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3.1: My Breathing Machine

Difficulty Level: At Grade Created by: CK-12
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What do my lungs look like, and how do they work?

Remember that your lungs are not just balloon-like air sacs. The lungs are made up of millions of tiny air sacs. In fact, the lungs look like an upside down tree. The trachea is like the trunk that branches to your two lungs. In this section you will pretend to be a tiny explorer who explores the human breathing machine.

Now let's start to explore inside the breathing machine to see how air gets to the lungs. The airway to the lungs begins with the nose and mouth. Again, pretend that you are a tiny explorer only as big as the period at the end of this sentence. You begin your exploration at the nose. You crawl into a nostril and find yourself in a large chamber. It feels like the inside of a cave with wet, slippery walls.

The slimy walls are warm and moist. The slime is called mucus. The mucus is a secretion produced by the cells that line the airways. Mucus helps wet the air and traps dust and dirt to help keep the lungs clean. The curves in the walls of the nostril cave swish the air around to help moisten it. You don't always breathe through your nose. Sometimes you breathe through your mouth. When you do, the air goes straight into the pharynx. It doesn't swish around through curves. You may notice that breathing through your mouth, for even a short time, makes your mouth and throat feel dry and scratchy.

What Do You Think?
Cold medicines such as decongestants slow the production of mucus. As you learned, mucus is the body's way of catching and getting rid of germs and foreign materials. Do you think it is wise to take cold medications like decongestants? Why or why not?

Voice Box Make a “guitar” from a cardboard box, a stick, and rubber bands. The stick keeps the bands away from the cardboard so they can vibrate. A thin, tight band makes a high sound. A thicker, looser band makes a low sound. Now blow across the bands. Describe the sound. What happens when you blow hard and when you blow lightly?

Figure 2.1 Your airways make it possible for your lungs to get lots of warm, moist, and clean air.

Figure 2.2 Make a cardboard guitar like this one.

The nose passages and mouth cavity come together in the pharynx. The pharynx is the throat. The pharynx is the tube in back of the nose and mouth where the nose passages and mouth cavity meet. You crawl down the front side of the pharynx and come to a trapdoor that opens into a chamber strengthened with cartilage. You are now entering the voice box called the larynx. The larynx connects the pharynx with the windpipe that will take you to the lungs. You notice that there are bands of stiff tissue strung across the larynx. These bands of tissue are the vocal cords.

Did You Know?
Your head and chest act as the cardboard box does that you built in the Mini Activity. The chest and head like the cardboard box create a resonating chamber. The resonator moves with the vibrating string and makes a bigger sound. The shape of every person's skull and chest is different. So different people have different voices. No two human voices are exactly alike.

Let's find out how the vocal cords work. There is a slit-like opening called the glottis between the vocal cords. You draw your vocal cords apart when you breathe to let air into your windpipe. You draw the vocal cords together to speak or sing. You make sounds when you breathe out because the air flowing through the glottis causes the vocal cords to vibrate. The vibrating vocal cords make sounds like the vibrating strings on a guitar.

Now look at the side view of the head shown in Figure 2.1 on page 9. Find the tube just below the larynx or voice box. The tube that attaches to the larynx is the main breathing tube or windpipe called the trachea. The trachea is the main breathing tube to the lungs. Now look behind the trachea. There's another tube called the esophagus. The esophagus is the tube that carries what you eat and drink to your stomach. You can see that the trachea and esophagus are very close together. Also, they are both connected to the mouth and nasal passages.

So the esophagus and the trachea are both connected to the mouth cavity and nasal passages. But food doesn't go into the lungs. Let's find out why. To keep what you eat and drink from going into the trachea, a trap door sits over the larynx. The trap door closes when you eat or drink to keep food out of the airways. But it opens when you breathe in or cough out. This flap-like trap door is called the epiglottis. Notice the word epiglottis has the word glottis in it. The epiglottis is an extension of the glottis. Have you ever laughed so hard that milk you were drinking came out your nose? That happens when the epiglottis opened, letting air out of your lungs when you were drinking the milk and starting to laugh at the same time.

Did You Know?
When you hit a tennis ball really hard, it travels around 50mph. When you hit a baseball really hard it travels about 85mph. When you cough, particles and water bits can travel about 100mph.

Sometimes food can get around the epiglottis and into your windpipe when you eat and talk or laugh at the same time. You cough when this happens. Food around your epiglottis triggers a cough. Then the blasts of air from the cough usually blow bits of food clear of your trachea. We will explore more about coughing and choking later in this unit. But the important concept to learn now is that the epiglottis covers your vocal cords and trachea when you swallow. That's why you can't swallow and talk at the same time. That's also why it's not a good idea to laugh when you have food or beverages in your mouth.

Did You Know?
When you get a really bad cold, your airways get infected. That infection is called bronchitis since it is the bronchi and bronchioles that are being attacked by the cold virus or bacteria.

Now let's get back to our exploration. By now you've gone past the epiglottis trap door and entered the trachea. You notice that the walls of the trachea contain rings of cartilage. Even from the outside you can feel the trachea in the front, low part of the neck. Below these rings of cartilage the trachea branches into two tubes-one tube for each lung. These tubes are called the bronchi. Like a branching tree, each of the bronchi branches again and again into smaller tubes called bronchioles. Each time the tube branches, it gets smaller. After about 16 forks in the path little clusters of air sacs called alveoli begin to appear.

What is the function of the rings of cartilage in your trachea?

Activity 2-1: Building Clusters of Balloon Alveoli


What do your lungs look like? In this activity each member of your class builds a balloon alveolus or air sac. Then you work with your team to build a balloon lung by clustering your individual balloon alveoli together. The airway into the alveolus is represented by yarn and the larger airway is represented by the rope.


  • Balloon
  • Yarn
  • String
  • Glue
  • Rope
  • Activity Report


Step 1 Blow up a balloon. Each balloon represents an alveolus filled with air.

Step 2 Tie off the neck of your balloon with yarn so the air won't escape. Tie a piece of yarn to the neck of your balloon. The yarn represents the part of each airway that supplies the alveolus with air.

Step 3 Gather the balloons from all of the students in your group. Tie the free ends of the yarn together. Then tie the yarn ends to one end of the rope. (See Figure 2.3.) The rope represents a primary bronchus. The yarn represents the bronchioles. The balloons represent the alveolar sacs.

Step 4 Soak strings with glue. Then wrap the glue-soaked strings around the balloons. The strings represent small blood vessels around the lung called capillaries.

Step 5 Answer the questions on the Activity Report.

Figure 2.3 The balloons represent alveoli. The yarn represents small branches of the airways or bronchioles. The rope represents a bronchus. The strings represent the capillaries.

Figure 2.4 The airways called bronchioles branch to the air sacs called alveoli.


The alveoli are very important to breathing because that's where oxygen is taken in and carbon dioxide is released. To take in enough oxygen you need a lot of alveoli. Let's take a closer look at the alveoli to find out why we need so many.

How many air sacs or alveoli do you have? How large are the alveoli? First think about the tiny air tubes that open into the alveoli. Remember the airways branch many times, and each time they branch they get smaller. If your windpipe branches into two smaller tubes and each of them branches into two, how many branches would you have? Now if the branching happens twenty more times, how many branches would you have? It's a big number, isn't it? But the number of alveoli is even bigger because each final air tube opens into 60 or 70 alveoli. This is like a bunch of grapes on one stem. So how many alveoli do you have?

What do air sacs do?

Air sacs make it possible for your blood to get oxygen (O2) from the air you breathe in. The alveoli also let your blood release carbon dioxide (CO2) into the air you breathe out.

Blood that is low in oxygen and high in carbon dioxide comes from the heart to the lungs in big blood vessels called arteries. These arteries branch into smaller and smaller vessels called arterioles until they branch into the tiniest of all blood vessels called capillaries. These small capillaries surround each air sac or alveolus. Remember that the string you glued to the balloon alveoli in Activity 2-1 represented capillaries. The blood in the capillaries exchanges gasses through the thin walls of the air sacs. The gases pass through the thin walls of the capillaries and the thin walls of the alveoli. This process is called diffusion. Diffusion is the natural movement of particles from an area of high concentration to an area of low concentration. For example, suppose there is a lot of carbon dioxide in the blood and only a little oxygen. But there is a lot of oxygen in the alveoli and only a little carbon dioxide. As a result of diffusion, the oxygen will move from the lungs where there is a lot into the blood where there is very little. The carbon dioxide will move from the blood where there is a lot into the alveoli where there is only a little. In this way, diffusion results in an even distribution of the particles.

Did You Know?
Some animals such as certain kinds of frogs can exchange oxygen and carbon dioxide across their skin. They have thin, wet skin. Because they can obtain oxygen through their skin, they can stay under water for long periods of time.

Figure 2.5 If you could count at the rate of 5 air sacs per second, it would take 694 days to count all the air sacs when they are squished-out flat- even if you counted 24 hours a day.

Different molecules diffuse at different rates. Give an example of different molecules that diffuse at different rates. Even one kind of molecule can diffuse at different rates depending on the conditions under which it is diffusing. Give an example of a molecule that can diffuse at a different rate when the temperature is cold than when the temperature is warm.

Did You Know?
Your body cells are never more than two cells away from a capillary. As the blood passes the cell, it brings food and oxygen and takes away carbon dioxide.

The small capillaries leave the alveoli and join into larger and larger blood vessels called venules. Eventually the venules join larger blood vessels called veins. The larger veins take blood that is high in oxygen (O2) and low in carbon dioxide (CO2) back to the heart. Then the heart pumps the oxygen-rich blood throughout the body to all the cells that need oxygen. You will learn more about why you need to exchange these gases in the next section.

Activity 2-2: The More the Airier


How would you design a lung to get enough oxygen into your body? Would you create small or big air sacs? How many air sacs would be required for the most efficient exchange of gases? In this activity you investigate how the number and the size of the air sacs can make a difference.


  • 1 volleyball
  • Ruler or tape measure (metric)
  • Calculator
  • 4-liter freezer bag
  • 16 Tennis balls
  • 60 Golf balls
  • Activity Report


Step 1 First, you're going to calculate the diameter squared of different sized spheres. Begin by measuring the diameter of a volleyball. Next, measure the diameter of a tennis ball. Then measure the diameter of a golf ball. Record this information in Data Table 1 on your Activity Report. Then square the diameter number to give you the diameter squared. Record this information in Data Table 1 on your Activity Report. Note that the information for the alveoli is provided.

This is the equation you will use to calculate the diameter squared:

\begin{align*}\text{diameter} \times \text{diameter} = \text{diameter}^2\end{align*}

We'll do one for you as an example. The diameter of a volleyball is 19cm. To calculate its diameter squared, you use this equation:

\begin{align*}19 \ cm \times 19 \ cm = 361 \ cm^2\end{align*}

So the diameter squared of a volleyball is \begin{align*}361 \ cm^2\end{align*}.

Step 2 Now you are going to calculate the surface area for each type of ball. To calculate the surface area you will multiply the diameter squared of each ball by 3.1 \begin{align*}(\pi)\end{align*}. \begin{align*}\pi\end{align*} is the Greek letter pi. It represents the number you get if you divide the circumference of any circle by its diameter. Since this number is always the same, it is called a constant. The actual value of \begin{align*}\pi\end{align*} is 3.1416. However for the purpose of the calculations in this activity we are using 3.1.) The equation would look like this.

\begin{align*}D^2 \times \pi = SA\end{align*}


\begin{align*}\text{diameter}^2 \times 3.1 = \text{surface area}\end{align*}

We calculated that the volleyball has a diameter squared of \begin{align*}361 \ cm^2\end{align*}. Now let's calculate the surface area of the same volleyball.

\begin{align*}361 \ cm^2 \times 3.1 = 1119.1 \ cm^2 \ \text{or} \ 1119 \ \text{rounded out}\end{align*}

This is the surface area of the volleyball.

Now calculate the diameter squared, then the surface areas of the tennis ball and the golf ball. Record this information in Data Table 1 on your Activity Report. Then write the surface area of each golf ball in the appropriate column in Data Table 2 of your Activity Report.

Step 3 Your lungs can hold as much air as the 4-liter freezer bag can hold. How many volleyballs can be placed in the 4-liter bag? You do not need to squeeze the volleyball into a freezer bag to prove that only one volleyball will fit. Record your results in Data Table 2. Repeat this step for both tennis and golf balls.

Step 4 Calculate the total surface area of the lung volume by multiplying the surface area of one ball by the number of balls in the lung volume. You will use this equation to calculate the total surface area of the number of balls that fit in the 4-liter bag.

\begin{align*}\text{surface area of ball} \times \text{number of balls} = \text{total surface area}\end{align*}

Now calculate the total surface area for one volleyball, since only one will fit into the bag.

\begin{align*}1119 \ cm^2 \times 1 \ \text{Volleyball} = 1119 \ cm^2\end{align*}

Calculate the total surface area for the number of tennis balls that will actually fit in the 4-liter bag. Then calculate the surface area for the number of golf balls that will fit into the 4-liter bag. Record this information in Data Table 2.

Step 5 Look at the total surface area of the alveoli in the lungs in Data Table 2. \begin{align*}60 \ m^2\end{align*} is a little more than \begin{align*}7.5 \ m\end{align*} by \begin{align*}7.5 \ m\end{align*}. Use chalk or string to mark off an area \begin{align*}7.5 \ m\end{align*} by \begin{align*}7.5 \ m\end{align*} outside or on the classroom floor. This area represents the surface area of the lungs. Imagine all the alveoli are flattened out in this area.

Step 6 Answer the questions on the Activity Report.

Words from the Latin Language Research the word origins of the following words.

alveoli, glottis, and trachea

How Much Air Do You Breathe?

Activity 2-2 gave you an idea about how much air will fill your lungs. Let's see how much air your lungs really can hold. But before you measure the air that you move in and out of your lungs, consider that there is always some air in your lungs and airways. So if you breathe in a huge breath and then breathe out as much air as you could, would your lungs be completely empty? All of the airways starting with the ones with stiff rings of cartilage cannot be collapsed completely. Even the alveoli don't collapse completely when you breathe out. Now that you know that, here's the same question again. If you breathe in a huge breath and let it out, would your lungs be completely empty?

After you breathe out, there is always air left in your lungs and airways. There is a reason this last bit of air stays behind between breaths. Your breathing tubes, the trachea, bronchi, and bronchioles do not collapse when you breathe out. Your alveoli do not collapse entirely when you breathe out, either. Therefore, there is always some air in your lungs. The air left in your lungs between breaths does have some oxygen in it. However, that amount of oxygen wouldn't last long if you didn't breathe in fresh air again soon. Let's find out how much air you do breathe in and out.

Activity 2-3: Building and Using a Spirometer


How much air do you breathe out each time you exhale? How much difference in volume is there between a normal breath and a deep breath? In this activity you build a spirometer to answer these questions. You use the spirometer to measure exhaled air after breathing normally and breathing deeply. Then you estimate how much gas you exhale in a normal breath and in a deep breath.


  • A large plastic container, basin, or washtub 1/2 to 2/3 full of water
  • Two plastic gallon jugs
  • A funnel
  • Rubber tubing
  • 100 ml (milliliter) beaker
  • Waterproof marker pens
  • Calculator
  • Plastic straw
  • Activity Report

Part A: Procedure Build Your Spirometer

Step 1 Follow these steps to mark the volume on the jug.

  • Fill the jug with water 500 ml (milliliters) at a time
  • Mark the water level on the side of the jug for each 500 ml of water added.
  • Number the 500 ml marks starting with the mark at the bottom of the jug.

Figure 2.6 Mark each 500 ml on a jug.

Step 2 Place your straw into the end of the tubing. The straw will serve as a mouthpiece.

Step 3 Fill the basin or washtub half full of water. Fill the gallon jug so it is full of water. Invert the gallon jug in the basin while covering the opening so the water stays in the jug. If the jug doesn't stay up by itself, ask a partner to steady it for you. Then continue the procedure.

Step 4 Insert the end of the tubing that is opposite the straw into the opening of the gallon jug, and far enough in so the opening of the tube is close to the bottom of the jug, which is now on top.

Part B: Measuring Your lung Volume Measure your normal breaths.

Step 5 Take a normal breath. Breathe out through the straw into the hose as you would normally exhale. Don't force more air out than you inhaled.

Step 6 Record in Table 1 the volume of the air bubble at the top of the jug.

Step 7 Repeat and measure the volume 2 more times. Complete column 1 of the data Table 1. This is your tidal volume.

Figure 2.7 This drawing shows how a student uses a spirometer.

Step 8 DISCARD YOUR STRAW for sanitary reasons, so no one else uses it. Take a new straw for measuring deep breaths. Measure your rate of air exhaled for a normal breath.

Step 9 Calculate the average volume of air exhaled per minute. Record the rate in column 3 of Data Table 1. This calculation will give you an idea of how much air you normally breathe per minute.

Measure your deep breaths.

Step 10 Fill the jug full of water. Cover the mouth of the jug and invert in the basin. Take a deep breath. Force as much air out through the straw into the hose as possible. You may have to use two gallon jugs. When one is almost full of air, stop exhaling long enough to switch the breathing tube to a second inverted jug full of water.

Step 11 Record in Table 2 the volume of the air in the 1 or 2 jugs you used.

Step 12 Repeat and measure the volume two more times. This is your vital capacity. How does your vital capacity compare with the volume from a normal breath?

Step 13 DISCARD YOUR STRAW for sanitary reasons, so no one else uses it.

Measure your rate of breathing.

Step 14 With the help of your lab partner, determine how many times at rest you breathe in a minute. Record the rate in column 2 of Data Table 1. Repeat two more times. Record the rate in column 2 of Data Table 1. After averaging these results, you will have an idea of how much air you breathe in one minute.

Step 15 Measure your rate of air exhaled for a deep breath. Calculate the average volume of air exhaled per minute. Record the rate in column 3 of Data Table 2. This will give you an idea of how much air you breathe when taking a deep breath.

Figure 2.8 This graph shows lung volumes and capacities.

People who were born and live their whole lives in the high Andes Mountains of South America are described as being “barrel-chested.” Why do you think this is so? What is the effect of altitude on the oxygen you breathe?

If you take a very deep breath and then exhale as much as you possibly can, the volume may be 6 to 8 times your tidal volume. This biggest possible breath is your vital capacity.

Use your spirometer to see how much of the difference between your vital capacity and your tidal volume is due to your ability to breathe more air in (your inspiratory reserve volume) or to breathe more air out (your expiratory reserve volume.)

Don't forget about the air in your lungs that you can never breathe out. That is your dead space. Your dead space may be as much as one liter. Therefore, your total lung volume is your vital capacity plus your dead space.

The air you breathe is 20% oxygen. Do you think the concentration of oxygen is the same in your alveoli when you breathe in? Explain.

Summary of the Parts of Your Breathing Machine

Your lungs are just one part of your breathing system. You have two lungs containing a total of 300 million alveoli (air sacs). If all 300 million air sacs in your lungs were squished out flat they would occupy a space about 7.5 m by 7.5 m. That's large enough to carpet half a tennis court. Calculate how many square meters there are in an area 7.5 m by 7.5 m.

Pretend you are a small quantity of air. Describe your journey through your breathing system. Begin your journey entering the nose and mouth and stopping when you leave the breathing system to enter the blood capillaries. Your response can be in the form of a short story, cartoon, or poem.

The huge number of alveoli helps to increase the surface area that is available for gas exchange. But remember that there is more to your breathing system than only your lungs. In addition to your lungs, your breathing system includes your nose, larynx, trachea, and all the branches of your airways. And don't forget your diaphragm, and the muscles between your ribs that power breathing.

Review Questions

  1. Explain how each major part of your respiratory system from your nose to your alveoli helps you to breathe.
  2. How are your air sacs designed to maximize the amount of oxygen you can get from the air you breathe?
  3. What happens in your lungs when you hold your breath?

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