Do you ever stop to contemplate the wonder of your body?
No?
No, neither do I.
But, actually, it's the most complicated piece of engineering
known to us, and it throws up some very puzzling questions.
Why do I catch colds?
Where does wind come from?
And what will we look like in 1,000 years?
For the answers, stick with me,
as we've uncovered the Things You Need To Know about the human body.
Right, let's get this show on the road, starting at the beginning...
Daddy, where did I come from?
So, your dad told you that he and your mum loved each other,
and decided to have a baby.
Actually, it was much more complicated than that.
If your father was an average guy, about 280 million of his sperm
entered the race to fertilise your mother's egg.
They were blasted in at an impressive 28mph,
but almost instantly, 210 million had dropped out.
What happened?
Well, up to 70% of human sperm is dodgy.
Some have three tails.
There are two-headed sperm chasing their tails,
and others with no tails at all.
Humans compare pretty badly with the animal kingdom.
Let's compare me with a rat and a pig.
100% of the rat's sperm will be strong swimmers
compared with only about 30% of mine.
And the pig will produce two pints of sperm,
where as I will produce about a teaspoonful.
Anyway, back to the sperm's voyage through your mother.
Those that made it into her cervix found themselves in a sticky maze.
This mucus protects a uterus from germs,
but it's also a hostile environment for sperm.
Most will never get through.
The 100,000 that did then had to race across the uterus -
an exhausting marathon.
By the fallopian tubes, we're down to just 200,
and half of them swim up the wrong tube.
Meanwhile, the egg is only fertile for 12 to 24 hours.
Chemicals in the female body encourage the sperm to keep swimming.
As they near the egg, they turn up the wick and accelerate.
But there can only be one winner.
As one super-sperm enters the egg,
a chemical message seals its surface shut.
So, if you ever feel down, remember this -
you are the product of an Olympian sperm.
Your father's sperm and your mother's egg
contained all the information needed to make you.
We are all unique products of our parents' genetic codes.
In which case...
How did I get my granny's chin?
Since way back when, families have argued about who the kids look like.
So, it was a nasty shock for your parents
when you were born with your granny's smooth, pointy chin,
instead of their elegant clefts.
Your DNA is to blame.
In 1953, James Watson and Francis Crick discovered that DNA,
or Deoxyribonucleic acid,
is contained in a structure called the double helix.
Lying at the centre of virtually all our cells.
Within your DNA are around 23,000 genes,
which you inherited in two packs -
one from your mother, and one from your father.
Your genes come in pairs, one from each parent.
Some pairs are what's called dominant recessive.
A dominant gene - like the one that causes a cleft chin -
always trumps its recessive pair,
making sure its characteristic is turned on.
But hang on, if your parents both have dominant cleft chins,
then how can yours be smooth?
Well, actually, we only inherit half of each of our parents' genes.
That's one gene from each of their pairs.
So we get half of our mum's genes and half of our dad's,
so it must follow that we get a quarter of each of our grandparents'.
Now here comes the important bit -
genes passed down from our grandparents
may be turned off in our parents, but turned on again in us.
To see how this works, let's take a look at your family tree.
Your dad got a dominant cleft gene from your grandfather,
with a paired recessive gene from your grandmother.
The same is true for your mother.
So, in both of them, the dominant cleft gene was switched on.
The eggs and sperm your parents produce could only carry
one gene from each pair.
It just so happened that the egg and sperm that made you
both got the recessive chin gene,
so you could only get your granny's pointy, smooth chin.
The genetic mix your family gave you is unique,
but the genes themselves aren't uniquely yours.
That's because DNA is the blueprint for all life,
from bacteria to human beings.
All your organs are made up of cells,
instructed by the genes within them to work together to keep you alive.
We share this basic method of staying alive
with every animal, and plant, on earth.
Which means you share 95% of your genes with rats,
60% with chickens,
and 50% with bananas.
Not all your features are decided by a single gene.
Genes are divided across structures called chromosomes,
and your eye colour, for example, was created by genes
from different chromosomes working together in harmony.
Now, this is very complex science, but here is a simple question...
How are you seeing this?
In Ancient Greece,
some people believed that eyes worked by projecting beams of light.
This idea was knocked on the head
by the 11th century scientist, Alhazen.
He figured out that we see by the light entering our eyes.
Objects reflect light rays which are focused on to the retina,
a layer at the back of the eyeball that's packed with 125 million
light receptor cells called cones and rods.
It's the red, blue and green cone cells that give you colour vision.
In total, we see ten million different shades.
What's more, your eyes are perfectly positioned to give you
two separate fields of vision.
Each eye shows your brain the same object from a different angle,
allowing you to see in 3D and judge distance and depth.
Human eyes move at over 30 mph,
giving you more information than the rest of your senses combined.
Impressive, till the sun drops
and human vision starts to look rather shoddy -
something you'll have noticed if you stub your toe
when you get up to go to the lavvy in the middle of the night,
while your cat wanders around as if he owns the place.
Your eyes are missing something.
Line up a human, a cat, a sheep, a seal and a deer,
and it's only human eyes that don't glint at night.
That's because the others have some awesome night-vision technology -
a layer of shiny cells called the tapetum lucidum.
When just a tiny bit of light trickles into the eye,
this layer reflects it back,
giving the receptor cells a second chance to pick up the light.
So, it's a myth that your cat sees in total darkness,
but it can see in one-sixth of the light that you can.
I love cats.
It's a pity they don't really like me.
Actually, they don't really like anybody,
they just want some food.
Food is what keeps your cells working and keeps you on the road.
Cells need fuel - that's why we eat, and this can cause pollution.
So...
where does wind come from?
You eat 2,000 to 3,000 calories a day.
You also let loose up to two and half pints of wind.
We're told the Emperor Claudius thought releasing wind so important,
he encouraged it at banquets.
He was right - it's an essential by-product of digestion.
Within ten seconds of swallowing,
your stomach is whisking your food into a liquid called chyme,
which is pushed into your small intestine,
where most of the goodness is absorbed.
The small intestine is actually huge -
a folded, 21-foot tube which, pressed flat,
would be bigger than a tennis court.
So, while you're busy sipping coffee after your dinner,
your stomach is already churning your shepherd's pie up into a soup,
which your small intestine can break down
into nutrients your cells can use.
Unfortunately...
HE BREAKS WIND
there are some leftovers.
Two and a half pints of watery gruel.
This is poured into your large intestine,
home to billions of bacteria known as gut flora.
These account for over two pounds of your body weight.
To your bacteria, this gruel is a feast.
As it pours in, they set to work digesting the plant fibres
your small intestine can't and extracting vitamins and fatty acids.
This vital work produces gas, including carbon dioxide,
hydrogen, methane and the odorous hydrogen sulphide...
which builds up in your large intestine, along with solid waste,
up to 50% of which is dead bacteria and cells from your own body.
Your intestinal muscles then squeeze
to create a zone of high pressure that moves the gas and waste
down towards your rectum...
forcing you to let out wind around ten times a day.
FART SOUNDS
Embarrassing when not expected, but at least you can do it on the move,
whereas getting rid of solid waste takes time.
According to a recent survey, by the end of our lives
we'll have spent 90 days sitting on the lavvy...
STRAINING NOISES
..giving us plenty of opportunities for a nice quiet read.
Now, if I'm Mr Average, I'm made up of two stone of protein,
two stone of fat, and around eleven pints of blood.
Sounds disgusting.
But to other life forms,
our bodies are actually an irresistible all-day buffet,
which begs a rather unusual question...
Is there life on me?
As the proud owner of a human body, you see yourself as an individual.
But, if an alien examined you,
he'd see a walking zoo of bacteria and parasites.
Your bacteria outnumber your cells ten to one.
Numerically, you're 90% microbe.
You house around 500 species in your gut,
128 in your lungs,
and up to 200 in your mouth.
Gross!
But as we've seen in the intestine, some bacteria are vital.
More than you can say for...
parasites.
At some point, face mites will almost certainly feed off your skin.
Right now, they may diving down into your follicles,
using their needle-like mouths to hoover up some of the eight pounds
of skin cells you lose every year.
SOUND OF A VACUUM CLEANER
Pull the plug, it's eating me alive!
Scary, but better than feeding bloodsucking bedbugs.
Vampire fiction is, of course, all the rage,
but the real twilight bloodsuckers are rather less romantic.
A few years ago, we believed we had the bedbug beaten.
But now, all across the world, the bedbug is back.
Bedbugs use their six legs to scuttle out as you sleep.
One feeding tube administers anaesthetic
to keep you in the land of nod, while the other sucks your blood.
Nasty.
But nowhere near as bad as hosting a tapeworm.
The tapeworm gets in as larvae in undercooked meat.
Once inside, it clings to your intestines,
spending up to 25 years happily bathing in your undigested food.
By absorbing your nutrients through its skin, it can grow up to 30 feet.
With any luck you'll manage to avoid tapeworm,
unless you're one of those blokes who believes he can do a barbecue.
Viruses, on the other hand, are most definitely going to get you,
and when they do, they will give you anything from a cold to Ebola.
And yet, technically, viruses have no life of their own,
so why do they cause so much trouble?
More to the point...
Why do I catch colds?
See that guy with the blocked nose?
When he sneezes, 40,000 droplets will fly twelve feet in the air,
infecting up to 150 people.
Ugh!
You should have ducked.
A cold virus just can't live without you.
WOMAN SCREAMS
With no cells of its own,
it needs to take over your cells and replicate.
Here's something to think about -
the virus can only travel around inside a blob of mucus,
and that means that when you catch a sniffle,
someone else's snot has been up your nose.
Anyway...
your immune system is constantly on the prowl for attackers like these.
If it wasn't, you could end up with fatal pneumonia.
So, when it spots a viral invasion, it grabs a sample
and takes it to the nearest lymph node, home of your killer T cells.
Here, a T cell first identifies the invader,
and then deploys an army of tailor-made immune cells to your nose.
These provide specialist backup
for the standard immune cells already fighting your cold.
Your nose has become a battleground.
Meanwhile, to stop infection spreading to your lungs,
you're manufacturing a daily pint of mucus.
This snot gives you a headache, while the virus irritates your nose,
so you have to blow it around 45 times a day.
Beating a cold takes you about seven days,
and you'll catch about four a year.
This is the golden age for the cold virus.
It hops on planes with its human hosts, visits new cities,
and finds hundreds of new homes with every sneeze.
As it replicates, it mutates,
so by next year it may be back in a different form.
SNEEZING
Children catch more colds than adults
because their immune systems are less experienced.
We also take ages to grow up -
the human being is the only species with a long adolescence -
and this life phase developed around 500,000 years ago,
just before the human brain developed into its current large size.
And all of this makes it much easier to answer our next question...
Why are teenagers so moody?
At around eight or nine, an alarm goes off
in the hypothalamus region of the brain,
waking up powerful hormones and triggering seismic changes,
which eventually erupt as breasts,
body hair, and bodily functions.
GROANING
Along with volcanic surges of temper,
which, until recently, were also put down to hormones.
Now experts believe the brain is to blame.
We used to think the human brain was mature by 18 months.
Not any more.
When scientists used magnetic resonance imaging
to scan the brains of young adolescents,
they discovered multiplying connections
between the cells of the prefrontal cortex.
That's the bit of the brain
that makes decisions and controls emotion.
So, what happens when we ask a teenage
and an adult brain a straightforward question?
The young brain's freshly grown connections really slow it down.
The teenage brain - here's one we found earlier -
finds it tough to control impulses and emotions,
and this explains why teenagers think going out drinking
with their mates is a viable alternative to, say, exam revision,
and that it explains why they can't understand
that this will upset their parents.
Because research show that young adolescent brains struggle
to read facial expressions.
Thankfully, by adulthood, the troublesome extra connections are pruned.
Mind you, hormones aren't entirely off the hook
when it comes to teenage moods.
Hormones produce female curves and male muscles,
but also an oily substance called sebum,
which means 80% of teenage skin suffers from...
whiteheads, blackheads,
yellow, pus-sy pimples,
and, if they're really unlucky, bulging nodules.
At the exact moment they want to look attractive,
teenagers start resembling pizza.
And if they do manage to get a date,
their brains are so emotionally inept
they're almost bound to mess it up.
Teenagers can be forgiven for not being able to handle alcohol.
Adults, you'd think, would know better,
but I'm afraid we don't, because every day in Britain,
500,000 of us go to work with a hangover.
Although, mind you, it's not just us.
Human beings have been boozing like mad
for thousands and thousands of years,
which gives my next question truly international significance.
Mmm.
Why am I hung-over?
Headache?
Dizziness?
Nausea?
The evidence suggests you drink alcohol.
On average, we each sink more than a gallon of pure alcohol a year.
Worldwide, this adds up to a trillion pints of beer,
or 250 billion bottles of wine,
or 100 billion bottles of vodka.
To find out how it makes you feel so bad, we need to flash back.
90 seconds after your first sip, the alcohol hit your brain,
and interfered with your neurotransmitters,
making you talkative and self-confident.
By drink two, your inhibitions were really dropping.
Back inside your brain,
a chemical called vasopressin would normally be sending a signal
to your kidneys, to tell them how much water take from your blood.
Alcohol switches this chemical off.
So, your kidneys started channelling most of the water to your bladder.
For every drink, you expelled four times as much in urine.
By now, you were also tired and emotional.
Thankfully, someone took you home, where you crashed out.
And slept really badly.
That's because alcohol suppresses the production of glutamine,
one of the body's natural stimulants.
When you stopped drinking, production revved up again,
so you spent the night tossing and turning.
Overnight, your dehydrated liver had to process the alcohol toxins,
so it stole water from your brain, which shrank
and began to pull on the membranes attaching it to your skull.
Which is why you just woke up with a pounding head, dry mouth,
and nausea from the after-effects of the toxins.
A hangover can last for up to 24 hours,
giving you plenty of time to think about what you did last night.
If you can face it, a cooked breakfast might help.
But chances are you can't,
so you'll just have to spend the day feeling like death.
But then one day you'll be ill
and your body won't be able to make you better,
and that brings us to the greatest question in life...
Why do I have to die?
For most of history, human life has been dangerous and short.
Natural selection favoured the genes that made you strong,
not the ones that helped you get old.
Our bodies still peak between 20 and 35.
After that, it's a slippery slope to the grave.
To find out why, scientists are investigating our cells.
When you're young and healthy, your cells divide and replicate
50 billion times every day.
But each individual cell can only split 50 times.
When it hits this magic number, it's retired.
As you age, cell death starts to outpace cell birth.
And ageing cells are more vulnerable to attackers.
Amongst the worst are free radicals -
unstable oxygen molecules which stabilize themselves
by stealing electrons from your cells...
inflicting damage that can cause diseases like cancer.
Free radicals aren't the only problem -
old cells just get tired.
The elderly cell enlarges, and becomes less efficient
at turning oxygen and nutrients into energy.
A bit like a person, really.
You can see the results of ageing in your skin.
Inside your body, the same thing is going on.
At 40, your nervous system becomes less co-ordinated,
giving you heartburn and constipation.
By 60, your eyes let in two-thirds less light than they did at 20.
And at 85, your heart can only beat a fifth as fast as a 20-year-old's.
At which point, well,
you and your magnificent body are just about to hit journey's end.
Although science is helping more of us to live to be old,
humans have a natural lifespan.
85 is about it, really,
and conventional medicine isn't likely to extend this very much.
But of course, medicine is becoming more and more unconventional,
so for our descendants, life and death may be very different.
So what will we look like in 1,000 years' time?
Two million years ago, Homo habilis made stone tools.
These days, Homo sapiens design computer chips.
Next, we'll be taking charge of our own evolution.
In 2008, scientists announced that they had taken a rat's heart,
stripped it to its scaffolding, and re-seeded it with cardiac cells.
Within four days, the cells were contracting.
Within eight, they had begun to pump.
This was a big step on the road to replacement organs.
Also in the pipeline are eyes, lungs and limbs.
This technology could make us immortal
IF we can also work out how to regenerate our brains.
But that will take a while,
because the brain is a mind-bogglingly complex thing.
Inside here there are a hundred billion cells,
all communicating through tiny waves of electricity.
And somewhere within all those cells and all those connections
is the mind, the essence of you and me.
Scientists are determined to unlock the mysteries of the mind.
When they do, we may achieve not just immortality,
but a higher intelligence than we can imagine.
Here's how -
in 2004, a computer chip was implanted
into the brain of a paralysed man.
When he wanted to watch TV, the chip read his thoughts,
and switched the channels.
By 2050, it's predicted that computer-brain interaction
will be so advanced, that we'll use PCs to store our surplus memories.
Beyond that, we'll aim to devise technology to merge
the creativity of human thought with the speed of computers.
So, while your descendants might choose to look human,
inside they could be more machine than man.
So, back to my first question -
do you ever contemplate the wonder of your body?
Well, we just have,
and now I'm incredibly proud to own one,
a marvellous vehicle in which to journey through life.
And one day that journey may last forever,
but I'm afraid you and I were born too early for that.
All we can do is sit back and enjoy the rest of the ride.
Subtitles by Red Bee Media Ltd
E-mail subtitling@bbc.co.uk