to absorb nutrients.

After an hour and a half, the small intestine has absorbed most of the nutrients from the

food.

What's left enters the large intestine through this, the ileocecal sphincter.

It's a valve that prevents our food going back.

There are few nutrients left to absorb now.

The large intestine's main job is to extract water.

What remains is a mixture of waste food, dead cells and lots of bacteria.

These bacteria in the large intestine aren't an infection.

They produce enzymes that break down complex carbohydrates in our food.

Carbohydrates we couldn't otherwise digest.

Finally, after about 12 hours, we expel what remains of the meal.

By the age of one, we start to interact with our world.

We are more independent from our mother, and now we can crawl.

We begin to explore the world on our own.

We can crawl because our bones have got stronger.

They need to be.

We're getting pretty heavy.

At birth, our skeleton is mostly cartilage, the same material our ears are made of.

Cartilage is flexible.

It's what enables us to squeeze through the birth canal.

But after birth, our soft skeleton is a problem.

Cartilage is too weak to support the growing body and protect vital organs.

So it gradually hardens into bone.

Cells called osteoblasts lay down minerals that turn soft cartilage into hard bone.

Some bones also fuse together.

When we are born, we have gaps between the plates of our skull.

These allow the skull to deform during birth.

Through our first year, these gaps called fontanels gradually close, until our skull

is finally complete.

As our skeleton develops, so does our desire to get around.

We're about to hit one of the major milestones in our life, learning to walk.

The key to walking isn't strength, it's balance, and the secret to standing is hidden

deep inside the ears.

Beyond the ossicles, the bones used for hearing, there are three looping structures.

Each loop is about the size of a small coin and orientated in three planes.

These semicircular canals are part of the ear but have nothing to do with hearing.

They're filled with liquid and they work like a spirit level.

They tell us what's up and what's down.

The liquid inside sloshes against sensory hairs lining the tubes.

The hairs send data to our brain about how we are oriented and our direction of movement.

They help us balance and stand on two feet and once we've mastered balance, we're one

step closer to learning to walk.

Now there's no limit on where we can go and what we can do.

From a baby to a toddler, we're about to begin our formative years, a time when we

will put our growing brain and developing immune system to the test.

From infancy to childhood, we're still growing fast and learning to stand on our own two

feet.

Next is a uniquely human challenge, learning to talk.

Most of us learn to talk by our first birthday but by two years old, we're learning ten new

words a day.

This is the Broker's area, the region at the side of the brain involved in processing language.

With it, we can create sentences and communicate complex thoughts.

Language is what separates us from other animals.

By talking, we can exchange thoughts and ideas.

We can teach our children not just by showing, but by telling.

As our brain develops, we gain other uniquely human qualities.

We're aware of our own identity and individuality.

We learn to think for ourselves.

We become conscious of ourselves as an individual.

And we're forming memories that will last a lifetime.

Human childhood is unique.

Few other mammals spend such a huge portion of their lives preparing for adulthood.

To do so would be a costly waste of time.

By the equivalent age in their lifespan, most mammals like dogs would already be reproducing.

Not so with us humans.

The timeline set by our DNA clock puts a delay on our sexual development.

To give us enough time for one thing, learning.

It takes well over a decade to learn the complex skills we need to become adults and

eventually parents ourselves.

Throughout childhood we are primed to learn.

Our brain is a mass of 100 billion nerve cells called neurons.

Between them, they generate enough electricity to keep a light bulb burning for a day.

The neurons communicate using electric impulses.

Each impulse is a tiny fraction of a thought and memory.

When we hear a new word, our ears convert the sound into electrical impulses in our

brain.

The brain can learn because the connections between the brain cells, the neurons, aren't

permanent.

The brain rewires itself.

The neurons send out tendrils, constantly forming new connections.

Where they meet is a synapse.

Chemicals called neurotransmitters bridge the gap to allow the impulse to continue the

chain.

The new connections form a pattern, a new memory.

We learn by making new connections between brain cells and then reinforcing them through

repetition.

The stronger the reinforcement, the more likely the memory will stick.

What does the word veteran mean?

And when later, someone asks us to recall that memory.

The same pattern of brain cells fires.

And we remember the answer.

Because the brain grows quickly in childhood, millions of new connections are being made

all the time, making this the perfect time to learn.

Learning takes many forms, not only in the classroom, but also in the world at large.

We learn by experience, and sometimes these experiences are unpleasant.

Because we're programmed to try out new things when we're young, we take risks with our body.

Fortunately, it's a finely tuned machine, capable of repairing itself when injured.

A cut knee is part of childhood.

As soon as the skin is broken, the body's repair mechanism swings into action.

Blood capillaries constrict to reduce their diameter and slow the bleeding.

Next, platelets, a type of blood cell, make the blood in the cut become viscous and clot.

Eventually the clot forms a plug that stops blood leaking out.

Our cells multiply at a prolific rate, not only to increase the number of cells in the

body, but also to replace worn out tissue with newer, stronger, bigger versions.

It's the same process of cell growth that heals wounds.

In fact, we're so good at healing in our youth, even the scar disappears.

Childhood is also the time when our horizons expand.

We interact with more and more people.

But this increased sociability exposes us to an increased risk of infection.

Fortunately, our body has an arsenal of defenses against germs.

Eyebrows and eyelashes, ear hairs and nasal hairs catch airborne bacteria.

Sweat, tears and mucus wash them off and skin constantly sheds its top layer of cells, taking

bacteria with it.

The mouth is especially vulnerable.

Recent discoveries have shown that pathogens found here are so potent that if they ended

up in vital organs like the heart, they could even prove fatal.

Fortunately, the body has a secret weapon.

Saliva contains lysozyme, an enzyme specially developed to destroy bacteria.

Though tiny, saliva glands create nearly one and a half liters of saliva every day.

When a pathogen like chicken pox does break through these external defenses, our immune

system must react to prevent serious infection.

Thousands of tiny viruses travel through the blood.

The virus hijacks a cell and manufactures thousands of copies of itself.

Then the infected cell ruptures, spreading yet more viruses around the body.

The outward signs of chicken pox are a rash and a high temperature.

The fever that comes with infection is a sign that our body is fighting back.

The heat slows down the spread of the disease.

These don't reproduce so well when it's hot.

The immune system kicks into action.

White blood cells latch onto the infected cells and pump in poisonous proteins.

It kills the cell, but it kills the viruses too.

The crusty skin pustules are the remnants of the battle between the immune system and

the chicken pox.

Each one contains cell debris and thousands of dead viruses.

It may be unpleasant, but getting diseases like this when we're young is vital for our

developing immune system.

Our body creates memory molecules against the virus's antibodies.

We used to rely on antibodies from our mother's milk.

Now we make our own.

If we catch the same disease when we're older, the antibodies enable our body to recognise

the virus instantly.

White blood cells wipe it out before it ever takes hold.

This makes childhood the ideal time for vaccination.

Vaccines are harmless doses of certain types of virus like rubella, mumps or polio.

They prompt the body to create antibodies, just as it would if infected.

By the age of 11, childhood is nearly over and we're about to change like never before.

We're on the brink of adulthood.

But first, the hormone surge of puberty.

Puberty doesn't stick to a schedule.

It begins at any time, depending on our DNA clock, our lifestyle, even our diet.

Whether female or male, for all of us, it begins in the brain.

At the base, the hypothalamus.

The same region that controls our body temperature.

Puberty starts when the hypothalamus releases the protein kisspeptin into the brain.

Kisspeptin triggers the release of other hormones throughout the body.

This stimulates the sex organs to mature.

In girls, that means ovulation.

For the first time on television, a high-definition view of the ovaries, these off-white organs.

This unique footage from Gold Coast IVF Fertility Center shows something incredible.

An egg inside a protective blister of fluid.

At ovulation, the blister ruptures.

The egg inside will travel down the fallopian tube, where it will be either fertilized or

lost in menstruation.

From the onset of puberty, ovulation occurs once a month, but the ovaries begin another

equally important role.

They start releasing hormones into the bloodstream, including oestrogen.

They have dramatic and lasting effects on both emotional and physical development.

Both boys and girls experience a growth spurt.

But each day when we look in the mirror, we see a new face looking back.

And the entire body changes beyond recognition.

Girls become women, boys become men.

Real brains also release kisspeptin during puberty.

The flood of new hormones stimulates the testicles to start producing sperm.

The testicles also generate a tenfold increase in testosterone, the hormone that turns a

boy into a man.

The larynx opens up and tilts forward.

The vocal cords stretch wider.

The longer cords vibrate at a lower pitch.

The voice deepens.

And also stimulates the growth of body hair and doubles muscle mass.

These are changes we can see and hear, but we're changing inside too.

In the brain, nerve cells undergo extensive rewiring.

The result is a transformation of our mood and character.

Both sexes suffer a torrent of new emotions, and one new emotion beats them all.

For the first time, both sexes find the other sex attractive.

From the inside out, our bodies are overwhelmed by new sensations.

Our pulse races, our blood pressures rise.

Our lips gorge, our cheeks flush with blood.

These are all signals that we desire someone.

And sometimes, that desire turns out to be mutual.

In adolescence, another major life milestone, our first kiss.

In the first two decades of life, the human body accomplishes something close to miraculous.

We're nearly four times our original height.

We're 21 times heavier.

We've digested nearly nine tons of food.

Our heart has beaten over a billion times, and our lungs have drawn 200 million breaths.

Finally, we're ready, in mind and body, to become an adult.

From teens to twenties, the rapid growth of childhood and puberty gives way to a new

phase of life, adulthood.

The twenties are our peak years, both outside and in.

Our body has stopped growing taller, but it will never stop changing.

Trillions of cells make up our organs and tissues.

Over time, these wear out or get damaged.

New cells grow and divide to replace the old ones.

Over time, whole organs are renewed.

We get virtually a brand new body every two years.

Some tissues regenerate even faster.

That's why we need to come here, for our hair and nails.

These are made from modified, dense skin cells.

Each hair grows from a hair follicle embedded in the skin.

The modified cells grow here, then die as new cells push them upwards.

We grow an incredible 11 kilometres of hair over our whole body every year.

Our hair grows whether we want it to or not, however other parts of our body are partially

under our control.

Choices we make now, like taking exercise, affect us for the rest of our life.

Exercise strengthens the heart.

It's the body's hardest working muscle.

Exercise also makes our heart muscle more efficient.

Each contraction pumps more blood, so the heart can beat slower.

It's a similar story in the lungs.

Exercise stimulates extra capillaries to grow.

So we absorb more oxygen with each breath.

We also inhale bigger breaths, so every tiny air sac in the lungs gets filled.

The surface area of all these sacs is huge.

If you laid them all flat, they would cover a tennis court.

Working out also strengthens our skeleton.

Exertion puts pressure on the bones.

That encourages the bone cells to renew bone fibres.

Specialised cells called osteoclasts eat away at old or damaged bone.

Other cells, osteoblasts, rebuild it with newer, stronger material.

The result, dense, strong bone.

At the same time, our muscles also build.

This is a new imaging technique that combines the highest resolution CT scans with cutting

edge computer power.

It's called volumetric, and it shows how over 650 skeletal muscles make up to a third of

our adult body weight.

Muscles are made up from bundles of fibres.

A good workout rips these fibres apart.

When our cells repair the damage afterwards, they add extra material.

The muscles grow back bigger and stronger through choices we made.

Unfortunately, some choices are less beneficial.

Some choices expose us to damage that even our youthful cells can't repair.

Colors are part of life in your 20s.

We all know smoke can damage the lungs, but smoke is not the only hazard here.

From the day we were born, our ears have been gradually damaged by loud noise.

The problem is deep inside the ear, past the ossicle bones inside the cochlea.

These fine hairs, the stereocilia, turn sound into nerve impulses, but they are very fragile.

Loud noise destroys these irreplaceable cells.

The hairs that respond to high frequencies are most affected, possibly because loud,

high pitched sounds shape their foundations more violently.

The effect is still too small to notice, but the frequency range of our hearing is already shrinking.

Another source of damage is alcohol.

Though social drinking is very much part of growing up, alcohol is a poison.

As we absorb it into our bloodstream, it affects both our organs and our state of mind.

A few drinks raise the heart rate and blood pressure.

It makes us feel relaxed, we lose our inhibitions and our fine coordination.

These sensations are the result of chemical reactions inside the brain.

Especially in this region, the cerebellum, the region responsible for coordination and balance.

Alcohol causes a chemical imbalance in the neurons.

While some synapses accept signals more often, others become blocked.

And the more we drink, the more the synapses are affected.

Alcohol also fools the brain into thinking we've drank too much water.

It sends the kidneys into overdrive, expelling valuable liquid into the bladder.

We might not feel dehydrated now, but next morning it's a different story.

After a party, it's the liver that clears up the mass.

The liver is the body's biochemical control center.

It performs over 500 functions.

One of them is to convert poisons into harmless chemicals.

One of the poisons is alcohol.

The process demands water, and the liver doesn't care where it takes it from.

The brain is 75% water.

When the liver demands water, it's the brain that suffers.

Water and essential minerals are sucked out and the brain shrinks away from the skull.

We experience a very particular kind of headache, a hangover.

The best cure?

Drink some water.

Time passes and we get to know our limits.

Now new challenges are on the horizon.

It's time to find love and have children of our own.

Many of us meet our future partner at work.

We may think the attraction is social or physical, but a lot of it is biological too.

We use our eyes to size up our date, but looks aren't everything.

It's also about smell.

Inside our nose, the olfactory nerves do more than detect smells.

They also detect chemicals we can't smell.

Pheromones, odourless hormonal messengers we release in our sweat.

Pheromones are much more than the body's natural perfume.

They carry detailed information about our genetic health and our ability to resist disease.

Our brains detect these signals and help us choose a partner with the best possible genes.

But love is not just an emotion.

It's about chemistry.

When we see our partner, we release adrenaline into the blood.

For heart pounds, we can't sleep.

And then as we get closer and more involved with our lover, another hormone comes into

play.

Our brain floods with dopamine, the feel-good hormone.

As potent as cocaine, it induces euphoria and it's addictive.

It leaves us wanting more.

Eventually after a time together, we start thinking about the final stage of love.

Commitment, marriage and even kids.

Love, both chemical and emotional, wins the day.

It's a relationship we hope will last a lifetime and the process of long-term bonding is chemical

too.

Sex isn't just about procreation or recreation.

It also strengthens the bond between lovers.

During orgasm, both partners' pituitary glands flood their bodies with oxytocin, the bonding

hormone.

It's the very same hormone that binds us to our mother as a newborn baby.

The more sex we have, the more oxytocin we produce, the stronger the bond between us.

Some anthropologists believe oxytocin could be evolution's way of creating a bond that's

strong enough to keep parents together through the trials of parenthood.

And the time for parenthood is now.

An egg, the largest cell in the human body, ripens and bursts from the ovary.

It begins the same monthly journey hundreds of others made before it.

While earlier eggs were lost in menstruation, this one is destined to become a baby.

Three hundred million sperm, the smallest cells in the body, enter the vagina during

ejaculation.

They have a tough journey in front of them.

First they must survive the hostile environment of the vagina.

Its secretions are acidic to prevent bacterial infection, but they also kill sperm.

Most of the sperm are killed before they reach the cervix.

The surviving sperm swim on into the uterus and fallopian tube.

The contractions in the walls of the fallopian tube help the sperm towards the egg.

Only a few hundred make it this far, and only one will succeed in fertilizing the egg.

This truly is survival of the fittest.

Only the most viable sperm with the best DNA will survive to pass on its genes.

So far there's no sign of what has happened.

We're totally unaware that we're about to embark on a new chapter in our life.

Over the next forty weeks, a single cell will develop into a baby.

Often the first symptom is morning sickness.

No one knows for sure what causes the nausea, but one theory is that it protects the fetus

from toxins in food.

These could harm its tiny organs during this critical phase of development.

Another theory is that nausea is a side effect of the mother's immune system, as it weakens

to avoid attacking the developing embryo.

The fetus is effectively a parasite.

It saps the mother's energy as it draws what it needs from her body.

It has its own life support system, the placenta.

Here the mother's blood passes nutrients across a membrane into the fetal blood.

Thanks to this constant nourishment, the baby grows over 850 grams in ten weeks, and the

uterus expands up to a thousand times its normal size just to hold it.

That extra space has to come from somewhere.

The mother's body has to realign itself internally.

This woman isn't pregnant.

Even so, all her organs are a tight fit.

In a pregnant woman, those organs get squeezed to the back or pushed up into the chest.

Not only are the organs squashed, but now they are working for two.

The lungs and heart have to work harder than ever before.

To make space, muscles and tendons in the spine relax.

It curves out of normal shape.

The stomach too is compressed and rotated through 45 degrees.

It can only hold small amounts of food and drink, yet all the while, the growing baby

is demanding more.

Finally after nine months, it's time to give birth.

Getting out a three kilogram baby is quite challenging.

Softened tendons allow the pelvis to open up the birth canal, but even so, it's a tight

fit to push the baby out.

Volumetic shows the claustrophobic route it must take, with a tight twist to get round

the tailbone.

Sometimes it's too tight a squeeze for mother and baby.

The only option for a safe delivery is a caesarean section.

From a newborn baby to becoming a parent ourselves.

Raising kids puts a huge strain on us physically and emotionally.

Life will never be the same again.

At 20s are behind us, our body is about to enter a new period of change.

As the aging process takes hold.

The story of human life continues.

From the peak years of early adulthood to the joy of becoming a parent.

As our children grow older, so do we.

Our body is about to face the next challenge.

By this time of life, our body has already peaked.

We have been aging for many years, but it is only now in our forties we really notice

our changing appearance.

Our body is changing and beginning to show its age.

The cumulative effect of years in the sun causes the first symptom.

Our skin is getting wrinkles.

Since birth, we have been replacing our skin at an astonishing rate.

We can make up to 40,000 new skin cells every minute.

These replace the dead cells we're constantly shedding.

By 45, we've created more than 180 kilograms of dust from old skin cells.

Our skin cells are never more than a month old, even in middle age.

Wrinkles are not caused by damage to the cells.

The problem is the stuff that binds them together.

Collagen.

Ultraviolet radiation in sunlight triggers a molecular chain reaction which degrades

the collagen.

The fibres get thinner and break.

Our middle aged skin loses its elasticity, becomes saggy and wrinkly.

Our eyesight is also changing.

A few years ago we could easily read without glasses.

The problem is in the lens.

Inner lens cells, along with heart cells and most brain cells, are among the only cells

our body never replaces.

They're exactly the same lens cells we had as a baby.

As we get older, the lenses gradually stiffen.

They focus less well.

Our eyes also start to dry out.

We produce a film that lubricates the eye.

Now we're producing less of it and fewer tears to flush out debris.

Our body shape changes also.

Exercise alone is no longer enough to keep us in shape.

At age 20 it was easy to stay trim.

We could eat what we wanted, do what we wanted.

Now our metabolism is changing.

It's easier to put on weight.

The explanation for this is in our blood.

In middle age, levels of sex hormones, estrogen and testosterone and the levels of growth

hormones drop.

As a result, our body begins shedding muscle, around three kilograms during each decade

of adult life.

And this affects our metabolic rate.

Less muscle means we burn fewer calories.

So our body needs less fuel to run on.

If we continue to eat like we used to, the surplus food gets converted into this.

Men lay down fat on their hips.

It's the body's way of providing a steady energy supply during pregnancy.

Men lay down fat in a different area, their bellies.

Belly fat evolve for quick energy release.

It helps sustain our male ancestors during hunting trips.

The body's ability to metabolize fat slows down as we age.

It's a process that occurs deep inside our cells.

Inside each cell in our body there are tiny structures called mitochondria.

They are our body's power plants.

They combine nutrients from food with oxygen from the lungs to release energy.

As we get older, the number of mitochondria dwindles and with them the ability to metabolize

fat so efficiently.

Sometimes too many calories and a drop in metabolism can be a lethal combination.

Excess fat is much more than an extra inch on our waistline.

It fills almost every cavity in our body.

For the first time on television, a special high definition endoscope inserted through

the navel reveals the full extent of fat cover inside our abdomen.

The intestines are smothered in yellow fat deposits.

Fat finds its way into the body's internal cavity, even inside our blood vessels.

Deposits build up on the inner walls, narrowing their diameter.

The heart must work harder to pump the blood through the restricted vessels.

In extreme cases, vessels can become totally blocked.

And if fat blocks the arteries that supply the heart, the results can be fatal.

Our heart muscles are starved of oxygen and nutrients and risk going into spasm.

A heart attack.

Heart disease is the biggest killer in the Western world.

As we enter our fifties, our body's metabolism may be slowing down, but our lifestyle is

new.

We have a growing family and a demanding career, and these can create another killer, stress.

We all recognize the outward signs of stress.

Sweaty palms and shortness of breath are dizzy feeling.

But the real damage takes place inside our body.

When we're stressed, the body instinctively shifts into the fight or flight mode.

Adrenaline and cortisol flood from the adrenal gland into the bloodstream.

This makes muscles contract, arteries constrict, and heart pump faster, increasing blood pressure.

We evolve the fight or flight reflex to respond to attacks from predators.

But we can't escape our predator, the relentless pressure of being a working parent, and our

body has no release.

It's the constant triggering of fight or flight that can be damaging to our body.

Stress may cause irreparable harm to our cardiovascular network, speeding up the aging

process in the blood vessels.

The high blood pressure damages cells in the artery walls.

They become stiff and thick.

Especially here, in the biggest artery, the aorta.

Arteries with stiff walls restrict blood flow around the body.

Our blood pressure rises, so our heart must work harder.

It's a vicious circle.

The more stress we experience, the more we damage our blood vessels.

And the more damaged our blood vessels, the less able we are to deal with the effects

of stress.

If stress gets out of control, the heart can become enlarged as it struggles to force blood

through our narrowing, less elastic blood vessels.

High blood pressure can also rupture blood vessels in the brain, causing stroke.

Most of us learn to manage stress, but for women in their fifties, another factor compounds

the problem, menopause.

During menopause, the ovaries run out of eggs.

They also stop producing the sex hormones, oestrogen and progesterone.

It signals the end of a woman's reproductive life.

As the supply of hormones winds down, it upsets the balance in the regions of the brain responsible

for mood, sleep and temperature control.

When the hypothalamus is thrown off course, hot flushes occur, moments when the body can't

set its thermostat correctly.

It's not just the brain that's affected, bone and muscle tissues also weaken.

A woman's body spends its whole life getting accustomed to these hormones.

Now they're gone, and the aging process accelerates.

As children leave home and we retire from work, our body enters a new phase in life.

Old age has arrived.

In 70 years, we have grown from a tiny baby to an adult, from child to parent to grandparent.

The aging process began several decades ago.

Now we are in the grip of old age.

It's the final chapter in the journey of life.

When we retire from work, our lifestyle begins to slow.

Our body is slowing down too.

The outward signs of aging are only part of the story.

Our senses are affected.

We have been losing the sensory hairs we used to hear from our cochlea since birth.

Now most of the higher frequency hairs are gone.

We're even losing hairs in the lower frequencies.

The lubricating pads are stiffening between the ossicles attached to the eardrum.

They're slowly seizing up.

These days we struggle to hear.

Our sight continues to worsen.

The lenses in our eyes become stiffer and even change color.

From clear blue to a frosty yellow brown.

The result of a lifetime's exposure to ultraviolet light.

Most of us are never aware of this change.

Our brain just works harder to compensate for it.

Aging also has a dramatic effect on our skeleton.

Many of us are at risk from osteoporosis.

Bone cells are still hard at work destroying old bone and replacing it with new bone.

But old age upsets the balance between them.

Others are destroying bone faster than osteoblasts can rebuild it.

What remains is a hollowed out cluster of brittle bone fibers.

The bones slowly crumble and broken bones become a very real danger.

It happens in both sexes but the hormonal changes of menopause accelerate bone loss

faster in women.

Aging is one of the great mysteries of life.

Why does our appearance change so very much between the ages of 40 and 70?

It's more than wear and tear.

It's a process affecting every cell in our body.

Every day cells clone themselves in their billions.

The DNA inside is also copied.

As old cells die off, the new ones take their place.

The trouble is, this cloning system isn't perfect.

Any imperfections in the DNA are also replicated.

In a lifetime we make so many copies of our cells that even the tiniest errors accumulate

with time.

It's just like using a photocopier.

Copies made from copies degrade in quality.

In our face, we have totally replaced the bone every two years since we were born.

Our 70 year old face is a 35th copy of our baby face.

The imperfections get exaggerated with each copy.

So by the time we reach old age, our face looks very different indeed.

Another cause of aging is in the very air that we breathe.

We need oxygen to live, but throughout our lives it is slowly poisoning us.

Inside each of our cells, our mitochondria are like tiny power plants, combining nutrients

with oxygen.

They create the energy we need, but just like a power plant, they also produce pollution.

In this case, the pollutant is a form of oxygen itself.

The oxygen molecules change into unstable forms called free radicals.

Over a lifetime, these free radicals slowly suffocate the mitochondria and damage ourselves.

Our cells and DNA become more and more damaged and can't replicate themselves.

Our body can't repair its organs properly.

They fail.

Death, like life, is an amazing biological process engineered into the cells of the body.

Just as our DNA dictates the timeline of our development, it also puts a cap on how long

we can live.

Each time a cell copies itself, it leaves behind a tiny piece of DNA.

After billions of divisions, so much DNA is lost, the cells eventually lose the ability

to divide altogether.

Death is not instantaneous.

It is a gradual winding down of tissues and organs.

It's thought that the heart's final pump flushes the bloodstream with endorphins, the body's

natural painkillers.

Now starved of oxygen, tissues cannot function.

Within ten seconds, our brain's electrical activity drops.

Within four minutes, it is damaged irreversibly.

Our hearing is the last sense to go.

Even after death, some cells stay alive.

It can take 24 hours for skin cells to stop dividing, and an amazing 37 hours before our

last brain cell finds its final impulse.

There is a saying that life goes on.

For some of us, it could even go on for some time.

Current trends suggest that children born in the West today can expect to live into

their 80s and beyond.

And even after we've gone, we live on through our loved ones.

Our children and our children's children carry our genes in every one of their cells.

They also carry memories of us too, the moments they have shared from our extraordinary story.

All journeys must one day end.

And what a journey it has been.

Thank you.