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Breathing Through the Lungs and the Intestine in Mammals

Linda Crampton is an experienced teacher with a first-class honors degree in biology. She writes about the scientific basis of disease.

Anatomy of the large intestine (sometimes known as the colon)

Anatomy of the large intestine (sometimes known as the colon)

Exciting and Hopeful Discoveries

Problems with our lungs can have major consequences. If their action is severely impaired, the use of a ventilator machine may be required in order to keep us alive. When an outbreak of a disease causing breathing problems is present in a community, there may not be enough ventilators available for everyone who needs them. Recent research may lead to a solution to this problem.

Researchers have discovered that mice and pigs can absorb oxygen through the lining of their intestine when the substance is administered through the anus and rectum. Other researchers have found that the nasointestinal administration of oxygen can be helpful for humans.

The intestine is part of the digestive system and lacks the high surface area of the tiny air sacs (or alveoli) in the lungs. Nevertheless, in the future, doctors might be able to supply enough oxygen via the intestine for a person’s needs. It may be possible to keep the person alive and avoid permanent health problems caused by poor oxygenation of tissues. The experiments suggest that these goals might be achieved, but more research is necessary.

Terminology related to the digestive tract varies. The word “gut” may mean just the stomach or the entire digestive tract. The term “colon” may mean the same thing as large intestine. On the other hand, the combination of the longer colon and the much shorter rectum and anal canal may be called the large intestine.

Breathing and Respiration

A healthy adult at rest generally takes between twelve and sixteen breaths per minute. This means that a person may take around 20,000 breaths in twenty-four hours. Exercise, illness, and age can affect the breathing rate. For everyone, obtaining oxygen and getting rid of carbon dioxide is an essential activity that must be done in a repeating cycle throughout life.

The airways and lungs are said to form the respiratory system. The word “respiration” is sometimes used as a synonym for breathing. Traditionally, however, respiration is the name of a process that involves four steps, as shown below.

  1. breathing
  2. external respiration (gas exchange between the alveoli and the capillaries)
  3. internal respiration (gas exchange between the capillaries and the tissues)
  4. cellular respiration (a multistep reaction inside cells in which oxygen is used to obtain energy from a food molecule such as glucose)

The last step can be considered the ultimate goal of respiration. The energy that’s obtained from food is stored in the bonds of a simpler molecule called ATP (adenosine triphosphate) and then used as required. Carbon dioxide is a waste product created by the cellular respiration process and is removed from the body during exhalation.

Side view of structures involved in breathing

Side view of structures involved in breathing

Airway Facts

The following structures are classified as airways. They form a continuous connection, as can be seen by combining the illustration above and the one below. The inhaled air travels from the larynx shown in the first picture into the trachea shown in the second. During inhalation, air travels along the airways and into the alveoli of the lungs.

The epiglottis shown in the illustration above is a flap of tissue that covers the glottis every time we swallow and momentarily prevents us from inhaling or exhaling. The glottis is an opening between the vocal cords in the larynx. It allows air to enter the larynx and the rest of the respiratory system. The cavities and tubes through which inhaled air travels when the route is open are listed below.

  • nose
  • mouth
  • pharynx
  • larynx or voice box
  • trachea or windpipe
  • two large bronchi, one going to each lung
  • narrower bronchi that branch from the two wide ones
  • bronchioles (even narrower tubes that go to the alveoli located in the lungs)

The muscle below the lungs in the illustration above is the diaphragm. It’s the most important muscle involved in inhalation.

Facts About the Alveoli

The alveoli are tiny sacs whose walls are very thin. The sacs are bordered by capillaries, which are the smallest blood vessels. Inhaled oxygen passes through the wall of the alveoli and into the capillaries. Carbon dioxide waste moves in the opposite direction. Each alveolus is lined with surfactant to prevent it from collapsing.

Determining the number of alveoli in a lung is challenging. An article published in the American Journal of Respiratory and Critical Care Medicine reported that based on the examination of six adult lungs, the mean alveolar number is 480 million.

The huge number of alveoli in each lung provides a large surface area for gas exchange between the outer and inner environment. This is an important point to be considered in any attempt to provide intestinal absorption of oxygen.

An Overview of Inhalation and Exhalation

The trigger for inhalation (or inspiration) and the control of breathing rate is involuntary, so it works whether we are awake or asleep. We don’t need to think about it, though we can control the breathing rate voluntarily to some extent.

The Process of Inhalation

The stimulus for inhalation comes from the medulla oblongata, which is located at the base of the brain. The region receives relevant signals about the status of the body from other structures. The increased concentration of carbon dioxide in the blood is a major trigger for the activity of the medulla oblongata.

The lungs expand due to the action of the muscles around them. The diaphragm under the lungs moves downwards as it contracts, increasing the amount of space for the lungs. Intercostal muscles between the ribs are also triggered to contract, though this has a smaller effect on the volume of the chest or thoracic cavity.

The lungs themselves don’t contain muscles, but they are connected to the muscles of the chest by a thin layer of pleural fluid on their surface. When the chest cavity expands, so do the lungs. This decreases the pressure of the residual air left inside them. Air pressure is now higher in the external environment, causing air to flow into the lungs.

The Process of Exhalation

Exhalation or expiration is often a passive process. The diaphragm relaxes and moves upwards, and the intercostal muscles also relax. These actions push air out of the lungs and cause the lungs to deflate.

We can voluntarily control the depth and frequency of inhalation or exhalation, at least until this creates problems for our body. Fortunately, the body can take charge of the process without conscious control.

The information about ventilators given below is intended for general interest. A doctor should be consulted if someone has questions about the use of the device or about its application and drawbacks in specific situations.

Use of a Ventilator Machine

A ventilator may be used when a person is experiencing respiratory failure. This term doesn’t mean that the person has stopped breathing. Instead, it means that the person can’t obtain enough oxygen for their needs or can’t expel a sufficient quantity of carbon dioxide to prevent harm. A doctor will know whether a ventilator might be helpful for a particular patient.

The ventilator pushes air containing oxygen into the lungs and, in some cases, pulls the air containing carbon dioxide out of the lungs. Some ventilators do their job while a patient is wearing a mask, but in others, a tube is inserted into the patient’s respiratory tract in order to transport the gases.

The video above discusses people with serious breathing problems who must be hospitalized while they are using a ventilator. Hospital ventilators are generally expensive and, therefore, limited in number. Today some people use a ventilator at home. These people may have spinal cord injuries, damaged lungs due to heavy smoking, or lung disease. The ventilator may be used all of the time or only at certain times.

If a ventilator is needed and is used in a hospital or at home, it can save a life. If a ventilator is needed and can’t be obtained, quality of life may decrease dramatically. In the worst-case scenario, it may not be possible to survive. Learning how to provide sufficient oxygen to people through the intestine without a ventilator could be very useful and might save lives.

Obtaining Oxygen Through the Intestine

The intestine is tubular and has a lower interior surface area than a lung. Supplying sufficient oxygen via the organ, therefore, requires some thought and planning. Scientists are exploring ways to make the process helpful.

Researchers know that some animals naturally absorb oxygen through their rear ends. The animals include sea cucumbers, at least some turtle species, some species of catfish, a freshwater fish called a loach (Misgurnus anguillicaudatus), and a spider relative named Tetragnatha praedonia.

Though the recent research about intestinal oxygen absorption in humans has been reported as a novel discovery by several sources, Russian scientists published the results of related research in 2015. They performed a clinical trial involving twenty-one patients with acute respiratory distress syndrome. The oxygen was sent into the intestine of the patients through a nasointestinal tube instead of the anus and was found to be helpful. A nasointestinal tube is inserted through the nose and into the digestive tract. As the second illustration in this article shows, there’s a route to the esophagus at the back of the nasal cavity.

One of the scientists involved in the latest research hopes that clinical trials of rectal oxygen administration will be performed soon. It will be interesting to see which form of intestinal oxygen administration is the most successful.

The digestive, gastrointestinal, or GI tract is a continuous passageway from the mouth to the anus. The esophagus, stomach, small intestine, and large intestine are connected, though sometimes the route of the passageway and the folds and loops of the intestine make this hard to see.

Description of the Mammal Experiments

A team composed of Japanese and American researchers supplied oxygen to the anal canal of the animals used in their experiments. This area has a relatively thin lining compared to the rest of the intestine. In addition, the lining in the area contains many blood vessels, which could be useful for absorbing oxygen.

Without an intestinal supply of pure oxygen, no mice in a test group survived for 11 minutes in a low-oxygen environment. Oxygen was then sent into the rectum of another group of experimental mice. 75% of these animals survived for fifty minutes in the low-oxygen environment. The intestines of the animals were abraded to allow a sufficient quantity of the inserted oxygen to be absorbed. The researchers say that the abrasion wouldn’t be suitable in a clinical setting involving human patients.

Another experiment was performed on pigs who were exposed to a low-oxygen environment. The level of oxygen was not low enough to be deadly, however. Some of the animals were supplied with a liquid containing an oxygenated perfluorocarbon via their intestine but received no intestinal abrasion. The compound is already used to help people with respiratory problems, though it’s administered to the lungs, not the intestine.

Compared to the animals who didn’t receive extra oxygen, the mice receiving intestinal oxygen were able to walk further, and more oxygen reached their heart. The same benefits were noticed in the pig experiment. In addition, the pigs receiving the perfluorocarbon lost their skin pallor and coldness. There seemed to be no side effects from the treatment. Postmortem tissues from the animals showed no unexpected or serious changes.

Intra-rectal delivery of a liquid form of O2 known as conjugated perfluorocarbon, a compound historically used in clinics for liquid ventilation through airway administration, is highly tolerable and efficacious in ameliorating severe respiratory failure.

— Ryo Okabe at al, "Cell Press"

An Ethical Dilemma

Reading the reports about the recent experiments described in this article is interesting biologically, but it’s also problematic for an animal lover. Using animals in laboratory experiments can be a dilemma, though, for some people who have a strong opinion about the process, there isn’t any dilemma.

The thought of experimenting on captive animals, potentially causing them pain, discomfort, or fear, and deliberately killing them at some point in the experiment is a horrible one for some people. On the other hand, some supporters of animal experiments say that if we didn’t use animals in scientific investigations, we would have to use humans as experimental subjects. This could cause suffering in us and lead to death either as a direct result of the treatment or because the treatment didn’t cure a patient’s health problem.

Though the term “animals“ is often used to represent other beings, biologically, humans are animals, too. We belong to the kingdom Animalia in biological classification and share many features with our nearest mammal relatives. We have the most advanced brain and culture in the animal kingdom, but like us, other mammals are sentient beings. Some people believe that our comparatively advanced state gives us some responsibility for our relatives.

Further Investigation Is Necessary

The discoveries about rectal breathing could be very helpful for certain medical problems, but the technique isn't ready for general use in humans. Hopefully, the technique will soon be as helpful for us as it was for the animals in the experiments. Perhaps scientists will be able to make the process even more effective in humans than it is now. This might be a challenge, but it’s an important one. It could also be important to further explore nasointestinal administration of oxygen.

Obtaining sufficient gas exchange between the body and the external environment is a vital activity. Depending on the mechanism of oxygen administration, it could also increase the quality of life. These would be valuable outcomes of the research.


  • Vital signs with respect to our body from John Hopkins Medicine
  • How the lungs work from the NIH (National Institutes of Health)
  • The number of alveoli in the human lung from the American Journal of Respiratory and Critical Care Medicine
  • Information about ventilators and their use from the NIH
  • Large (and small) intestine facts from the National Cancer Institute training modules
  • Mammals can use their intestines to breathe from the ScienceDaily news service (Note that the title of the press release should really say that at least some mammals can use their intestines to breathe.)
  • Some mammals can breathe through their intestine from Live Science (This article includes comments from one of the researchers.)
  • Mammalian enteral ventilation ameliorates respiratory failure (the scientific report about the discovery) from Cell Press
  • Enteral oxygen administration in human patients from the European Journal of Anesthesiology

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2021 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on June 23, 2021:

Thank you very much for the comment and for sharing your opinion, Flourish. I was actually surprised that there was so little response in the other comments about the section discussing the ethics of the situation. Medical discoveries can be fascinating and can be helpful for us, but the treatment of lab animals is a major problem.

The creation of small organoids from a patient’s stem cells may one day be a replacement for the use of animals when testing treatments, The organoid research and the required technology seem to be progressing well. That doesn’t help the animals that are currently being used in research, though.

FlourishAnyway from USA on June 23, 2021:

I was glad to see the section on ethics of animal testing. As interesting as this advance may be, there comes a point when our vulnerable and fragile bodies simply give out, and we just need to let go without the aid of artificial methods like potentially ventilating through the intestines. Just because we can doesn't mean we should. Death is not the worst thing that can happen to a sentient being. What was done to those mice and pigs was terribly unjust. So much happens in the name of science that is.

This isn't a criticism of you. Your research is impeccable, as is your writing. Please don't take it that way.

Linda Crampton (author) from British Columbia, Canada on June 21, 2021:

Thank you for the comment, Mary. I hope your friends have recovered well. I’m looking forward to progress in the research, too. Testing procedures on animals can certainly be a problem, though.

Mary Norton from Ontario, Canada on June 21, 2021:

I look forward to progress in this, especially because of my friends who were intubated when they got Covid. We have always wrestled with the ethical dilemma of animal testing. Yet, somehow we can find ways to further knowledge without bringing damage to other animals.

Linda Crampton (author) from British Columbia, Canada on June 17, 2021:

Hi, Adrienne. The discoveries could be very useful, but as you say, minimizing or eliminating suffering in lab animals is important.

Adrienne Farricelli on June 17, 2021:

This is very interesting and sounds like a great option for the future. The ethical dilemmas of experimenting with animals is a touchy subject though. Hopefully, if they must push through, they'll find ways to minimize suffering.

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

I’m so sorry about your dog’s health, Heidi. I wish dogs had a longer lifespan. I’m glad she’s doing better at the moment. I hope the improvement lasts for a long time. Thank you for commenting.

Heidi Thorne from Chicago Area on June 16, 2021:

This is very interesting! Actually, the question of breathing and oxygen intake has been an issue in our house recently with our girl dog who the vets suspect has heart cancer, though the procedures to verify would be highly invasive and stressful for an older dog. They also suspected laryngeal paralysis, a neurological issue, which made her cough and have difficulty breathing. Her heart sac and abdomen were filled with fluid. Once the fluid was drained, she's pretty much back to her usual self, though the long-term prognosis is not good. She's 12. So we understand that her health will likely decline. She's on an herbal supplement which the ER vets recommended to help control the fluid buildup. She's breathing easily now, but also tires easily. Just enjoying any time we have with her.

Always appreciate your articles since they often provide perspective on things I see in my area of the world. Thanks so much!

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

Thank you for such a kind comment, Bill!

Bill Holland from Olympia, WA on June 16, 2021:

It's like going to science class where my favorite teacher is expounding wisdom. Love your articles, Linda! I always come away smarter because of them.

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

I hope the research saves and improves lives, too, Vanita. It might be very helpful for us. Thanks for commenting.

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

Hi, Fran. Science is certainly full of surprises! It’s a fascinating area to explore.

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

Thanks, Devika. I think it’s a fascinating topic. I hope the research leads to new medical treatments for us.

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

I hope it saves lives too, John. I’ll be watching for news about the research with respect to human health with great interest.

Linda Crampton (author) from British Columbia, Canada on June 16, 2021:

Hi, Chitrangada. I hope the research helps people, too. It might be very useful. Thank you for commenting.

Vanita Thakkar on June 16, 2021:

Very interesting, informative and useful article, Linda. Let us hope these discoveries are researched further so that they can save lives and cure ailments. Thanks for sharing.

fran rooks from Toledo, Ohio on June 16, 2021:

Alicia, who knew? Fascinating article. Science is full of surprises and you gave a detailed article about our lungs. Thank you so much.

Devika Primić from Dubrovnik, Croatia on June 16, 2021:

AliciaC An incredible hub! Information that affects lives and well-focused on the topic as well. Unique information and in detail. You have done a great research and provided me with what I had no idea of.

John Hansen from Australia (Gondwana Land) on June 16, 2021:

Very interesting research. I hope it saves lives. Thanks for sharing about this research, Linda.

Chitrangada Sharan from New Delhi, India on June 16, 2021:

An interesting and informative article, and as always I have learnt something new from your well written and researched article.

Hopefully, the scientific research will help people further.

Thank you for sharing this wonderful information.

Linda Crampton (author) from British Columbia, Canada on June 15, 2021:

Thank you very much, Rozlin.

Rozlin from UAE on June 15, 2021:

This is a well researched, informative and helpful hub, Linda. Thanks for sharing.

Linda Crampton (author) from British Columbia, Canada on June 15, 2021:

Hi, Peggy. I hope the discoveries do save people’s lives. Thanks for the visit.

Peggy Woods from Houston, Texas on June 15, 2021:

This is fascinating information! Medical science is ever-evolving. Hopefully, these discoveries will end up saving more people's lives.

Linda Crampton (author) from British Columbia, Canada on June 15, 2021:

Hi, Bill. I hope to hear more about the topic in the future, too. It will be interesting to discover the types of disorders that the treatment might help.

Linda Crampton (author) from British Columbia, Canada on June 15, 2021:

Thank you for the visit and the kind comment, Misbah. I’m hoping that new advances in science, such as the creation and use of organoids, will eventually end the use of animals in experiments.

Blessings to you as well.

Bill De Giulio from Massachusetts on June 15, 2021:

This is incredible, Linda. I have never heard of any animal being able to breathe through the intestines. If this is possible for humans I imagine the implications will be huge. Many thanks for the education, I hope to hear more about this in the future.

Misbah Sheikh from — This Existence Is Only an Illusion on June 15, 2021:

Linda, it was a great read. Well-written and well-researched article. I never knew that some animals can absorb oxygen through the lining of their intestine. It makes me sad that animals are used in laboratories for studies, and they die. Of course, I know it is important, but I feel bad for them. I'm hoping that it will be useful to humans as well in the future.

Thank you for sharing your knowledge.

Love and Blessings

Linda Crampton (author) from British Columbia, Canada on June 15, 2021:

Thank you, Penny. I appreciate your comment very much.

Penny Leigh Sebring from Fort Collins on June 15, 2021:

Fantastic information. Thank you for putting this together!

Linda Crampton (author) from British Columbia, Canada on June 15, 2021:

Thank you very much, Pamela. I always appreciate your comments.

Pamela Oglesby from Sunny Florida on June 15, 2021:

Linda, this is certainly a well-documented, and well-written article. I had no idea that any animal observed oxygen through the intestines. This is so interesting. Thank you so much for this article, and I hope to learn some more about this.