Science: Sorted! Brains, Bodies, Guts and Stuff

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1. How to Build a Body

Okay, so let’s imagine that you want to build a human body . . .

What, out of bits of dead people? Like Frankenstein or something?

No, not like that. You’re not allowed to use whole arms, legs and heads. They’re already half-built, aren’t they? I meant a whole human body, built from scratch. Where would you start?

Hmmm . . . let’s see. I s’pose I’d start with a skeleton. I’d wire a bunch of bones together, like they do with dinosaur bones in museums, and make a skeleton.

That’s a good start. But a skeleton can’t stand up or move by itself. You can dangle it from a hook to stop it collapsing into a heap of bones, but that’s about it. How are going to get it standing and moving?

Easy. I strap some muscles on there. String ’em between all the bones, so they can hold the skeleton up and pull the arms and legs about. Oh, and then cover the whole lot with skin, so it doesn’t look too hideous.

Nice work. This body is really starting to take shape. But here’s the thing – if those muscles are going to do any work at all, they’ll need energy. Where are you going to get that?

Ah. Good point. Right, then – we stick in a stomach and some guts in it, so it can digest food, and get energy from that. Oh, and stick some teeth in the gob, so it can mash the food up. And a food tube, I s’pose, to get food from the mouth to the stomach.

Great idea. So now you’ve got a digestive tube for absorbing nutrients to feed the muscles. And, once you’ve got all the energy you can from your food, you can – ahem – ‘drop’ what’s left out of the other end of the tube. Excellent.

Thank you.

But working muscles need oxygen too, otherwise they can’t use the chemical food-energy you’ve just absorbed. Where are you going to get that?

Easy – shove some lungs in. And some air tubes leading to the nose and mouth. Sorted.

Not quite.

Eh? (Sigh) What now?

Now you have two lungs full of oxygen, and a gut full of chemical energy from your food. But how are you going to get the oxygen and energy to muscles spread all over your body, from head to toe?

Oh, yeah. Hmmmm . . . tricky one, that. Hang on . . . got it!

On you go . . .

You pipe the oxygen and energy there in little blood tubes! You link all the muscles, guts, lungs and stuff together with veins and arteries. Then you stick a heart in the middle to pump the blood around.

Well done. You’ve created a bloodstream, which will now carry oxygen and energy all over the body – a brilliant solution. But there’s still a problem. What’s going to keep the whole system working together? How are you going to coordinate and control all this eating, breathing, pumping and moving about?

Doh! Of course! I forgot the brain! Okay – here’s what we do. We shove a brain in the skull to control everything, and we wire it up to all the body bits we already have with nerves. Oh, and while we’re about it let’s stick some eyeballs and eardrums in the head too. And wire up the nose, mouth and skin so they can smell, taste and touch things. Howzat?

A masterstroke. Now your body can control itself and get information about the outside world through your five senses of sight, sound, touch, taste and smell. (And, as a bonus, your brain also lets you think and learn about stuff as you go.) We’re nearly there.

Nearly?

Yep, nearly. Just a couple more things to deal with, and we’re done. Now think hard, because this next one’s a biggie.

You’ve just built a body using most of the major organ systems needed for life. You have a movement (or musculoskeletal) system made of bones and muscles. You have a digestive system for absorbing energy. You have a breathing (or respiratory) system for absorbing oxygen. You have a blood (or circulatory) system for moving the oxygen and energy around. And you have a brain and nervous system with which to control the whole lot. Now, what are you going to build all these organs and systems out of?

Errr . . . what?

Well, so far you’ve built a muscly skeleton, skin, guts, lungs, blood vessels, nerves, eyes, ears, a heart and a brain. What will you actually use to build these body organs? Wood? Plasticine? Lego? What?

Oh. Wow. I . . . errr . . . never thought about that. Well, bones are made of bone, aren’t they? And muscles are made of . . . well . . . muscle?

That’s right, they are. But it goes a bit deeper than that. In biology, we call these organ-building materials tissues. Most tissues are made of stringy protein fibres, fatty membranes and watery, sugary gels. Some special tissues, like bone tissue, also contain minerals like calcium, to make them tougher and stronger. But, for the most part, your tissues are woven from proteins, fats, sugars and water.

This is, of course, why you have to eat these things – your food isn’t just for energy, it also provides the building blocks for body tissues – you literally are what you eat! Inside the body, proteins, fats and sugars are combined in hundreds of different ways to produce scores of different tissue types. But now we have a new problem.

What’s that?

Now your body is delicious and nutritious. Proteins, fats and sugars aren’t just good grub for us. They’re also good grub for bacteria and other microscopic parasites – which just love to invade your body and munch away on your yummy tissues. Before long, your precious body will be chewed up, rotten and decayed. So how are you going to stop them?

Wrap yourself in cling film? Take some antibiotics?

Okay, not bad. In a way, your skin acts like cling film – covering your delicate, watery tissues with a protective layer that stops bacteria getting in. And while your body doesn’t make antibiotics, it does make antibodies and other bacteria-busting defence systems, which lie in the tissues and bloodstream, waiting to do battle.

But even with all these defences your tissues will eventually break down by themselves. Just as cogs, wheels, circuits and pipes wear out inside machines, so too do your bones, joints, muscles, nerves and blood tubes. How are you going to fix that?

Errr . . . repair them? Or replace the worn-out parts with new ones?

Excellent. That’s exactly what we’ll do. In fact, we’ll go one better. We’ll make the tissues repair and regenerate themselves. As old parts wear down, we’ll grow new parts to replace them.

We can do that? How?

By building our tissues out of living, growing, regenerating cells.

Cells are the most basic building blocks of life. The tissues of your body may be made of proteins, fats and sugars, but they’re organized into tissues by layers of cells. Muscle tissue is built with layers of muscle cells, bone tissue with bone cells, brain and nerve tissue with nerve cells. And so on.

But don’t cells wear out too?

Most of them do, yes. But cells also grow and divide, replacing layers of old, dead cells in a tissue with fresh, new ones from beneath. Inside the nucleus (or command centre) of each cell, instructions coded into DNA are decoded and used to direct the assembly of cells into tissues.

As your tissues age and wear down, the healthy cells within them grow and divide to replace the ageing ones (which conveniently self-destruct). In this way, old blood, skin, nerve, muscle and bone cells are shed (or passed) from your body every day, each one replaced by a shiny new cell. In the absence of a nasty disease or accident, the whole process keeps your body ticking along for decades, with no mechanic, plumber or electrician required. Let’s see any other machine top that!

So that’s it, then? We’ve done it? We’ve built a body?

Yep – we’re done.

Hooray! We rule!

Good job. We now have a living, breathing, moving body that can take care of itself. We’ve used cells to build tissues, tissues to build organs and organs to build organ systems. Now just add food, water and oxygen, and you’re away!

Now you understand how the body is built, you’re ready to explore the really good stuff. In the chapters that follow, we’ll tackle the body system by system, and discover why we sweat, why we itch and why we have toes. We’ll find out how farts become eggy, how karate masters chop through concrete, and how you could live with half a brain.

You ready, Dr Frankenstein?

Then let’s do it . . .

2. Blood, Breath and Body Pumps

How do babies breathe before they’re born?

They don’t. Because they don’t have to. While they’re in the womb, their mothers do their breathing for them. And while babies do ‘practise’ breathing in the womb, they don’t take their first breath until they’re pushed out into the cold, airy world.

What? Babies don’t breathe? But how is that possible? Wouldn’t they suffocate?

Nope. They do just fine. They get all the oxygen they need from their mothers, so they don’t need to breathe. At least not until they’re out of the womb and the umbilical cord that connects them to their mothers is cut.

So they breathe through that tube? Like a snorkel or something?

In a manner of speaking, yes. Only they’re not actually breathing. They’re just receiving oxygen through the cord, and making use of it.

I don’t get it. If you stop breathing, you die. Everybody knows that.

Ah, but that’s not strictly true, you see. At least not for everybody, all the time. Think about it – how long can you hold your breath?

I dunno – about a minute?

Right. And did you die last time you tried?

Don’t be stupid. If I did, I couldn’t say so, could I?

Exactly. So you stopped breathing for a full minute and yet here you are, alive and well.

Eh? So you’re saying that babies hold their breath? For, like, nine months?

No, no, no, no, no. Not at all. I’m saying that you don’t necessarily have to be breathing to be receiving and using oxygen. That’s because there’s a big difference between breathing (moving air in and out of your lungs) and respiration (using oxygen from your bloodstream to power your brain and other tissues). If you stop respiring, your cells cannot use oxygen to power themselves, and you die. But you can stop breathing for several minutes before you stop respiring. That’s why you can hold your breath without dying. What’s more, if you can find another way to get oxygen into your bloodstream, then you don’t need to breathe at all.

So how do babies do it?

For the nine months that it’s inside the womb, a baby (or rather, the foetus¹) receives oxygen through the umbilical cord. This fleshy cord is made up of three thick blood vessels (two umbilical arteries and one umbilical vein). The umbilical vein passes into the foetus’s belly (where the navel or belly button will eventually be), and carries nutrients and fresh oxygen to the baby’s liver and heart. On the return route, carbon dioxide and other wastes are carried from the baby’s hip (or iliac) arteries, up past the bladder and back out through the belly into the umbilical veins. In this way, oxygen and nutrients pass in and out of the body of the growing foetus without it ever having to breathe (or, for that matter, eat).

So where does the umbilical cord come from? From the mother’s blood?

Not directly, no. Mixing the mother’s blood and the baby’s blood would be dangerous, as it could expose the foetus to all sorts of viruses, bacteria or toxic chemicals, against which it has no defence. So, instead, blood vessels from the mother and from the foetus meet inside the womb, within a special organ called the placenta. The placenta is formed from the same fertilized egg the foetus developed from, and it floats beside the foetus in the womb. Inside the placenta, capillaries from the mother’s bloodstream and from the foetus’s umbilical cord twine around each other like a pair of hands with interlacing fingers.

Arranged in this way, oxygen and nutrients can move back and forth through the spaces between the two bloodstreams without them ever actually flowing into each other. So the placenta works like a fleshy sieve, filtering out any nasties before they get into the umbilical cord and the foetus’s blood. Which is pretty nifty, if you think about it.

So the baby doesn’t use its lungs at all?

Not really, no. Right up until it’s born, the foetus’s lungs are filled with fluid. From about three months onwards, it makes small breathing movements with the lungs. But it’s not really using the lungs to breathe with at all – it’s just preparing the breathing muscles for use later on. Like a breathing ‘workout’ inside the womb.

But if the lungs were filled with fluid, wouldn’t the baby drown when it tried to use them?

Well spotted. It would, but it doesn’t. Right after the baby is pushed out of the womb, it gives a mighty heave, coughs up all the fluid, and starts breathing with its first cry (doctors, nurses and midwives sometimes hold the baby upside down and pat it gently on the back to help kick-start this first breath once the baby is delivered). That’s why the first thing the baby does is cry. Well – that, and because it’s chilly and scary out in the big, bad world.

Wow. That’s kind of amazing. Okay – one more thing. What happens to the umbilical cord and the