Chapter 7: The Bronchi: Branches of the Breathing Tree

A Fork in the Road

Think of a big tree in winter, when all its leaves have fallen. You can see the thick trunk rising from the ground. Then the trunk splits into a few large branches, and each large branch splits into smaller branches. Those split again into twigs, and the twigs split into even thinner twigs, until the very tips are so fine that they look like lace against the sky. Now turn that picture upside down in your mind. You have just imagined the inside of your lungs.

In the last chapter, you followed the air down the windpipe, riding past the waving cilia. The windpipe is like the trunk of the tree. At the bottom of the windpipe, behind the upper part of your breastbone, the trunk splits into two. One tube turns toward your left lung, and the other tube turns toward your right lung.

These two tubes are called the main bronchi. They carry every breath into your lungs and every used breath back out again. One of them alone is called a bronchus. You say bronchi like "BRONG-kye" and bronchus like "BRONG-kus."

The place where the windpipe splits has its own name: the carina, said like "kuh-REE-nuh." The carina is a small ridge, like the pointed edge where a road divides into two. It is one of the most sensitive spots in all your airways. If a crumb or a drop of water tumbles down this far, the carina feels it at once, and it sets off a very strong cough. In this way, the fork in the road is also a lookout post.

Put both hands flat on your chest, one on each side. Under your left hand is your left lung, and under your right hand is your right lung. Your two lungs are not quite the same. Your heart sits a little toward the left side of your chest, so your left lung is slightly smaller to make room for it.

Each lung is divided into big parts, called lobes, a bit like the sections of an orange. The left lung has two lobes. The right lung has three lobes. Each lobe gets its own branch from the main bronchus, so the air can reach every part.

The two main bronchi are not twins either. The right main bronchus is a little wider and shorter than the left one, and it points more steeply downward. Because of this, if a small object is breathed in by accident, it is more likely to slip into the right side. This is one more reason why it is safest to sit still while eating and to keep small toys and coins out of your mouth.

Branching Again and Again

Once a main bronchus enters a lung, it starts to split. It divides into a branch for each lobe. Each of those branches divides again, and again, and again. With every split, the tubes become narrower, and there are more of them. Scientists sometimes call this whole system the bronchial tree, because it looks so much like a tree turned upside down.

Here is a fun way to think about how quickly the branches add up. Imagine one tube that splits into two. Then each of those two splits into two, making four. Then four become eight, and eight become sixteen. If you kept going like this twenty times, you would end up with more than a million tiny tubes. Your real airways do not split in a perfectly even way like this, but they do split again and again, about twenty times or more, before they reach the very end of the tree.

The bigger branches near the top are still called bronchi. Like the windpipe, they have pieces of cartilage in their walls to keep them from collapsing. But as the branches get smaller, the cartilage pieces get smaller too, and then they disappear altogether. The tiny branches without any cartilage are called bronchioles, said like "BRONG-kee-ohlz." The word simply means "little bronchi."

The bronchioles are very narrow indeed. Many of them are less than one millimeter wide, which is thinner than the lead inside a wooden pencil. Without cartilage to hold them open, the bronchioles stay open in a different way. The spongy lung tissue all around them is joined to their outside walls. When your lungs fill with air, this tissue gently pulls the bronchioles open, like many tiny ropes tied to the sides of a tent.

All these branches give your lungs a surprising feel. Your lungs are not hollow bags, like balloons at a party. They are soft and spongy, a bit like a bath sponge, because they are filled with branching tubes and tiny air spaces. Healthy lungs are pink and very light for their size. All those tubes are folded neatly inside your chest, and every single one has a job: to carry air in and out, day and night, without ever getting tangled.

Why the Air Takes a Winding Path

You might wonder why your lungs need such a complicated tree. Why not just one big tube leading into one big bag? The answer is that a tree shape can reach everywhere.

Think about the roads in a town. A big main road carries traffic into town, and then smaller streets branch off it, and even smaller lanes branch off those, until every single house can be reached. The bronchial tree does the same thing for air. It brings fresh air to every corner of your lungs.

The tree shape also helps to protect you. In the windpipe, air rushes along quite fast. But each time the airways split, the total space for the air grows larger, so the air slows down. By the time air reaches the tiniest bronchioles, it is drifting along very gently. When air slows down like this, some of the tiny bits floating in it have time to sink and settle onto the sticky walls, where the mucus can catch them.

The branching points help in another way too. Imagine riding a bicycle very fast and then coming to a sharp bend in the path. If you do not slow down, you might keep going straight and bump into the hedge. Heavier bits of dust in the air behave in a similar way. When the air turns a corner at a fork, the air can make the turn, but some of the heavier bits cannot. They keep going straight and smack into the wall of the airway, right onto the sticky mucus.

So every fork in the bronchial tree is like a little catching net. The air twists and turns as it travels deeper, and at each turn, more dust, pollen, and germs get caught. The larger and heavier a bit is, the sooner it is trapped. Only the very smallest bits travel all the way to the end of the tree. In a later chapter, you will meet the special cleaners that deal with those smallest bits.

This catching system works best when it gets help from the very start. When you breathe through your nose, the nose filters out many of the biggest bits before they even reach the windpipe. It also warms and moistens the air, which keeps the lining of the bronchi comfortable. Breathing through your mouth is fine when you are running fast or when your nose is blocked, but for quiet, everyday breathing, the nose is the best front door.

Guards Along the Branches

The moving staircase you learned about in the windpipe does not stop at the fork. The cilia and the mucus carry on down into the bronchi and into many of the bronchioles too. So when dust lands on the wall of a branch deep inside your lung, the cilia there begin to push it upward. The dust travels from a small branch to a bigger branch, then to a main bronchus, and then up the windpipe to your throat, where you swallow it.

Imagine a river system seen from high above. Tiny streams flow into bigger streams, and bigger streams flow into a wide river. The mucus in your airways flows in a similar way, except that it flows upward. Little trickles of mucus from thousands of small branches join together, and they all head toward the same place: the top of the windpipe.

The walls of the small bronchioles also have special helper cells. These cells make protective substances that keep the lining of the airways healthy and help to calm things down when the lining is irritated. Some of these substances can stick to germs and make them easier for other guards to catch. The lining of your airways is not just a smooth wall. It is a living, working surface full of helpers.

There are also guard stations along the branches. Where the main bronchi enter each lung, there are lymph nodes, the same small, bean-shaped guard posts that you met in your neck in Chapter 4. You also have lymph nodes under your arms and in many other places. Lymph nodes are like meeting rooms for the white blood cells that defend your body. Messages about germs found in the lungs are carried to these meeting rooms, and there the defenders plan how to fight back.

Sometimes germs manage to make the lining of the bronchi sore and swollen. This often happens after a cold. The bronchi make extra mucus, and a person may have a chesty cough that brings up mucus for days. Doctors call this kind of swelling of the bronchi bronchitis. Most of the time, the body's guards win the fight and the cough slowly fades. If a cough goes on for a long time or breathing becomes hard, a grown-up should talk to a doctor.

Young children have narrower airways than grown-ups, simply because their bodies are smaller. In a narrow tube, even a little swelling or a little extra mucus makes a big difference. This is one reason why very young children sometimes have a noisy, wheezy chest when they catch a cold. As children grow, their airways grow wider and longer, and colds usually affect their breathing less.

Muscles Like Rubber Bands

There is something else wrapped around your bronchi and bronchioles: thin bands of muscle. These bands circle the tubes a little like rubber bands wrapped around a roll of paper. When the muscle bands relax, the tubes stay wide open. When the muscle bands tighten, they squeeze the tubes and make them narrower.

This muscle is different from the muscles in your arms and legs. You can decide to lift your arm or kick a ball. But you cannot decide to tighten the muscles around your airways, no matter how hard you try. They are controlled automatically, by nerves and by natural signals in your body. The muscles in the walls of your stomach and your blood vessels work in the same automatic way. Scientists call this kind of muscle smooth muscle.

Why would your airways need muscles at all? Scientists are still learning about this. The muscles help to keep the airways firm and steady as you breathe. They may also help to adjust how much air flows to different parts of the lungs. When you run and play hard, signals in your body help the airway muscles relax a little, so the tubes open wider and more air can flow in and out.

Your airways also change a little with every breath. Try this: sit up straight and take a slow, deep breath in through your nose, then let it out gently. As your lungs filled, the spongy tissue around your bronchioles stretched, and the tubes became a tiny bit wider and longer. As you breathed out, they settled back again. Your bronchial tree is quietly moving with every breath you take.

When a person breathes in something irritating, such as smoke or strong fumes, the airway muscles may tighten a little. This narrows the tubes for a short time, and a cough may follow. Some scientists think this helps to protect the deeper parts of the lungs by keeping out more of the irritating air. In most people, this tightening is small and passes quickly, and you would not even notice it.

Asthma: When the Branches Are Extra Sensitive

For some people, the airways are extra sensitive. Things that would not bother most people can make their airways react strongly. This condition is called asthma, said like "AZ-muh." Asthma is very common. In almost any classroom, there may be one or more children who have it. You might have asthma yourself, or you might know someone who does.

When the airways of a person with asthma react, three things can happen at once inside the bronchi and bronchioles. First, the lining of the tubes can become swollen and puffy. Second, the tubes can fill with extra mucus that is thick and sticky. Third, the bands of muscle around the tubes can squeeze tighter. Each of these three things makes the tubes narrower, and all three together can make them much narrower.

These changes do not usually last all the time. For much of the day, a person with asthma may breathe just as easily as anyone else. The trouble tends to come and go, a bit like weather that is calm for days and then turns stormy. When the airways are calm again, the swelling goes down, the extra mucus is cleared away, and the muscle bands relax.

Think about blowing air through a wide drinking straw. The air flows out easily. Now think about blowing through a very thin straw, the kind used to stir a drink. You have to push much harder, and the air comes out slowly. When the airways become narrow during asthma, breathing feels a bit like that. The air has to squeeze through the narrow tubes, and breathing out can become especially slow and hard.

A person having trouble with asthma may cough, especially at night or after running. The person may feel that the chest is tight, as if something is squeezing it. Sometimes there is a whistling sound when the person breathes out. Doctors call this sound a wheeze. The air makes the sound as it squeezes through the narrowed tubes, a little like the whistle you make when you blow through tightly pressed lips.

Many different things can set off asthma, and they are not the same for everyone. For one person, it might be a cold. For another, it might be dust, pollen, animal fur, smoke, cold air, or running very hard. People with asthma and their families learn which things affect them, so that they can avoid some of those things and be ready for others.

Asthma is not something you can catch from another person, the way you can catch a cold. It is a way that some people's airways work. Doctors know a lot about asthma, and they help families make a plan to keep the airways calm and to know what to do if breathing becomes hard. With that kind of care, children with asthma can run, swim, play games, and do all sorts of things that they love.

You can be a good friend to someone with asthma. Never tease a person for coughing or for needing to rest during a game. If a classmate seems to be struggling to breathe, tell a teacher or another grown-up right away. Being patient and kind helps a friend with asthma feel safe, and feeling calm and safe matters a lot when breathing is hard.

There is one important rule for everyone, whether you have asthma or not. If you or a friend ever find it hard to breathe, tell a grown-up straight away. Do not wait to see if it gets better, and do not feel shy about asking for help. Grown-ups would always rather know, and they can get help quickly if it is needed.

Now take a slow, comfortable breath and imagine the air flowing down your windpipe and splitting at the fork. Half of it turns left and half of it turns right. It flows into the branches, then into smaller branches, then into the thinnest bronchioles, slowing down all the while. Along the way, sticky walls catch the dust, waving cilia carry it back up, and guards stand ready at every station.

But what happens at the very tips of the tree, where the thinnest twigs end? On a real tree, the tips of the twigs grow leaves. In your lungs, the tips of the bronchioles end in something just as wonderful: clusters of tiny, bubble-like sacs where oxygen finally leaves the air and enters your body. In the next chapter, you will visit these tiny balloons, and you will find out why there are so many hundreds of millions of them.

Key Words

Bronchi: The two large tubes that split off from the bottom of the windpipe, one going into each lung, along with the bigger branches that come from them. One of them is called a bronchus.

Bronchioles: The tiny branches at the ends of the bronchial tree. They have no cartilage in their walls and are often less than one millimeter wide.

Bronchial tree: The whole system of airways inside your lungs, which branches again and again like a tree turned upside down.

Carina: The ridge at the bottom of the windpipe where it splits into the two main bronchi. It is very sensitive and can set off a strong cough.

Lobes: The large parts that each lung is divided into. The right lung has three lobes, and the left lung has two.

Smooth muscle: A kind of muscle that works automatically, without you deciding to move it. It wraps around the airways and can make them narrower or wider.

Lymph nodes: Small, bean-shaped guard stations found in many parts of your body, including near the lungs. White blood cells gather there to fight germs.

Asthma: A common condition in which the airways are extra sensitive. When they react, the tubes become swollen, full of mucus, and squeezed tight, which makes breathing harder.

Wheeze: A whistling sound made when air squeezes through narrowed airways, often while breathing out.