Before We Begin: A Note for Children and Grown-Ups

Take a slow breath in through your nose. Now let it out. You just did something that you will do about twenty thousand times today, mostly without noticing. Each breath brings your body the oxygen it needs. Each breath also brings in tiny things you cannot see: specks of dust, bits of pollen, and germs that would like to make you sick.

So why are you not sick all the time? Because your body has guards. They stand at every step of the path that air travels, from the tip of your nose to the deepest corners of your lungs. Some guards are hairs. Some are sticky mucus. Some are quick reflexes, like a sneeze or a cough. Some are cells that move around and clean up. They work all day and all night, even while you sleep.

This book visits those guards one at a time. Each chapter stops at one part of the body, such as the nose, the windpipe, or the tiny air sacs inside the lungs. In each chapter, you will find out what that part does and how it helps protect you from germs and dirt. At the end of every chapter, there is a short list of key words with simple explanations, so you can look back at them whenever you like.

You can read the chapters in order, like a journey that follows a breath of air down into the lungs and back out again. You can also open the book at any chapter that makes you curious. Some children like to read one chapter at bedtime. Some families like to read a chapter together and then talk about it.

A note for grown-ups: this book explains how the body works. If a child, or anyone in your family, has symptoms that worry you, please talk to a doctor or another health professional. They can look at the person in front of them, which no book can do.

Now, take one more deep breath. Feel your chest and your belly rise. Somewhere inside, the guards are already at work. Let us go and meet them.

Chapter 1: What Is in the Air?

A Breath You Never Think About

Take a slow breath in. Now let it out. You just did something you have done many thousands of times today, and you probably did not even notice most of them. Your body breathes while you read, while you play, while you eat your lunch, and even while you are fast asleep.

A child your age breathes somewhere around fifteen to thirty times every minute when resting. That adds up to more than twenty thousand breaths in a single day. If you could pour all the air you breathe in one day into bathtubs, you would fill dozens of them. That is a lot of air going in and out of one small body.

Here is something to try. Sit still and put one hand flat on your chest and the other hand on your belly. Breathe in slowly through your nose. Can you feel your chest and belly gently rise? Now breathe out and feel them sink back down.

That gentle rise and fall is a sign that your body is working hard for you, quietly and without complaint. Every breath brings something your body needs. But every breath also brings in things your body does not need at all. This book is about both of those things, and about the clever guards inside you that keep you safe.

To understand those guards, we first have to answer a simple question. When you breathe in, what exactly are you breathing? Air looks like nothing at all. You cannot see it in front of your face. But air is full of surprises.

What Air Is Made Of

Air is not empty. It is made of gases, which are things so light and spread out that they float around everywhere and fill every space. You cannot see most gases, but you can feel air move when the wind blows on your face or when you wave your hand quickly back and forth.

The biggest part of air is a gas called nitrogen. About seventy-eight parts out of every hundred parts of air are nitrogen. Your body breathes nitrogen in and breathes it right back out again without using it. Nitrogen is a bit like a crowd of people walking through a train station who never stop to buy anything.

The second biggest part of air is the gas your body truly needs. It is called oxygen. About twenty-one parts out of every hundred parts of air are oxygen. That might not sound like very much, but it is plenty. With every breath, your body takes some of that oxygen and keeps it.

The last small part of air is made of a few other gases. One of them is carbon dioxide. There is only a tiny amount of carbon dioxide in the air around you. But when you breathe out, the air leaving your body has much more carbon dioxide in it than the air you breathed in. Your body has added it, and you will soon see why.

The air you breathe out still has quite a lot of oxygen left in it. Your body does not take every bit of oxygen from each breath. It takes only a part, and the rest goes back out into the room. That is one reason why many people can share the air in a classroom all day long without any trouble, as long as fresh air can come in from time to time.

Think of air like a big bowl of mixed fruit. Most of the bowl is apples, which you look at but do not eat. That is the nitrogen. A smaller part of the bowl is oranges, and the oranges are what you came for. That is the oxygen. And a few other little bits are mixed in among them.

Air also holds water in a form you cannot see, called water vapor. On a cold day, when you breathe out and see a little white cloud in front of your mouth, you are seeing that water vapor from your body. The cold air turns the vapor into tiny drops, just for a moment, before the cloud disappears.

Why Your Body Needs Oxygen

Your body is made of tiny living parts called cells. Cells are so small that you need a microscope to see them, and your body has trillions of them. There are skin cells, bone cells, muscle cells, brain cells and many more. Each kind of cell has its own job, the way each worker in a busy town has a different job.

Every one of those cells needs energy to do its work. A muscle cell needs energy to help you run and jump. A brain cell needs energy to help you think and remember. A heart cell needs energy to keep your heart beating all day and all night.

Some parts of your body use more oxygen than others. Your brain is only a small part of your body's weight, but it uses about one fifth of all the oxygen your body takes in. Thinking, dreaming, remembering and noticing the world around you are all hard work. Your brain needs a steady supply of oxygen every moment of the day and night.

When you run, jump or climb, your muscles suddenly need much more energy than when you sit still. To make that energy, they need more oxygen. That is why you breathe faster and more deeply when you play hard. Your heart beats faster too, so that it can carry the extra oxygen to your busy muscles.

Your cells get their energy from the food you eat. But food alone is not enough. To get the energy out of food, cells also need oxygen. You can think of food as the wood for a campfire and oxygen as the air that helps the fire burn. Without air, a fire goes out. Without oxygen, cells cannot get the energy they need from food.

When cells use oxygen and food to make energy, they also make a leftover gas. That leftover gas is carbon dioxide. Your body does not need carbon dioxide, so it has to get rid of it. That is why the air you breathe out carries more carbon dioxide than the air you breathe in.

So breathing has two big jobs. Breathing in brings fresh oxygen into your body. Breathing out takes carbon dioxide away. Your lungs, which are two soft, spongy organs inside your chest, are where this trade happens. You will visit the lungs and learn how they do this trade in later chapters of this book.

Your body can go for a long time without food and for a shorter time without water, but it needs fresh air every few seconds. That is why your body never stops breathing, not even for one night. Your brain keeps breathing going all by itself, so you never have to remember to do it.

You may wonder where all that oxygen comes from, and why it never runs out. Green plants make it. Trees, grass, garden plants and even tiny green living things floating in the sea use sunlight to make their own food, and as they do this, they release oxygen into the air. People and animals breathe that oxygen in. Then people and animals breathe out carbon dioxide, which plants take in and use again. It is a giant sharing circle that goes around and around the whole world.

The Tiny Travelers in the Air

If air were only nitrogen, oxygen and a few other gases, breathing would be very simple. But air carries passengers. Tiny bits of all kinds of things float in the air, too small or too spread out for you to notice them most of the time.

Have you ever seen a beam of sunlight shining into a quiet room? If you look closely at the beam, you can often see little specks drifting and twirling inside it. Those specks are dust. They were floating in the air the whole time, but you could only see them when the bright light lit them up.

Dust is made of many things mixed together. It can include tiny pieces of soil, bits of cloth from clothes and blankets, flakes of dead skin, and crumbs too small to see. Most dust is not dangerous, but your body would still rather not have it deep inside your lungs.

Pollen is another tiny traveler. Pollen is a fine powder that plants make to help them grow seeds. Many plants let the wind carry their pollen far away. A single plant can release a huge number of pollen grains, and each grain is smaller than the dot at the end of this sentence.

Some people's bodies react strongly to pollen, even though pollen itself is usually harmless. Their noses run, their eyes itch and they sneeze a lot. This kind of reaction is called an allergy. It happens when the body's guards treat a harmless thing as if it were a danger.

Smoke is another traveler. Smoke from fires, from cooking or from burning things carries tiny black bits called soot. There are also little spores from mold, which is a fuzzy growth that can appear on old bread or in damp places. And then there are the smallest travelers of all, which are germs.

Indoor air has travelers too. Cooking sends tiny drops of oil and steam into the air. Pets shed bits of fur and flakes of skin. Carpets, curtains and pillows slowly give off tiny threads. Opening a window for a while lets fresh air come in and helps carry some of these travelers away.

The size of each traveler matters a great deal. Bigger bits, like most pollen grains and large pieces of dust, are fairly easy for your body to catch near the start of the breathing path. Smaller bits can drift deeper. The very smallest bits, like some smoke particles and germs, can travel a long way down if nothing stops them. That is why your body has guards at every stage of the path, not only at the beginning.

Germs: Most Are Harmless, Some Are Not

Germs are living things, or almost living things, that are much too small to see without a microscope. The two kinds you will hear about most often are bacteria and viruses. They are everywhere: in the air, in the soil, in water, on your skin and inside your body.

Bacteria are tiny living things made of just one cell each. Dozens of them lined up side by side would still be narrower than one of your hairs. Viruses are even smaller than bacteria. Hundreds of viruses could fit across the width of a single hair.

Here is something that might surprise you. Most bacteria do not make people sick at all. Many of them are actually helpful. Huge numbers of friendly bacteria live in your belly and help your body break down food. Others live on your skin and help keep harmful germs from settling there.

Some germs, though, can make you sick if they get inside your body and start to multiply. A cold is caused by viruses. So is the flu. Other illnesses can be caused by bacteria. These germs are like weeds in a garden. A few weeds may not matter much, but if they spread everywhere, the garden starts to suffer.

Germs that make people sick are a problem mostly because they can multiply so quickly. When bacteria find a warm, wet place with food, one bacterium can split into two. Then those two can split into four, and four can become eight. Viruses work differently. A virus slips inside one of your cells and makes the cell build many new copies of the virus, which then go off to find more cells.

Many of the germs that cause colds and coughs travel through the air. When a sick person coughs, sneezes or even talks, tiny drops of wet spray fly out of the person's mouth and nose. Some of those drops carry germs. Other people nearby can breathe in those drops.

Germs can also land on things like door handles, tables and toys. If you touch one of those things and then touch your nose, mouth or eyes, the germs can get inside. That is one of the reasons grown-ups remind children to wash their hands. You will learn more about how to help your body in a later chapter.

So every breath you take is a little like opening the front door of a house. The door has to open to let in fresh air. But when the door is open, dust, pollen, smoke and germs might try to sneak in too. A house with an open door needs someone watching it.

A Body Full of Guards

Your body has many guards that work together to stop unwanted visitors. These guards are not people, of course. They are body parts, special cells and clever actions that protect you. Together they form your body's defense system, and much of that system is called the immune system.

Think of a castle in an old story. A castle has high walls and a gate. It has guards at the gate who check who comes in. It has soldiers inside who chase away any intruders who slip past. And it has workers who clean up after any trouble. Your body has all of these.

Your breathing passages are full of these protectors. Some are like walls and gates. Some are like sticky traps that catch dust and germs. Some are like tiny brooms that sweep dirt away. Some are like alarms that ring when something is wrong. And some are like soldiers and cleaners that find germs and destroy them.

The guards work in layers. If one layer misses something, the next layer has a chance to catch it. Most of the time, the guards near the start of the path catch almost everything. Only very small things travel deep into the lungs, and even there, more guards are waiting.

Sometimes germs do get past the guards and make you sick. That happens to everyone, children and grown-ups alike. When it does, your body fights back, and you will read about how it fights back in later chapters. If you feel unwell, always tell a grown-up, so the grown-up can decide whether to talk to a doctor.

It is normal for children to catch several colds every year. A young body is still meeting many germs for the first time. Each time the immune system meets a new germ, it learns what that germ looks like. The next time the same germ comes along, the immune system can often recognize it and fight it off faster. In this way, your guards grow wiser as you grow older.

Most of the time, though, your guards win without you ever knowing there was a battle. Right now, while you are reading these words, your body is quietly catching dust, sweeping away dirt and watching out for germs. All of this happens with each breath, from morning until night and from night until morning.

The Journey Begins at the Nose

In this book, you will follow the path that air takes through your body. You will start where air comes in, and you will travel all the way down to the deepest parts of your lungs. At each stop, you will meet a different part of your body and learn how that part keeps you healthy.

You will meet the nose and its sticky traps. You will learn why sneezes and coughs are so useful. You will visit the throat and the tonsils, and a little door that keeps your food from going the wrong way. You will ride down a windpipe that works like an escalator, and climb through branches like the branches of a tree.

You will find hundreds of millions of tiny balloons deep in your lungs. You will meet cleaner cells that eat dust and germs. You will follow red blood cells as they carry oxygen like a train carries passengers. You will learn about the muscles that pump air in and out, and about what happens when germs make you sick.

At the end of the journey, you will also learn how you can help your guards. Small everyday habits, like washing your hands and getting enough sleep, make the guards' work easier. Your body does most of the work, but you can be a helpful partner.

Now, take one more slow breath in through your nose. Feel the cool air enter. The very first place that air touched was the inside of your nose. And the nose, as you are about to discover, is far more than a bump in the middle of your face. It is the first gate of your body's breathing castle.

Key Words

Air: The mix of gases all around us, mostly nitrogen and oxygen. Air also carries water vapor and tiny floating bits such as dust and pollen.

Oxygen: A gas in the air that every cell in your body needs. Cells use oxygen to get energy from food.

Carbon dioxide: A gas that your cells make as a leftover when they use food and oxygen. Your body gets rid of it when you breathe out.

Cell: One of the tiny living parts that make up your body. Your body has trillions of cells, and each kind has its own job.

Lungs: Two soft, spongy organs in your chest. They take oxygen from the air and send carbon dioxide out of your body.

Pollen: A fine powder made by plants to help them make seeds. The wind can carry pollen a long way through the air.

Germs: Tiny living things, such as bacteria and viruses, that are too small to see without a microscope. Most are harmless, but some can make people sick.

Bacteria: Tiny living things made of a single cell. Many are helpful, and only some cause illness.

Virus: A germ that is even smaller than bacteria. A virus can only multiply inside the cells of a living thing, and viruses cause colds and the flu.

Immune system: The body's team of special cells and body parts that find and fight germs. It works like the guards and soldiers of a castle.