Chapter 6: The Windpipe: An Escalator of Tiny Hairs

Down Past the Little Door

Take a slow breath in through your nose. The air you just pulled in has already traveled a long way. It squeezed past the hairs in your nose, swirled through your throat, and slipped past the epiglottis, the little door that flips down over your airway whenever you swallow. Now the air is heading somewhere new, deep inside your neck and chest, where you cannot see it.

The next stop is a tube called the windpipe. Doctors and scientists call it the trachea, which is said like "TRAY-kee-uh." The windpipe is the main road for air going down to your lungs and coming back up again. Every breath you have ever taken has traveled along this road, and every breath you take tonight while you sleep will travel along it too.

At first, the windpipe might sound like a plain, empty pipe, a bit like a drinking straw. But the windpipe is much cleverer than a straw. Its inside walls are covered with tiny hairs, so small that you would need a strong microscope to see a single one. These hairs move all day and all night, and together they work like a moving staircase that carries dirt and germs up and away from your lungs. In this chapter, you will find out how that staircase works, and why it is one of the hardest-working guards in your whole body.

A Ringed Tube You Can Feel

You can find your windpipe right now. Put one finger very gently on the front of your neck, just below your chin, and slide it slowly down toward the top of your chest. Do not press hard; a light touch is all you need. Somewhere in the middle of your neck, you may feel a firm, slightly bumpy tube under the skin. That tube is your windpipe.

Keep your finger there and swallow, just as you did in the last chapter. You may feel the voice box, or larynx, move up and then down again. The voice box sits at the very top of the windpipe, and it lifts up when you swallow so that the epiglottis can close neatly over the opening. A moment later, the voice box settles back down, and breathing goes on as usual.

In a grown-up, the windpipe is about 10 to 12 centimeters long, which is a little longer than a new crayon. It is about as wide as a grown-up's thumb. Your windpipe is smaller than that, because your whole body is smaller. As you grow taller, your windpipe will grow longer and wider too, so that it always fits the size of your lungs.

The windpipe runs down from your neck into your chest. It sits just in front of the esophagus, the food pipe you met in the last chapter, which carries food and drinks down to your stomach. So there are two tubes side by side: one for air at the front, and one for food at the back. This is why the epiglottis matters so much. The little door makes sure that food goes into the back tube and air goes into the front tube.

Your windpipe is also well protected from bumps. In your neck, it is surrounded by muscles and other soft parts that cushion it. Lower down, inside your chest, it passes behind your breastbone, the flat bone in the middle of your chest. Your body keeps this important road tucked safely away, just as people keep precious things in a safe place.

Those bumps you felt with your finger are rings. The windpipe has about 16 to 20 of them, stacked one above the other like a pile of bracelets on an arm. The rings are made of cartilage, the firm but bendy material that you met in your nose and ears in Chapter 2.

Why does the windpipe need rings? Think about a tube made of very thin, soft plastic. If you sucked hard on one end, the sides would cave in and the tube would flatten. The windpipe has the same problem to solve. Each time you breathe in, air rushes through quickly, and the rings keep the walls from folding inward. Thanks to the rings, the road for air stays open whether you are sleeping, running, laughing, or singing.

The rings are not complete circles. Each ring is shaped like the letter C, with the open part facing the back of your neck. The gap in each ring is closed by a soft band of muscle. This is a clever design. The food pipe lies right behind that soft band, so when you swallow a big bite of food, the food pipe can bulge out a little into the soft space. If the rings were closed circles, the hard cartilage would get in the way of every mouthful.

The windpipe can also bend and stretch. When you look up at the sky, your windpipe gets a little longer. When you look down at your feet, your windpipe gets a little shorter. The soft parts between the rings let the windpipe move along with your neck, a bit like the folds of a bendy straw. It can twist and stretch, yet it never pinches shut.

A Carpet of Tiny Hairs

Now let us look inside the windpipe. Imagine that you could shrink down to the size of a speck of dust and stand on the inside wall. You would find yourself in a strange and busy place. The floor under your feet would be a thick carpet of tiny hairs, all waving back and forth. These hairs are cilia, said like "SIL-ee-uh," the same kind of tiny hairs that you met inside your nose. One single hair is called a cilium.

The cilia grow on the cells that line the inside of the windpipe. A single one of these cells can have around two hundred cilia sticking up from its top, like the bristles on a brush. Millions upon millions of these cells cover the walls of your windpipe and the smaller airways below it. All together, there are so many cilia that no one could ever count them one by one.

The cilia do not just stand still. Each cilium beats very quickly, more than ten times every second. Every beat has two parts. First comes a fast, strong stroke in one direction, like an oar pushing through water. Then comes a slow, gentle return, with the cilium bending low so that it does not push anything back the wrong way. Fast and strong one way, slow and soft on the way back: the cilia repeat this over and over, all day and all night.

Even more amazing, the cilia work as a team. They do not all beat at exactly the same moment. Instead, each cilium beats just a tiny bit after its neighbor. This makes waves that travel across the carpet of hairs, the way a field of tall grass ripples when the wind blows across it. You may have seen people in a big crowd stand up and sit down one after another to make a wave. The cilia do something similar, and their waves always push in the same direction: up, toward your throat.

Picture a single grain of pollen that slipped past your nose. It drifts down into the windpipe and lands on the wall. Right away, it is carried along by the rippling hairs below, a little like a ball passed over the heads of a crowd from one pair of hands to the next. The pollen grain cannot hold on, and it cannot go back down. Little by little, it travels upward, away from your lungs. To see how this ride really works, you need to meet the partner of the cilia.

Sticky Mucus on Top

The cilia have a partner, and that partner is mucus. You already know about the mucus in your nose. The windpipe makes mucus too, though usually much less than you might think, and most of the time you never notice it at all. Special cells in the lining of the windpipe, along with tiny glands in its walls, make the mucus a little at a time.

The mucus in the windpipe lies in two layers, a bit like a sandwich without the top slice of bread. Right next to the cells, around the cilia, there is a thin, watery layer. The cilia can wave freely in this watery layer, the way your legs can kick freely in a swimming pool. On top of the watery layer lies a thicker, stickier layer of mucus, like a blanket floating on water. The tips of the cilia just reach up and touch the underside of this sticky blanket.

The sticky layer is a trap. Think back to all the things that float in the air: dust, smoke, pollen, and germs such as bacteria and viruses. The nose catches many of them, but some always get past. When these tiny things reach the windpipe, many of them land on the sticky mucus and get stuck there, like a crumb on a piece of sticky tape. Once they are stuck, they cannot drift down any deeper into your lungs.

Mucus is more than just glue. It contains natural substances that can weaken or harm germs, and it carries helpful proteins called antibodies, which grab onto germs. So the mucus does two jobs at once. It catches troublemakers, and it starts to fight them while they are trapped. Mucus may seem a bit yucky, but it is one of the best friends your lungs have.

Most of the time, the mucus in your airways is clear and thin. When you have a cold, it may become thicker and turn cloudy, yellow, or even green. Part of that color comes from the white blood cells that rush in to fight the germs. The color shows that your body is busy, but it does not tell anyone exactly which germ is causing the trouble. Only a doctor can work that out, and most colds get better on their own with rest.

The Escalator at Work

Now put the two partners together: the waving cilia below and the sticky mucus on top. With each fast, strong stroke, the tips of the cilia push the sticky blanket a tiny bit upward. With millions of cilia pushing, wave after wave, the whole blanket of mucus slides slowly up the windpipe, carrying everything that is stuck in it. Scientists call this the mucociliary escalator, said like "MEW-koh-SIL-ee-air-ee." That long name simply means "a moving staircase made of mucus and cilia."

Think of an escalator in a large building, the kind of moving staircase that carries people from one floor to the next. You step on at the bottom, stand still, and the stairs carry you up. The windpipe escalator works in a similar way, except that it carries dust and germs instead of people. And unlike a real escalator, it never switches off. It keeps moving while you eat breakfast, while you play, and while you are fast asleep.

The escalator is slow but steady. In a healthy windpipe, the mucus moves about a centimeter every minute, sometimes a little faster. That may not sound like much. But in one hour, the mucus can travel the whole length of a grown-up's windpipe several times over. Most of the dust and germs that land on the escalator are carried out of the airways within a day or so, long before they can cause trouble.

So where does all that mucus go when it reaches the top? It arrives at the back of your throat, near the place where the windpipe and the food pipe meet. Then, without even thinking about it, you swallow it. You do this many times every day and never notice. It might sound strange, but it is perfectly normal and safe. Once the mucus reaches your stomach, the strong juices there break down most of the germs and dust that came along for the ride.

Remember that your nose has cilia too. The cilia in your nose push mucus backward, toward your throat. So there are two moving staircases that meet in the same place. One carries mucus down from your nose, and the other carries mucus up from your windpipe and lungs. Both end at the back of your throat, where a swallow sends everything safely down to your stomach.

Sometimes the escalator gets extra help. When there is a lot of mucus, or when something larger lands in the windpipe, a cough can push it up much faster. The walls of the windpipe have nerve endings that are very good at feeling when something does not belong. When these nerve endings sense a crumb or a drop of water going the wrong way, they send a quick message to the brain, and the brain answers with a strong cough. The escalator is the slow, steady cleaner, and the cough is the fast emergency push. Together, they keep the road to your lungs clear.

Have you ever noticed that people with a cold often cough more just after they wake up? While you sleep, you cough much less than when you are awake, so a little extra mucus can gather in the airways overnight. The escalator keeps working, but it moves slowly. When you wake up, sit up, and start moving, a few coughs help clear away the mucus that collected during the night.

You may notice this team at work when you have a cold. Your airways make more mucus than usual, and the mucus can become thicker. The escalator works hard to carry it all up, and you may cough more to help it along. The coughing can be annoying, especially at night, but it is part of the way your body clears out germs. If a cough lasts a long time, makes breathing hard, or worries you, that is something for a grown-up to talk about with a doctor.

When the escalator and the cough push germs up and out, you can help protect the people around you. As you learned in Chapter 3, cough into a tissue or into the bend of your elbow, and then wash your hands. Your own guards have done their job by pushing the germs out, and these simple habits keep those germs from starting a new journey into someone else's nose.

Taking Care of Your Escalator

The cilia are strong workers, but they are also delicate. Some things make them slow down, and other things help them keep a good rhythm. The good news is that many of the things that help your cilia are simple parts of everyday life.

Smoke is one of the worst things for cilia. When a person breathes in tobacco smoke, the smoke can make the cilia beat more slowly, and over many years it can damage them so that fewer are left. Then the mucus does not move up as well, and dirt and germs can stay in the airways longer. That is one reason why people who smoke often cough a lot. It is also why breathing in other people's smoke is not good for children. You will learn more about smoke and clean air near the end of this book.

Very dry air can also make the escalator work less smoothly. Remember the thin, watery layer around the cilia. If that layer becomes too thin, the cilia cannot wave so freely, and the sticky blanket on top becomes thick and hard to move. Your nose helps by adding moisture to the air before the air reaches the windpipe. Drinking enough water during the day helps your whole body, including the moist linings of your airways.

Moving your body is good for your airways too. When you run, jump, and play, you breathe more deeply and more quickly, and air moves through every part of your windpipe and lungs. Many scientists think that this kind of active play helps the airways stay clear. Playing outside when the air is clean is good for your lungs in many ways, and it is fun as well.

The next time you take a breath, think about the busy carpet of cilia in your windpipe. Even now, as you read or listen to these words, millions of tiny hairs are rowing together. They are carrying a slow, sticky river upward and away from your lungs. You do not have to tell them what to do. They work on their own, day and night, and they have been working ever since you were born.

At the bottom of your windpipe, inside your chest, the road comes to a fork. The windpipe splits into two smaller tubes, one heading to your left lung and one heading to your right lung. From there, the tubes split again and again, like the branches of a tree. In the next chapter, you will travel into those branches and find out how they carry air to every corner of your lungs.

Key Words

Windpipe (trachea): The tube that carries air from your throat down into your chest. At its bottom end, it splits into two tubes, one for each lung.

Cartilage: A firm but slightly bendy material in your body. It makes the rings of the windpipe, and it also shapes your ears and the tip of your nose.

Cilia: Tiny hairs on the cells that line your airways. They beat in waves to push mucus up toward your throat.

Mucus: A sticky, slippery liquid made by the lining of your nose and airways. It traps dust and germs and helps to fight them.

Mucociliary escalator: The team of mucus and cilia that works like a moving staircase. It slowly carries trapped dirt and germs up and out of your airways.

Voice box: The part at the top of the windpipe that helps you make sounds when you talk or sing. It moves up when you swallow.

Food pipe: The tube behind the windpipe that carries food and drinks from your throat down to your stomach.

Cough: A strong, fast push of air from your lungs. It clears your airways when something gets in that does not belong there.

Breastbone: The flat bone in the middle of the front of your chest. The lower part of the windpipe passes behind it, where it is well protected.