13.3 Breathing

Created by CK-12 Foundation/Adapted by Christine Miller

13.3.1 Butterfly Stroke
Figure 13.3.1 How long can you hold your breath?

Doing the ‘Fly

The swimmer in the Figure 13.3.1 photo is doing the butterfly stroke, a swimming style that requires the swimmer to carefully control his breathing so it is coordinated with his swimming movements. Breathing is the process of moving air into and out of the lungs, which are the organs in which gas exchange takes place between the atmosphere and the body. Breathing is also called , and it is one of two parts of the life-sustaining process of respiration. The other part is gas exchange. Before you can understand how breathing is controlled, you need to know how breathing occurs.

How Breathing Occurs

Breathing is a two-step process that includes drawing air into the lungs, or inhaling, and letting air out of the lungs, or exhaling. Both processes are illustrated in Figure 13.3.2.


13.3.2 Inhalation and Exhalation
Figure 13.3.2 Breathing depends mainly on repeated contractions of the diaphragm.


Inhaling is an active process that results mainly from contraction of a muscle called the diaphragm, shown in Figure 13.3.2. The is a large, dome-shaped muscle below the lungs that separates the (chest) and cavities. When the diaphragm contracts it moves down causing the thoracic cavity to expand, and the contents of the abdomen to be pushed downward. Other muscles — such as intercostal muscles between the ribs — also contribute to the process of , especially when inhalation is forced, as when taking a deep breath. These muscles help increase thoracic volume by expanding the ribs outward. The increase in thoracic volume creates a decrease in thoracic air pressure.  With the chest expanded, there is lower air pressure inside the lungs than outside the body, so outside air flows into the lungs via the respiratory tract according the the pressure gradient (high pressure flows to lower pressure).


Exhaling involves the opposite series of events. The diaphragm relaxes, so it moves upward and decreases the volume of the thorax. Air pressure inside the lungs increases, so it is higher than the air pressure outside the lungs. , unlike inhalation, is typically a passive process that occurs mainly due to the elasticity of the lungs. With the change in air pressure, the lungs contract to their pre-inflated size, forcing out the air they contain in the process. Air flows out of the lungs, similar to the way air rushes out of a balloon when it is released. If exhalation is forced, internal intercostal and abdominal muscles may help move the air out of the lungs.

Control of Breathing

Breathing is one of the few vital bodily functions that can be controlled consciously, as well as unconsciously. Think about using your breath to blow up a balloon. You take a long, deep breath, and then you exhale the air as forcibly as you can into the balloon. Both the inhalation and exhalation are consciously controlled.

Conscious Control of Breathing

You can control your breathing by holding your breath, slowing your breathing, or , which is breathing more quickly and shallowly than necessary. You can also exhale or inhale more forcefully or deeply than usual. Conscious control of breathing is common in many activities besides blowing up balloons, including swimming, speech training, singing, playing many different musical instruments (Figure 13.3.3), and doing yoga, to name just a few.


13.3.3 Conscious Control of Breathing
Figure 13.3.3 Playing the trumpet is hard work. Exhaled air must be forced through the lips hard enough to create a vibrating column of air inside the instrument.

There are limits on the conscious control of breathing. For example, it is not possible for a healthy person to voluntarily stop breathing indefinitely. Before long, there is an irrepressible urge to breathe. If you were able to stop breathing for a long enough time, you would lose consciousness. The same thing would happen if you were to hyperventilate for too long. Once you lose consciousness so you can no longer exert conscious control over your breathing, control of breathing takes over.

Unconscious Control of Breathing

Unconscious breathing is controlled by  in the and of the brainstem (see Figure 13.3.4). The respiratory centers automatically and continuously regulate the rate of breathing based on the body’s needs. These are determined mainly by blood acidity, or . When you exercise, for example, carbon dioxide levels increase in the blood, because of increased cellular respiration by muscle cells. The carbon dioxide reacts with water in the blood to produce carbonic acid, making the blood more acidic, so pH falls. The drop in pH is detected by  in the medulla. Blood levels of oxygen and carbon dioxide, in addition to pH, are also detected by chemoreceptors in major arteries, which send the “data” to the respiratory centers. The latter respond by sending nerve impulses to the , “telling” it to contract more quickly so the rate of breathing speeds up. With faster breathing, more carbon dioxide is released into the air from the blood, and blood pH returns to the normal range.

13.3.4 Nervous Control of Respiration
Figure 13.3.4 Clusters of cells in the pons and medulla of the brain stem are the respiratory centers of the brain that have involuntary control over breathing.

The opposite events occur when the level of carbon dioxide in the blood becomes too low and blood pH rises. This may occur with involuntary hyperventilation, which can happen in panic attacks, episodes of severe pain, asthma attacks, and many other situations. When you hyperventilate, you blow off a lot of carbon dioxide, leading to a drop in blood levels of carbon dioxide. The blood becomes more basic (alkaline), causing its pH to rise.

Nasal vs. Mouth Breathing

Nasal breathing is breathing through the nose rather than the mouth, and it is generally considered to be superior to mouth breathing. The hair-lined nasal passages do a better job of filtering particles out of the air before it moves deeper into the respiratory tract. The nasal passages are also better at warming and moistening the air, so nasal breathing is especially advantageous in the winter when the air is cold and dry. In addition, the smaller diameter of the nasal passages creates greater pressure in the lungs during exhalation. This slows the emptying of the lungs, giving them more time to extract oxygen from the air.

Feature: Myth vs. Reality

Drowning is defined as respiratory impairment from being in or under a liquid. It is further classified according to its outcome into: death, ongoing health problems, or no ongoing health problems (full recovery). Four hundred Canadians die annually from drowning, and drowning is one of the leading causes of death in children under the age of five. There are some potentially dangerous myths about drowning, and knowing what they are might save your life or the life of a loved one, especially a child.

Myth Reality
“People drown when they aspirate water into their lungs.” Generally, in the early stages of drowning, very little water enters the lungs. A small amount of water entering the trachea causes a muscular spasm in the larynx that seals the airway and prevents the passage of water into the lungs. This spasm is likely to last until unconsciousness occurs.
“You can tell when someone is drowning because they will shout for help and wave their arms to attract attention.” The muscular spasm that seals the airway prevents the passage of air, as well as water, so a person who is drowning is unable to shout or call for help. In addition, instinctive reactions that occur in the final minute or so before a drowning person sinks under the water may look similar to calm, safe behavior. The head is likely to be low in the water, tilted back, with the mouth open. The person may have uncontrolled movements of the arms and legs, but they are unlikely to be visible above the water.
“It is too late to save a person who is unconscious in the water.” An unconscious person rescued with an airway still sealed from the muscular spasm of the larynx stands a good chance of full recovery if they start receiving CPR within minutes. Without water in the lungs, CPR is much more effective. Even if cardiac arrest has occurred so the heart is no longer beating, there is still a chance of recovery. The longer the brain goes without oxygen, however, the more likely brain cells are to die. Brain death is likely after about six minutes without oxygen, except in exceptional circumstances, such as young people drowning in very cold water. There are examples of children surviving, apparently without lasting ill effects, for as long as an hour in cold water. Rescuers retrieving a child from cold water should attempt resuscitation even after a protracted period of immersion.
“If someone is drowning, you should start administering CPR immediately, even before you try to get the person out of the water.” Removing a drowning person from the water is the first priority, because CPR is ineffective in the water. The goal should be to bring the person to stable ground as quickly as possible and then to start CPR.
“You are unlikely to drown unless you are in water over your head.” Depending on circumstances, people have drowned in as little as 30 mm (about 1 ½ in.) of water. Inebriated people or those under the influence of drugs, for example, have been known to have drowned in puddles. Hundreds of children have drowned in the water in toilets, bathtubs, basins, showers, pails, and buckets (see Figure 13.3.5).
13.3.5 Supervision of Children Near Water
Figure 13.3.5 Young children should never be left unattended around sources of water that pose a risk of drowning, including water in toilets, bathtubs, and buckets. Here, there are clearly two adults supervising within arm’s reach.

13.3 Summary

  • Breathing, or , is the two-step process of drawing air into the lungs (inhaling) and letting air out of the lungs (exhaling). is an active process that results mainly from contraction of a muscle called the diaphragm. is typically a passive process that occurs mainly due to the elasticity of the lungs when the diaphragm relaxes.
  • Breathing is one of the few vital bodily functions that can be controlled consciously, as well as unconsciously. Conscious control of breathing is common in many activities, including swimming and singing. There are limits on the conscious control of breathing, however. If you try to hold your breath, for example, you will soon have an irrepressible urge to breathe.
  • Unconscious breathing is controlled by respiratory centers in the and of the brainstem. They respond to variations in blood by either increasing or decreasing the rate of breathing as needed to return the pH level to the normal range.
  • Nasal breathing is generally considered to be superior to mouth breathing because it does a better job of filtering, warming, and moistening incoming air. It also results in slower emptying of the lungs, which allows more oxygen to be extracted from the air.
  • Drowning is a major cause of death in Canada, in particular in children under the age of five.  It is important to supervise small children when they are playing in, around, or with water.

13.3 Review Questions

  1. Define breathing.
  2. Give examples of activities in which breathing is consciously controlled.
  3. Explain how unconscious breathing is controlled.
  4. Young children sometimes threaten to hold their breath until they get something they want. Why is this an idle threat?
  5. Why is nasal breathing generally considered superior to mouth breathing?
  6. Give one example of a situation that would cause blood pH to rise excessively. Explain why this occurs.

13.3 Explore More

How breathing works – Nirvair Kaur, TED-Ed, 2012.

How do ventilators work? – Alex Gendler, TED-Ed, 2020.

How I held my breath for 17 minutes | David Blaine, TED, 2010.

The Ultimate Relaxation Technique: How To Practice Diaphragmatic Breathing For Beginners, Kai Simon, 2015.



Figure 13.3.1

US_Marines_butterfly_stroke by Cpl. Jasper Schwartz from U.S. Marine Corps on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 13.3.2

Inhale Exhale/Breathing cycle by Siyavula Education on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 13.3.3

Trumpet/ Frenchmen Street [photo] by Morgan Petroski on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 13.3.4

Respiratory_Centers_of_the_Brain by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 13.3.5

Lily & Ava in the Kiddie Pool by mob mob on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.


Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure 22.20 Respiratory centers of the brain [digital image].  In Anatomy and Physiology (Section 22.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/22-3-the-process-of-breathing

Kai Simon. (2015, January 11). The ultimate relaxation technique: How to practice diaphragmatic breathing for beginners. YouTube. https://www.youtube.com/watch?v=Vca6DyFqt4c&feature=youtu.be

TED. (2010, January 19). How I held my breath for 17 minutes | David Blaine. YouTube. https://www.youtube.com/watch?v=XFnGhrC_3Gs&feature=youtu.be

TED-Ed. (2012, October 4). How breathing works – Nirvair Kaur. YouTube. https://www.youtube.com/watch?v=Kl4cU9sG_08&feature=youtu.be

TED-Ed. (2020, May 21). How do ventilators work? – Alex Gendler. YouTube. https://www.youtube.com/watch?v=yDtKBXOEsoM&feature=youtu.be


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Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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