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5.1: Breathing Mission Control

Difficulty Level: At Grade Created by: CK-12
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How is breathing controlled to match the needs of the cells?

You breathe hard and fast when exercising. You breathe more quietly and slowly when you are sleeping. So how does your body know how much to breathe? Breathing is not usually a conscious decision. You can make yourself breathe faster or slower and sometimes hold your breath. But most of the time breathing happens automatically without having to think about it. The interesting question is, “How does your body know when to breathe and how hard to breathe?” How does it control oxygen intake and carbon dioxide release? In this section you will explore your body's control systems-the systems in your body that help regulate and maintain body functions at a normal level.


Your body has to keep track of many things such as temperature, oxygen, energy, water, salt, nutrients, growth, balance, reproduction, and much more. Your body has control systems that maintain the functions in your body. Control systems detect changes from normal. When a change is detected, the controller sends messages to the organs of the body telling them to go into action to return conditions to normal. The controller for breathing is located in the parts of your brain just above the spinal cord. This area of the brain is called the brain stem. The breathing controller is within parts of the brain stem called the pons and the medulla. There are other controllers in the pons and the medulla, and there are many other controllers in the brain!

Homeostasis Look up the word homeostasis. What does it mean? Where does the word come from? How does it relate to feedback?

You will study a temperature controller in Activity 4-1 by controlling the temperature of a water bath. When you finish you will know more about how controllers use feedback information to keep something constant. Feedback systems can be based on positive or negative feedback. Positive feedback allows an action to continue and to get bigger and bigger. Negative feedback causes an action to stop or reverse. Most of the feedback systems in your body are negative feedback systems. For example, if you start to lose your balance and fall to the left, the balance detectors in your ears sense that you are falling to the left. They send this information to the balance controller in the brain. The balance controller sends instructions to your muscles to push you back to the right and restore your balance. Because the information from the sensors-falling to the left-caused an opposite response-move to the right-we say the sensors provided the controller with negative feedback information.

Another controller in the brain works like the thermostat in your home. A thermostat controls temperature. The thermostat in your home responds to changes in room temperature, and activates either the heater or air conditioner to keep the room temperature at a constant level. The thermostat senses the room temperature and compares the room temperature with a set point temperature. If the room is colder than the set point, the thermostat turns on the furnace. If the room is warmer than the set point, the thermostat turns off the furnace.

Your body has a controller that is similar to the thermostat in your home. Your body's thermostat is in a part of the brain called the hypothalamus. Your body temperature stays rather constant in all the seasons, at rest, and during exercise. Like a home thermostat controls the furnace, the body thermostat makes you shiver when you are too cold or sweat when you are too warm. The shivering heats you up and the sweating cools you down. A rise in body temperature causes the body's thermostat to activate responses that reverse the rise in temperature. Therefore, this controller is also using negative feedback information. Similarly, a fall in body temperature causes the controller to issue commands that reverse the fall in temperature.

Your breathing control systems use negative feedback. Can you think of any other negative feedback systems in your body?

Activity 4-1: How Does a Controller Work?


There are many instruments that control variables and keep them constant. A good example is a thermostat that controls a water bath or furnace. In this activity you investigate how a controller works by becoming the thermostat. The purpose is to keep the temperature of the water at \begin{align*}37^\circ\end{align*} Celsius \begin{align*}(98.6 ^\circ \ F)\end{align*}. This is the normal temperature of your body.


  • Water bath; e.g. \begin{align*}1000-ml\end{align*} (milliliter) beaker
  • Thermometer
  • Ice cubes in container
  • Heater (hot plate)
  • Resources 1, 2, and 3
  • Activity Report

BE CAREFUL WITH THE HOT PLATE! Here's an important safety tip before you begin this activity. Make sure you are wearing safety goggles to protect yourself from hot water. Hot plates can be very dangerous. They get hot enough to burn. So be extremely careful.


Step 1 Use Resource 1 as a model. There are four jobs for the members of your group to do. They are the following jobs.

Student 1 watches the thermometer and tells another student when to add ice or heat the water to keep the temperature at \begin{align*}37^\circ\end{align*} Celsius.

Student 2 adds ice water to the bath if the thermometer reads greater than \begin{align*}37^\circ\end{align*} Celsius.

Student 3 turns on the hot plate if the thermometer reads less than \begin{align*}37^\circ \ C.\end{align*}

Student 4 records the temperatures and completes the temperature table.

Step 2 Design your own data table or use the Data Table on Resource 2.

Step 3 Heat the water in the beaker to a temperature of \begin{align*}37^\circ\end{align*} Celsius. When the temperature is exactly \begin{align*}37^\circ \ C\end{align*}, use this as time zero and start recording.

Step 4 Maintain this temperature at \begin{align*}37^\circ \ C\end{align*} for 20 minutes. Adjust the temperature with ice or the heater to maintain at the temperature of \begin{align*}37^\circ \ C\end{align*}. Take the temperature every 2 minutes and record it on the Data Sheet.

Step 5 Use Resource 3 to make a line graph showing the temperature changes during the twenty-minute period.

Controlling Your Breathing

Your respiratory system is responsible for the important exchange of the gases oxygen and carbon dioxide. Your breathing rate tells a lot about how much oxygen and carbon dioxide you have in your blood. The amount of carbon dioxide and oxygen in your blood is called your blood gas levels. The breathing controller in the pons and medulla of your brain receives information about how much oxygen \begin{align*}(O_2)\end{align*} and carbon dioxide \begin{align*}(CO_2)\end{align*} is in your blood. If the amount of \begin{align*}CO_2\end{align*} increases or the amount of \begin{align*}O_2\end{align*} decreases, your breathing controller makes you breathe faster and deeper. These same changes in blood gases cause your heart controller to speed up your heart rate.

The breathing controller works in a similar way as a thermostat works. The breathing controller compares existing levels of oxygen and carbon dioxide to set levels. Sensors pick up changes in levels of \begin{align*}CO_2\end{align*} and \begin{align*}O_2\end{align*} in the blood. The sensors send this information to the breathing controller in the pons and medulla. The breathing controller responds by sending instructions to the breathing muscles to breathe faster or slower and to the heart to beat faster or slower. For example, if there's too much carbon dioxide or too little oxygen, the lungs work harder and the heart pumps more.

Figure 4.1 The medulla and pons control your breathing, and the hypothalamus controls your body temperature. Both systems use negative feedback.

Your Amazing, Adapting Body

Remember that when you exercise, your cells use more \begin{align*}O_2\end{align*} and produce more \begin{align*}CO_2\end{align*}, That means the concentration of these gases in the blood changes. There is less \begin{align*}O_2\end{align*} and more \begin{align*}CO_2\end{align*}, Your breathing controllers sense those changes in the amount or concentration of gases. When the breathing controllers sense the \begin{align*}O_2\end{align*} level in the blood is low and the \begin{align*}CO_2\end{align*} level is high, the controllers speed up your breathing and your heart rate. The increased breathing and blood pumping reverse the blood gas changes caused by exercise. Thus, the controller works to keep blood gases at the same level.

Sometimes the concentration of gases in your blood changes when the concentration of gases in the air you breathe changes. For example, have you ever climbed a high mountain? As you go higher and higher up a mountain there is less \begin{align*}O_2\end{align*} in the air. Since there is less \begin{align*}O_2\end{align*} in the air, the concentration or amount of \begin{align*}O_2\end{align*} in your blood is less, too. When this happens, you can feel “short of breath” and must breathe harder.

Mountain climbers that go to the tops of the highest mountains take oxygen with them in pressurized bottles. Gas is very compressible. This means gas can be forced into smaller containers than can the same amount of solids or liquids. So you can get lots of gas in a small bottle. What about you when you fly over those mountains in a jet plane? The airplane is pressurized to 5,000 feet. That means that being in an airplane is like being on top of a 5,000-foot mountain, no matter how high you fly.

Just as mountain climbers carry oxygen with them to the highest mountains, so do scuba divers carry their air supply in pressurized bottles or air tanks. When divers go very deep, their bodies are under high pressure and they use the pressure in their air tanks to inflate their lungs. Breathing air at high pressure, however, causes gases to dissolve in their blood. Remember that as long as the bottle of champagne was corked the carbon dioxide stays dissolved in the liquid. But when the cork comes out, bubbles of gas come out of solution. If this happens in the blood of a diver who comes up too quickly, it is very serious. The gases dissolved in the diver's blood can come out of the blood as bubbles. This quick release of gas is called the bends and causes a lot of pain. The bends can be treated. A diver with the bends is put in a high pressure tank to get the gases back into solution. Then the pressure is lowered slowly so the gas can diffuse out safely in the lungs.

Figure 4.2 At higher elevations you can quickly use up your blood's oxygen supply. That makes you breathe faster and harder.

Figure 4.3 Climbers on the world's highest peaks take oxygen with them.

Scuba divers carry their air in tanks that they wear on their backs. But some divers depend only on their lungs. Divers in the Far East who dive for pearls use only their lungs for air. They train themselves to be able to take very deep breaths of air that will last for several minutes. They decrease the \begin{align*}O_2\end{align*} they need for swimming by holding onto heavy weights to go down. Now suppose you are in the Far East diving for pearls. You take a big breath and dive. As you swim your body uses up oxygen and makes carbon dioxide all the time you are under water. The carbon dioxide is building up and the oxygen is being used up. Finally, the amounts of both of these gases tell your brain that you need to take a breath. That's when you are forced to come to the surface to breathe.

The breathing controller in your brain is more sensitive to increases in \begin{align*}CO_2\end{align*} in the blood than to decreases in \begin{align*}O_2\end{align*} in the blood. As you just read, sometimes people who want to swim long distances underwater spend a minute or so breathing fast and deep before beginning to hold their breath. This is called hyperventilation. Hyperventilation does not add much \begin{align*}O_2\end{align*} to the blood, but it does eliminate a lot of \begin{align*}CO_2\end{align*}, So when the swimmer starts swimming underwater, he or she doesn't feel the need to breathe and swims farther before surfacing. But this can be very dangerous because the swimmer can use so much blood \begin{align*}O_2\end{align*} that he or she can black out before feeling the urgent need to surface for air. Swimmers drown every year because they do not know how hyperventilation influences their breathing controllers.

Figure 4.4 Swimmers and divers often feel carbon dioxide build up and oxygen deplete if they stay under water too long.

As you can see, your breathing controller can adapt to different circumstances such as high pressure or low oxygen in the air for a short time. But your body also can adapt to changes around it for a longer period of time or even permanently. For example, people manage to live in high places on the top of high mountains. As a matter of fact, in spite of the low oxygen associated with living in high places nearly 15 million people in the world live over 10,000 feet above sea level. Some people live all their lives in the Andes Mountains in South America at 16,000 feet above sea level. How do people do this? Their bodies have adapted in several ways. One way the body adapts to less oxygen at higher altitudes is by increasing the capacity of the lungs. The lungs of people who live at high altitudes can often take in more air than the lungs of people at low altitudes. Another adaptation results in the production of more red blood cells. Red blood cells are the cells that carry \begin{align*}O_2\end{align*} to the cells. Another way the body adapts to altitude is by increasing the capacity of the blood to deliver \begin{align*}O_2\end{align*} to cells.

Did You Know?

One red blood cell contains 280 million molecules of hemoglobin. Each molecule of hemoglobin can carry 4 molecules of oxygen. How much oxygen can one red blood cell carry?

Your Hard-Working Red Blood Cells

Let's find out what's in your blood. Your blood contains water, salts, and blood cells. Most of the cells are red blood cells. Some of the blood cells are white blood cells. The white blood cells do several jobs such as fight infection. But the red blood cells are the cells that carry oxygen. Look at Figure 4.6 to see the shape of a red blood cell. It contains a protein called hemoglobin. Hemoglobin is able to pick up and combine with molecules of \begin{align*}O_2\end{align*} as the blood flows through the lungs. Then the hemoglobin releases that \begin{align*}O_2\end{align*} into the cells as the blood flows through the body and by the cells. You might say that the red blood cells are really containers filled with this wonderful protein. Red blood cells are red because the hemoglobin turns red when it combines with \begin{align*}O_2\end{align*}.

Figure 4.5 This test tube of whole blood demonstrates that your blood is made up of 40% blood cells and 60% plasma.

Hemoglobin absorbs \begin{align*}O_2\end{align*} much like a sponge absorbs water. What happens when you whisk a damp sponge over a wet puddle on the floor? The sponge absorbs the water and the floor is dry. Now think of your lung model again. Imagine the blood filled capillaries coming close to the air filled air sac. Imagine that each red blood cell is like a sponge for \begin{align*}O_2\end{align*}. When the red cell moves by the air sac, the \begin{align*}O_2\end{align*} leaves the air sac and goes into the red cell like water being absorbed by a sponge.

Figure 4.6 \begin{align*}O_2\end{align*} binds to the protein in your red blood cells called hemoglobin.

Did You Know?

Your body makes 15 million new red blood cells every second to replace an equal number that die every second.

Figure 4.7 Hemoglobin in red blood cells soaks up \begin{align*}O_2\end{align*} in your lungs and delivers it to your cells.

Figure 4.8 This drawing shows the path of oxygen and carbon dioxide. Red blood cells unload \begin{align*}CO_2\end{align*} and take up \begin{align*}O_2\end{align*} at the lung. They unload \begin{align*}O_2\end{align*} and take up \begin{align*}CO_2\end{align*} at the cell.

What Do You Think?

What is your opinion about the procedure that some athletes use called “blood doping?” Some athletes have some of their blood withdrawn and put in cold storage weeks before an event. Their bodies make new red blood cells to replace the ones that were withdrawn. The athletes have their stored blood transfused back into their bodies just before the event. This increases the number of red blood cells in their blood and the amount of oxygen they are able to take up from each breath. Why do you think an athlete might do this? Do you think blood doping should be an illegal procedure? Why or why not?

The red cell absorbs as much \begin{align*}O_2\end{align*} as it can the same way a damp sponge soaks up as much water as it can hold. When there is a lot of \begin{align*}O_2\end{align*} available in the air sac at sea level, each red cell takes as much as it can. But the \begin{align*}O_2\end{align*} in air sacs is lower at high levels such as on Mt. Everest. When the \begin{align*}O_2\end{align*} is low in the air sac, each blood cell picks up less than a full load. Therefore, these red cells have less \begin{align*}O_2\end{align*}. So when they get to the body cells, there is less \begin{align*}O_2\end{align*} than is needed being delivered. To fix this problem, temporarily your body compensates by breathing faster and pumping more blood. Eventually your body will make more red blood cells if you stay at that elevation for a while. After a few weeks at high altitude, your body will have more red blood cells moving around in your blood than at sea level.

You know how the hemoglobin absorbs \begin{align*}O_2\end{align*} as the blood passes the air sac in the lung. Remember that the red blood cells loaded with \begin{align*}O_2\end{align*} are pumped back to the body and to the cells. Now let's find out what happens when the oxygen-saturated hemoglobin arrives at your cells. Remember that your cells are constantly using \begin{align*}O_2\end{align*} and making \begin{align*}CO_2\end{align*}. The \begin{align*}CO_2\end{align*} diffuses into your blood and into your red cells. Because there is less \begin{align*}O_2\end{align*} in the cells than there was in the red blood cell, the hemoglobin releases its \begin{align*}O_2\end{align*}. The \begin{align*}O_2\end{align*} diffuses into the cells that need it. When the \begin{align*}O_2\end{align*} has been released and the \begin{align*}CO_2\end{align*} is loaded, the blood takes the \begin{align*}CO_2\end{align*} back to the lungs. At the air sacs, the red blood cells release the \begin{align*}CO_2\end{align*} so you can breathe it out.

What happens when you hold your breath? Take a deep breath. Hold it. Try not to breathe. When you first breathe in, the amount of oxygen in the lungs is high. But as time passes, the \begin{align*}O_2\end{align*} content in your lungs decreases. This happens because the blood continues to take up \begin{align*}O_2\end{align*} from your lungs as it passes the air sac. But, since your mouth is shut and you are holding your breath, no new oxygen gets in to your lungs. As the \begin{align*}O_2\end{align*} is decreasing in your lungs the \begin{align*}CO_2\end{align*} is increasing. When you start holding your breath, the amount of carbon dioxide in your lungs is low. As more blood passes by the air sac, it takes away the \begin{align*}O_2\end{align*} but leaves the \begin{align*}CO_2\end{align*} it brought from the cells. So the amount of \begin{align*}CO_2\end{align*} increases in your lungs. Look at Figure 4.9. The graph shows the results of holding your breath.

Figure 4.9 When you hold your breath, your body uses up the oxygen available in your lungs and begins to build up carbon dioxide levels.

Review Questions

  1. How does the need for oxygen change as you exercise? How does the amount of oxygen in your blood change as you go to a higher altitude?
  2. Describe how your body's controllers work like thermostats by using negative feedback.
  3. How does your breathing controller work?
  4. Describe how red blood cells and diffusion relate to breathing.

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Date Created:
Feb 23, 2012
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