BBC James Mays Things You Need to Know

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.