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2.1: Breathing: Why and How?

Difficulty Level: At Grade Created by: CK-12
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What happens when I breathe?

Breathing moves air in and out of your lungs. Lungs are spongy tissue made up of many tiny air-filled sacs that are surrounded by blood vessels. Breathing moves air in and out of your lungs all your life. As you breathe in, your chest and lungs expand. As you breathe out, your chest and lungs contract (squeeze in). Look at the people around you. Can you see them breathe just by watching their chests move? Now observe your own breathing. Breathe in. How does your chest feel when you breathe in? How does your chest feel when you breathe out? This unit will help you learn about the breathing process-How air is moved in and out of the lungs-Why breathing is necessary-What breathing does for the body-And how the body knows how much and how fast to breathe. By the end of this unit it will be pretty obvious how important and how amazing your breathing system is. When you know how the breathing system works, you can explore how to keep your breathing system healthy.

Why Do You Breathe?

You breathe in to get the important gas called oxygen \begin{align*}(O_2)\end{align*} from the air. All the cells in your body need oxygen. Without oxygen your cells, especially your brain cells, start to die in just a few minutes. You breathe out to get rid of a waste gas your cells make called carbon dioxide \begin{align*}(CO_2)\end{align*}.

A person who can't breathe will faint or “black out.” What is fainting and why is it one of the first things to happen when you can't breathe?

How Do You Breathe?

Your lungs are enclosed in your chest cavity. Over the lungs are your shoulder bones. Around your lungs are your ribs. These bones together make the chest cavity stiff and strong to protect the lungs. The diaphragm is located beneath the lungs. The diaphragm is a large dome-shaped sheet of muscle. When the diaphragm muscle contracts, its dome shape is flattened and the chest cavity gets bigger. This change in the volume of the chest cavity pulls air into the lungs, and they expand. When the diaphragm relaxes, it moves back into its dome shape. When this happens, the chest cavity gets smaller and you breathe out.

I Think Breathing . . . Write a paragraph or a poem about breathing. Try to answer these questions in your writing. What is breathing? How can you tell if you are breathing or not? How does breathing look when you observe someone? How does it feel? How does it sound? What happens when you breathe more quickly? What happens when you breathe more slowly?

Figure 1.1 Air flows through the nose or mouth into the air passages. From there it flows to the air sacs in the lungs where oxygen and carbon dioxide are exchanged.

As you breathe in, the air flows through your nose and mouth, through your windpipe, and through many smaller airways to all parts of your lungs. When you breathe out, the direction of the airflow reverses. So when you breathe out, the airflow goes from your lungs into the small airways, back up the windpipe, and out your nose or mouth.

When you exercise, you breathe deeply. During exercise you take in more air per breath than when you are resting. Do you think this is because the diaphragm contracts more strongly when you exercise? What other muscles can help your diaphragm when you have to breathe deeply? Put your hands flat against the sides of your chest and breathe deeply. Can you feel other muscles helping you to breathe?

Let's make a model of your chest and lungs to see how your lungs inflate.

Activity 1-1: How Do You Breathe?


How do you breathe? How does air move in and out of your lungs? In this activity you build a model of a lung to help you find out how the air moves into and out of your lungs. In the activity a clear, plastic soda bottle represents the chest. A balloon represents the lung. Another balloon represents the diaphragm or breathing muscle. The model you build will show how your lungs expand inside your chest each time you breathe. Your model also will show what happens if the breathing system is damaged by a wound that makes a hole in the chest cavity.


  • Clear, plastic bottle (about 1 liter)
  • Scissors
  • Tape
  • 2 balloons, or one balloon and a rubber glove
  • Activity Report


Step 1 Cut the bottom third off of the bottle. Be careful to leave a smooth edge so that the balloon won't tear.

Figure 1.2 Cut and remove the bottom third of the bottle.

Step 2 Place the balloon inside the bottle and roll the open end of the balloon over the lip of the bottle opening.

Figure 1.3 The balloon in the upper half of bottle should look like this.

Step 3 Tie off the neck of the second balloon. Cut the balloon so you can cover the opening at the bottom of the bottle with the balloon. Make sure the tied-off neck is facing down or outside the bottle after the balloon is attached.

Step 4 Tape the balloon to the bottle and make sure there are no air leaks.

Figure 1.4 Cover the opening of the bottle bottom with another piece of balloon. Make sure the tied neck of the balloon faces out to use as a handle.

Step 5 Use the tied off neck of the balloon as a handle and pull down. Hold for a second. Let go.

Step 6 Repeat. Observe and record what happens to the balloon inside the bottle.

Step 7 Answer questions 1-3 on the Activity Report before continuing with Step 8.

Step 8 Make a hole in the plastic chest. Pull down on the diaphragm. Hold for a second, and then release the diaphragm.

Step 9 Repeat. Observe and record what happens to the balloon inside the bottle.

Step 10 Answer questions 4-5 on the Activity Report. Make sure you save the model of the lung you built. You will be using the same model in Activity 5-2.

Figure 1.5 Use the tied off neck of the balloon as a handle to pull down.

Your lungs are similar in some ways to balloons. Like balloons, they fill with air. Your diaphragm is somewhat like a suction device. Air rushes into the lungs when the diaphragm contracts. Air is forced out of your lungs when the diaphragm relaxes. What makes your diaphragm work? You can make yourself breathe. But most of the time your diaphragm works, and you don't even know it. Your brain sends a message to the diaphragm muscle. The message tells the diaphragm to contract, and air is pulled into your lungs. The diaphragm moves automatically as part of your body's internal maintenance program.

Did You Know?

Lizards don't have diaphragms. All of their breathing depends on muscles between their ribs. These muscles are also needed for locomotion. So the lizards cannot breathe while they're running. That is one reason that lizards can be great sprinters, but not good long distance runners. Maybe the same was true of dinosaurs.

Figure 1.6 When the diaphragm muscle contracts and pulls down, air rushes into the lungs. When the diaphragm relaxes and moves up, air moves out of the lungs.

Figure 1.7 Your lungs aren't just two empty sacs filled with air like balloons. They're actually millions of tiny sacs bunched together.

What are hiccups? What happens to your diaphragm when you get the hiccups? What causes hiccups?

What Do My lungs look like?

Now try some creative thinking to imagine what it's like inside your lungs.

The inside of your lungs is pretty fantastic. Suppose you could shrink yourself down to a person smaller than the period at the end of this sentence. Now, suppose you enter someone's lungs in a breath of air. First, you bounce around and rattle down some tubes. As you get swept along, you pass many branch points like forks in a road. As you move through the tubes, they get smaller and smaller. Finally you end up sitting at the bottom of a tiny, thin-walled sac. The only way to get out is the way you came in. This is true unless you can pass across the thin membranes of the tiny air sacs like molecules of oxygen can.

How Many Breaths in a Lifetime? Determine your resting breathing rate by counting how many breaths you take per minute while sitting at your desk. Then calculate how many breaths you will take during your lifetime if you live to be 85 years old.

How might exercise affect this number? Ask a partner to count the number of breaths you take after running or jumping in place for one minute. Then decide how many minutes you might exercise each week. Use what you just found out to recalculate your lifetime breaths.

Figure 1.8 A visit inside lungs would show you a world of nonstop action.

Why would the Olympic cross-country, running team train at high altitudes?

Write a poem or a song using the vocabulary in this section to describe what you think about breathing. You can relate your lungs to objects such as broccoli or relate your breathing to functions you commonly encounter such as wind outside your window.

Remember that you are still a tiny particle. Imagine that the floor of the air-sac you're in is bigger than the last tiny airway you came through. But the lining of the air sac is very thin-almost as thin as a soap bubble. You can see through it. You can see a lot of pipes under the thin, air sac floor. These pipes are the veins and arteries that carry blood to and from the heart. You hear blood rushing under you. You feel air whiz in and out with each breath. Oxygen gas pushes from the air around you, through the floor, and into the blood. Carbon dioxide gas bubbles up through the floor and out over your head. The walls of the air sac bounce in and out with every breath. It's very windy and noisy all around you.

Did You Know?

It wasn't until the 16th century that scientists actually began to test their theories about breathing in a systematic way. The theories of ancient thinkers such as Aristotle became the basis for practical experiments. In the mid-1600s, several scientists proposed that breathing actually supplied the body with a needed substance called air.

That imaginary trip helped show how you breathe. But there are some other important questions to answer. For example, “Why do you breathe? How do your lungs work? Where does the air go? What do you take out of the air you breathe in? What do you put into the air you breathe out? What does the air do in your chest? Why do you not pass out if you try to hold your breath?” These are some of the things you will learn in this unit. You'll also find out what exercising and climbing high mountains can do to your breathing. As you complete the unit, you'll learn about diseases that damage your ability to breathe. You will also find out how to keep your fantastic, breathing machine healthy.

Review Questions

  1. Why do you need to breathe continuously?
  2. Why do you exhale carbon dioxide \begin{align*}(CO_2)\end{align*}?
  3. Describe how your diaphragm works to make you breathe in and out.

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Date Created:
Feb 23, 2012
Last Modified:
Jan 30, 2016
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