Chapter 11: The Diaphragm and Ribs: The Breathing Pump

Who Fills the Lungs?

In the last chapter, you rode along with the red train of the blood. Red blood cells picked up oxygen in the lungs, rushed through the heart, and carried that oxygen to every corner of your body, from your brain to the tips of your toes. On the way back, the same red blood cells helped bring carbon dioxide, a waste gas, back to the lungs so that you could breathe it out.

But a train can only pick up what is waiting at the station. If the air inside your lungs never changed, the oxygen in that air would soon be used up, and the carbon dioxide would pile up like garbage that nobody takes away. Fresh air has to come in, and old air has to go out, again and again, all day and all night, for your whole life.

So here is a puzzle. Your lungs are soft and spongy, a bit like two large sponges filled with tiny balloons. They have no strong muscles of their own. They cannot squeeze themselves smaller or puff themselves up. So what makes your lungs fill with air and empty again, more than twenty thousand times every single day?

The answer is a team of muscles and bones that wraps around the lungs like a living machine. The most important member of the team is a muscle called the diaphragm. Its partners are your ribs and the small muscles between them. Together, they make up the breathing pump. In this chapter, you will find out how this pump works, how your body keeps it running even while you sleep, and how it helps protect you from germs and dust.

The Diaphragm: A Dome Under Your Lungs

Put your hand on your chest, then slide it down to the bottom edge of your ribs, just above your belly. Underneath your hand, hidden deep inside your body, lies the diaphragm. It is a wide, thin sheet of muscle that stretches all the way across the inside of your body, from front to back and from side to side. Say its name slowly: DIE-uh-fram.

The diaphragm is shaped like a dome, or like a bowl turned upside down. When it is relaxed, the middle of the dome bulges upward into the chest. Your lungs and your heart rest on top of it. Your stomach, your liver and the rest of your belly organs sit underneath it. In this way, the diaphragm works as a floor for the chest and a ceiling for the belly at the same time.

Like every muscle, the diaphragm has one main move: it can tighten. When a muscle tightens and gets shorter, scientists say that it contracts. When the diaphragm contracts, the dome pulls downward and becomes flatter, almost like the surface of a trampoline that someone has pulled tight. When the diaphragm relaxes, the dome rises back up into its curved shape.

This simple movement, down and up, down and up, is the heart of breathing. Each time the diaphragm flattens, the space inside your chest grows taller. Each time the diaphragm relaxes, that space becomes shorter again. As you will soon see, changing the size of that space is all it takes to move air in and out of your body.

The diaphragm is one of the hardest-working muscles you have. Your arm muscles rest when you sit still, and your leg muscles rest when you lie down. The diaphragm keeps going from the moment you are born, through every day and every night. Luckily, it is built for this kind of steady, gentle work. It rarely gets tired the way your legs do after a long run up a hill.

The Ribs: A Cage That Can Move

Now let your fingers walk along the side of your chest. You can feel bumps, one above the other, like the rungs of a ladder. These bumps are your ribs. Most people have twelve pairs of ribs, twelve on the left side and twelve on the right. The ribs curve around from your backbone at the back toward your breastbone, the flat bone in the middle of the front of your chest.

Together, the ribs form the rib cage. A cage may sound like something stiff and fixed, but your rib cage is not like that at all. Between each pair of neighboring ribs there are thin layers of muscle called intercostal muscles. The word intercostal simply means "between the ribs." When these muscles tighten, they lift the ribs upward and outward.

To picture how this works, think of the handle on a bucket. When the handle hangs down, it lies close against the side of the bucket. When you lift the handle up, it swings outward and makes a wider loop. Your ribs move in a similar way. As the rib muscles pull them up, the ribs swing out to the sides, and the front of your chest moves forward a little. The chest becomes wider from side to side and deeper from front to back.

The rib cage has a second job that is just as important. It is a shield. The ribs curve around your heart and lungs like the bars of a strong but flexible fence, protecting these soft organs from bumps and knocks when you run, tumble or play. Many of the ribs are joined to the breastbone by cartilage, the same firm but bendy material that shapes the tip of your nose and the outside of your ears. Because of the cartilage, the rib cage can give a little when it is pushed, instead of being stiff like a box.

There is one more clever detail. The outside of each lung and the inside of the chest wall are covered by thin, smooth layers called the pleura. Between these two layers there is a tiny amount of slippery fluid. This fluid lets the lungs glide against the chest wall without rubbing. It also makes the two layers cling together, the way two wet pieces of glass slide easily over each other but are hard to pull apart. Because the layers cling, the lungs follow every move the chest makes, like a shadow following you across the floor.

Breathing In and Breathing Out

Now you know the parts. Here is how they work together. Air always moves from a place where it is more squeezed to a place where it is less squeezed. Scientists call this squeeze pressure. When you let go of the neck of a blown-up balloon, the squeezed air inside rushes out into the open room. Your breathing pump uses this simple rule every few seconds of your life.

When you breathe in, the diaphragm contracts and moves down, and the rib muscles lift the ribs up and out. The chest becomes bigger in every direction: taller, wider and deeper. The lungs, which cling to the chest wall, stretch to fill the larger space. Now the air inside the lungs is spread out and less squeezed than the air outside your body. So outside air flows in through your nose or mouth, down your windpipe and into the lungs, until the pressure inside and outside is even again.

A medicine dropper works in a similar way. When you squeeze the rubber bulb and then let go, the bulb springs back and makes more room inside, and liquid is pulled up into the tube to fill that room. Your chest is like the bulb. Each time the diaphragm and ribs make the chest bigger, air is pulled in to fill the extra space.

Breathing out, when you are calm, is even easier. The diaphragm simply relaxes and floats back up into its dome shape. The rib muscles relax too, and the ribs settle back down. The lungs themselves are stretchy, like a rubber band that has been pulled, so they spring back to a smaller size on their own. The chest gets smaller, the air inside is squeezed, and out it flows. When you are resting, breathing out takes almost no effort at all.

When you need to blow hard, your body adds extra power. Muscles in your belly tighten and push the belly organs up against the diaphragm, while other rib muscles pull the ribs down. You use this strong push when you blow out candles on a birthday cake, blow soap bubbles, shout across a field to a friend, or play a whistle or a flute. You also use it when you cough, which you will read more about soon.

Here is something you can try safely right now. Sit or lie down comfortably and place one hand flat on your belly and the other hand on your chest. Breathe in slowly through your nose. Can you feel your belly gently push your hand outward? That happens because the diaphragm is moving down and nudging the belly organs out of the way. Now breathe out slowly and feel your belly sink back in: you are feeling your breathing pump at work.

You might notice that the hand on your chest moves less than the hand on your belly. During calm breathing, the diaphragm does most of the work. During big, fast breaths, the rib muscles join in much more, and you can feel your whole chest lift. Both ways are normal. Your body simply chooses the right amount of effort for each moment.

Fast Breaths, Slow Breaths

Try counting how many times you breathe in during one minute while you sit quietly. Ask a grown-up to watch a clock for you. It is a little tricky, because thinking about breathing can change it, so just breathe the way you normally do. Many children your age breathe somewhere around fifteen to thirty times a minute when they are resting. Most grown-ups breathe a little more slowly, about twelve to twenty times a minute. Newborn babies breathe much faster, often thirty to sixty times a minute, because their tiny bodies are growing so quickly.

Your breathing speed is not fixed. Run around the playground, climb a hill, or dance to fast music, and you will notice that you breathe faster and deeper. Your working muscles are using up oxygen and making extra carbon dioxide, so the breathing pump speeds up to keep the station stocked for the red train. When you sit down and rest, your breathing slowly calms down again.

Each breath also changes in size. In a quiet breath, the diaphragm moves down only a little, about the width of one or two of your fingers. In a very deep breath, it can move down several times farther. As you read in Chapter 8, a grown-up's quiet breath moves about half a liter of air. A really deep breath can move several times more than that.

Sometimes the pump does something unexpected. A hiccup happens when the diaphragm suddenly tightens with a jerk, all by itself. Air starts to rush in, and a split second later the vocal cords in your throat snap shut, making the "hic" sound. Hiccups can be a nuisance, but they are usually harmless, and they stop on their own after a while.

Your breathing pump also helps you talk, sing and laugh. Your voice is made when air from the lungs flows out past the vocal cords in your throat and makes them buzz. To sing a long note, you let the air out slowly and steadily. To laugh, your diaphragm and belly muscles give quick little pushes, one after another. Every word you say rides out of your body on a breath of air.

The Brainstem: A Guard That Never Sleeps

Think about your breathing for a moment. You can choose to take a big breath, or to breathe slowly, or to hold your breath for a few seconds before you put your face in the water at the pool. But most of the time, you do not think about breathing at all. You breathe while you read, while you eat lunch, while you play, and all night long while you are fast asleep. So who is in charge when you are not paying attention?

The answer is the brainstem, the part of the brain you met in Chapter 3 and again in the last chapter. The brainstem sits at the very bottom of the brain, where the brain joins the spinal cord, the long bundle of nerves that runs down inside your backbone. The brainstem looks after many jobs that keep you alive without your having to think about them, and breathing is one of the most important of those jobs.

Inside the brainstem, groups of nerve cells send out a steady rhythm of signals, like a drummer keeping the beat for a band. Each signal travels along nerves to the diaphragm and the rib muscles and tells them, "Tighten now." Between the signals, the muscles relax. Drumbeat, breathe in. Pause, breathe out. The beat goes on, hour after hour and year after year.

The brainstem does not only keep a steady beat. It also listens to reports from tiny sensors in the body. Some of these sensors are in the brainstem itself, and others sit beside large blood vessels in the neck and near the heart. The sensors keep checking the blood, especially how much carbon dioxide the blood is carrying. When carbon dioxide starts to build up, for example while you are running, the sensors send a message, and the brainstem tells the pump to work faster and harder.

This is why nobody can hold their breath for very long. If you hold your breath, carbon dioxide slowly builds up in your blood. Soon the sensors raise the alarm, and the brainstem sends stronger and stronger signals until you simply have to breathe. This safety system is stronger than your own wish. Even so, holding your breath for a long time is not a good game to play, especially in water. Breathing should always feel easy and comfortable.

Your brainstem is a guard that never takes a break. When you sleep, your breathing becomes slower and more even, but it keeps going all night. Even while you are dreaming of flying over mountains or swimming with whales, the drummer in your brainstem keeps the beat, and your diaphragm keeps moving the air in and out.

How the Pump Protects You

By now, you have met many guards that protect your lungs: the hairs and mucus in your nose, the tonsils in your throat, the epiglottis, the escalator of tiny hairs in the windpipe, and the cleaner cells in the alveoli. The breathing pump may not look like a guard, but many of those other guards could not do their jobs without it.

First, the pump gives power to coughs and sneezes. As you learned in Chapter 3, before a cough you take a quick breath in. Then the vocal cords close tight, and the belly and rib muscles squeeze hard, building up pressure inside the chest. Suddenly the vocal cords open, and a strong burst of air shoots up the windpipe, carrying mucus, dust or a stray crumb along with it. Without a strong breathing pump, the emergency exits you read about earlier in this book would be nothing more than weak little puffs.

Second, deep breaths help keep the lungs clean and open. When you breathe very gently for a long time, some of the tiny air sacs deep in the lungs may not fill all the way. Every few minutes, without noticing, you take a sigh, a breath that is a little bigger than usual. Sighs, along with the big breaths you take when you run, laugh or play, stretch those quiet corners open again. Big breaths also help the escalator of tiny hairs move mucus up and away. This is one reason why being active is so good for your lungs.

Third, the pump adjusts when your body is fighting germs. When you are sick, your body is working hard behind the scenes, and it often needs more oxygen than usual. The brainstem notices and may make you breathe a little faster. In this way, the breathing pump changes its speed to match what your body needs, whether you are racing a friend or resting in bed. If breathing ever seems hard or uncomfortable, it is important to tell a grown-up right away.

Fourth, the pump works together with your nose. When you breathe in calmly through your nose, the air moves slowly enough to be warmed, moistened and filtered before it reaches your lungs. The diaphragm sets the pace, and the nose does the cleaning. Calm nose breathing, by day and by night, lets these two guards work as a team.

Finally, remember that the pump needs care, like any member of a team. Sitting and standing tall gives the diaphragm room to move. Running, swimming, climbing and dancing make the breathing muscles stronger. And if breathing ever feels hard, tight or noisy in a way that is new, tell a grown-up, so that the grown-up can talk to a doctor if they are worried.

Every breath you take is a small team effort. The brainstem gives the signal. The diaphragm pulls down, and the ribs swing out. The lungs stretch, and fresh air rushes in, past all the guards, down to the tiny balloons where the red train is waiting. Then the pump relaxes, and the old air flows out again. In the next chapter, you will find out what happens on the days when germs manage to slip past the guards, and how your whole body works together to fight back.

Key Words

Diaphragm: A dome-shaped sheet of muscle under the lungs. When the diaphragm tightens and flattens, air flows into the lungs.

Contract: What a muscle does when it tightens and becomes shorter. Contracting is how muscles pull on parts of the body and move them.

Ribs: The curved bones that wrap around the chest. Together they make the rib cage, which protects the heart and lungs and moves when you breathe.

Intercostal muscles: Thin muscles between the ribs. They lift the ribs up and out to make the chest bigger when you breathe in.

Pleura: Two thin, smooth layers around each lung with a little slippery fluid between them. They let the lungs glide easily as the chest moves.

Pressure: How strongly air or water is being squeezed. Air always moves from a place where the pressure is higher to a place where it is lower.

Breathing rate: The number of breaths you take in one minute. It goes up when you are active and goes down when you rest.

Brainstem: The bottom part of the brain, where the brain joins the spinal cord. It keeps breathing and other life-supporting jobs going without your thinking about them.

Hiccup: A sudden, jerky tightening of the diaphragm, followed by the quick closing of the vocal cords, which makes a "hic" sound.