Chapter 8: The Alveoli: Hundreds of Millions of Tiny Balloons

Bunches of Grapes at the Tips

Have you ever held a bunch of grapes? There is one thick stem at the top. It splits into smaller stems, and each small stem ends in a round, plump grape. Now imagine a bunch of grapes so tiny that the whole bunch could sit on the head of a pin. Inside your lungs, there are millions and millions of bunches like that, and they are where the most important part of breathing happens.

In the last chapter, you followed the air down the branches of the bronchial tree, from the trunk of the windpipe to the thinnest twigs, the bronchioles. At the very end of each tiny bronchiole, the air finally arrives at a cluster of small, round air sacs. Each sac is called an alveolus, said like "al-VEE-oh-lus." When there are many of them, they are called alveoli, said like "al-VEE-oh-lye."

The alveoli are the reason you have lungs at all. Everything you have learned about so far, from the nose to the bronchioles, is a road that leads here. The nose cleans and warms the air. The windpipe and bronchi carry it deeper and keep it clean. But it is only here, in the alveoli, that the oxygen in the air leaves the air and enters your body.

Each alveolus is very small, only about a fifth of a millimeter across. That is about the size of a fine grain of sand. If you lined up five of them in a row, the line would be only about one millimeter long. Yet even though each one is so small, together they make up most of the inside of your lungs.

The number of alveoli is hard to imagine. A grown-up's two lungs hold about 300 to 500 million of them. Here is a way to feel how big that number is. Suppose you started counting the alveoli, one every second, and you never stopped to eat or sleep. To count 300 million of them, you would need almost ten years.

A newborn baby has far fewer alveoli than a grown-up. As a child grows, the lungs grow too, and new alveoli keep forming for many years. So right now, while you are growing, your lungs are still building new tiny balloons. That is one reason why clean air matters so much for children. Your lungs are not finished yet, and they are getting ready to serve you for your whole life.

How much air fills all those alveoli with each breath? When a grown-up is sitting quietly, each breath is about half a liter of air, roughly the amount in a small bottle of water. Your quiet breaths are smaller, because your lungs are smaller. When you take a really deep breath, you can pull in several times more air than in a quiet breath. Either way, the fresh air spreads out through the branches and into the waiting alveoli.

The alveoli are not floating separately like loose bubbles. They are packed closely together, like the bubbles in a sink full of soap foam. Each alveolus shares walls with its neighbors. There are even tiny openings in some of these shared walls, so a little air can pass from one alveolus to the one next door. If one small airway gets blocked, these little openings can let some air reach the alveoli beyond it by another route.

Why Not One Big Balloon?

You might wonder why your lungs have hundreds of millions of tiny balloons instead of just two big ones. The answer has to do with surface. The surface of something is its outside, the part that can touch the air around it. Oxygen can only enter your blood through a surface, so the more surface your lungs have, the more oxygen can get in.

Here is a way to understand this. Think about a big, round block of ice. If you put it in a glass of water, it melts slowly, because only its outside touches the water. Now crush the same block of ice into many small pieces. The crushed ice melts much faster, because all those little pieces together have far more outside surface touching the water. The amount of ice is the same, but the surface is much bigger.

Your lungs use the same trick. By dividing their space into hundreds of millions of tiny alveoli, they create an enormous surface. If you could flatten out the walls of all the alveoli in a grown-up's lungs and spread them across the floor, they would cover about 50 to 100 square meters. That is roughly as big as the floor of a whole classroom. All of that surface is folded up neatly inside your chest, behind your ribs.

A big surface means that a lot of oxygen can enter the blood at the same time. This is especially important when you run, jump, or climb. Your muscles need much more oxygen when they work hard, and your huge lung surface can supply it. Even when you are sitting still, every breath brings fresh air to millions of alveoli at once.

But a big surface is only half of the trick. On the other side of that surface, there must be blood waiting to take the oxygen away. Your lungs have that too. Blood flows right up against every one of those tiny balloons, as you will see next.

A Wall Thinner Than Paper

Wrapped around every alveolus is a net of very thin blood vessels called capillaries, said like "KAP-ih-lair-eez." The capillaries cover each alveolus like a string bag wrapped around a ball. They are so narrow that the tiny red blood cells inside them must travel in single file, one after another, like children walking through a narrow doorway. So much blood flows through these nets that the whole lung takes on a soft pink color. The blood is spread out in a thin layer right next to the air, which helps your lungs make the swap fast.

Between the air inside the alveolus and the blood inside the capillary, there is a wall. But this wall is astonishingly thin. It is made of the very thin wall of the alveolus pressed right against the very thin wall of the capillary. Together, they are often less than a thousandth of a millimeter thick. That is more than a hundred times thinner than a sheet of paper.

Why does the wall need to be so thin? Oxygen and carbon dioxide have to pass right through it, and the thinner the wall, the faster they can cross. Think of a mesh window screen. Air passes through it easily because the screen is thin and full of tiny gaps. If the screen were as thick as a brick wall, hardly any air would get through. The thin wall in your lungs is a little like that screen, but much, much finer.

Of course, a wall this thin is also delicate. It cannot have a thick layer of mucus or a carpet of cilia, because those would make the wall too thick and slow down the swap of gases. This is why all the guards higher up in your airways are so important. The nose, the throat, the windpipe, and the bronchi catch most of the dust and germs long before they can reach this delicate place.

The Great Swap

Now for the most important moment of the whole journey. When you breathe in, fresh air fills your alveoli. This air is full of oxygen. Meanwhile, the blood arriving in the capillaries has just come back from a trip around your body, where it gave away much of its oxygen. So there is a lot of oxygen in the alveolus and much less in the blood next to it.

When there is a lot of something in one place and only a little in the place next to it, it tends to spread out toward the place where there is less. Think about what happens when bread is baking in an oven. Soon the smell spreads from the kitchen into every room of the house. Or think about a drop of food coloring in a glass of water. Slowly, the color spreads until the whole glass is tinted. This spreading is called diffusion, said like "dih-FEW-zhun."

Oxygen does the same thing in your lungs. It spreads from the air in the alveolus, where there is a lot of it, through the thin wall, and into the blood, where there is less. Inside the blood, the oxygen is picked up by the red blood cells, which carry it away to the rest of your body. You will learn much more about those red blood cells and their journey in a later chapter.

Remember from Chapter 1 why your body needs this oxygen. Every cell uses oxygen to get energy from the food you eat, a little like a campfire that needs air to keep burning. Your cells release energy from food slowly and gently, with no flames at all, but they still need a steady supply of oxygen to do it. Now you know the exact spot where that oxygen enters your body.

At the same time, something else is moving the other way. As your body works, every one of its cells makes a waste gas called carbon dioxide. The blood collects this carbon dioxide and brings it back to the lungs. In the capillaries, there is a lot of carbon dioxide, and in the fresh air of the alveolus there is very little. So the carbon dioxide spreads out of the blood, through the thin wall, and into the alveolus. When you breathe out, it leaves your body.

This swap happens incredibly quickly. A red blood cell passing an alveolus spends less than a second in the capillary, and in that short time it picks up its oxygen and lets go of its carbon dioxide. Oxygen in, carbon dioxide out, over and over, in hundreds of millions of alveoli at once. It happens all day and all night, whether you are thinking about it or not.

In Chapter 1, you read that the air you breathe out still has quite a lot of oxygen left in it. Now you can see the numbers. Fresh air is about one fifth oxygen, and breathed-out air still has a little more than three quarters of that oxygen left. Your lungs take only part of the oxygen from each breath. But your breathed-out air has much more carbon dioxide than the fresh air, about a hundred times more. And it has a lot of water vapor too.

You can see that water vapor for yourself. On a cool day, breathe gently onto a cold window or a mirror. A misty patch will appear on the glass. That mist is made of tiny drops of water that came out of your lungs with your breath. The warm, moist air from your alveoli cooled down when it touched the cold glass, and the water in it turned back into droplets. Every breath you breathe out carries a little water away with it.

High up on a tall mountain, the air is thinner. Each breath there holds less oxygen than a breath taken near the sea. People who climb high mountains find that they breathe faster and their hearts beat faster, because their bodies are working to get enough oxygen into the alveoli. If they stay high up for several weeks, their bodies slowly adjust, for example by making more of the cells that carry oxygen in the blood.

Keeping the Balloons Open and Clean

There is a tricky problem with having so many tiny, wet balloons. The inside of each alveolus is covered with a very thin film of liquid. Wet surfaces tend to stick together. You may have noticed how two sheets of wet plastic wrap cling to each other, or how wet hair clumps together. If nothing stopped them, the walls of the tiny alveoli could stick together and collapse every time you breathed out.

Your lungs solve this problem with a special liquid called surfactant, said like "sur-FAK-tunt." Some of the cells in the walls of the alveoli make surfactant and spread it over the inside surface. Surfactant works a little like soap. It makes the watery film much less sticky, so the alveoli can stay open, even when you breathe all the way out. Then, when you breathe in again, the alveoli fill up easily.

Think about blowing up a party balloon. The first puff is often the hardest, and once the balloon has started to stretch, the next puffs are easier. If the alveoli collapsed completely each time you breathed out, every breath in would be like that hard first puff, over and over. Surfactant keeps the alveoli from collapsing, so each new breath is easy, and you hardly notice that you are breathing at all.

Babies start making surfactant before they are born. A baby who is born very early may not have made enough of it yet, and so the baby may find breathing hard work. Doctors and nurses who care for newborn babies know how to help in these cases. As the baby grows, the baby's lungs begin to make plenty of surfactant on their own.

Surfactant has another job too, and this one is about guarding your body. Some of the proteins in surfactant can grab onto germs. They clump the germs together and make them easier to find and remove. So even here, in the most delicate part of the lungs, there is a defense. The liquid that keeps the alveoli open also helps to keep them clean.

The walls of the alveoli can also mend themselves. If some of the thin cells lining an alveolus are damaged, for example by a germ, other cells nearby can divide and grow to replace them. In this way, the lining is patched up, a little like a road crew filling in a crack in the road. Your lungs are always quietly repairing and renewing themselves.

Still, the alveoli cannot use mucus and cilia the way the bigger airways do. So a few of the tiniest specks that make it all the way down need a different kind of cleaning. Some of these specks are dust, smoke, or pollen bits that are too small to be caught higher up. Others are germs. Without some way to clear them away, they could pile up in the alveoli.

When Germs Reach the Alveoli

Most of the time, the guards in your nose, throat, and airways stop germs long before they reach the alveoli. And the few germs that do get this far are usually dealt with quickly. But once in a while, germs manage to settle in the alveoli and start to multiply. When this happens, the body sends in many defenders to fight back.

During this fight, some of the alveoli can fill up with fluid and with the defenders themselves, instead of with air. Doctors call this kind of lung infection pneumonia, said like "new-MOH-nee-uh." The filled alveoli cannot swap oxygen and carbon dioxide as well as usual. A person with pneumonia may have a fever, a cough, and fast or hard breathing, and may feel very tired.

If a child has a fever along with a cough, fast breathing, or breathing that seems hard, a grown-up should talk to a doctor. A doctor can listen to the lungs with a stethoscope, a tool with a round end that is placed on the chest. Through the stethoscope, the doctor can hear the air moving in and out and can tell a lot about how the lungs are doing. Most people with pneumonia get better with the right care and rest.

There are many everyday ways to help keep germs away from your alveoli in the first place. Washing your hands, covering coughs and sneezes, and getting enough sleep all help your body's guards. Vaccines also help to train your body to recognize some of the germs that can cause lung infections. You will learn more about handwashing and vaccines later in this book.

So who is it that keeps the alveoli clean every single day? Who gobbles up the specks of dust that slip all the way down, and who stands ready to fight the germs that arrive here? The answer is a group of remarkable cells that actually live inside the alveoli. They crawl over the inner walls, searching for anything that does not belong.

Take a slow breath and picture the air arriving in your alveoli. Hundreds of millions of tiny balloons fill up together. Oxygen slips through the thin walls into your blood, and carbon dioxide slips out to be breathed away. And in the corners of those tiny balloons, the cleaners are busy at work. In the next chapter, you will meet these cleaners, and you will see how they keep the deepest part of your lungs safe.

Key Words

Alveoli: The tiny, round air sacs at the ends of the smallest airways, where oxygen enters the blood and carbon dioxide leaves it. One of them is called an alveolus.

Capillaries: The thinnest blood vessels in your body. They wrap around each alveolus like a net, and red blood cells pass through them in single file.

Oxygen: A gas in the air that every cell in your body needs in order to use the energy in your food.

Carbon dioxide: A waste gas made by the cells of your body. The blood carries it to your lungs, and you breathe it out.

Diffusion: The way something spreads out from a place where there is a lot of it to a place where there is less, like a smell spreading through a house.

Surface area: The total size of the outside of something. Your lungs have a huge surface area because they are made of hundreds of millions of tiny alveoli.

Surfactant: A soap-like liquid made in the alveoli. It keeps the tiny air sacs from sticking shut, and some of its proteins help to clump germs together.

Pneumonia: An infection deep in the lungs in which some of the alveoli fill with fluid instead of air, making breathing harder.

Stethoscope: A tool doctors use to listen to the sounds of your lungs and heart through your chest.