Updated: Apr 26

By: Carl Sadek
Posted on: February 4, 2022
Cover Image Artwork by: Clarissa Day
Classification: Photography


Breathing. A process that governs our ability to live. It is a process that we all think we know completely. We breathe in to oxygenate our organs, exhale to release carbon dioxide, and repeat this cycle in order to survive. In reality, there is much more to breathing than that… These three steps are merely a broad outline of what really happens in the process of breathing, which characterizes basically all living things.

What controls breathing? What triggers an increase in respiration rate? Is CO2 just a waste gas?

These questions can be answered by taking a look at the process of breath-holding, which can help simplify the mechanics behind regular breathing.

Whether you are a professional diver, a Navy SEAL, or just a regular teenager betting with friends, you may have realized that breath-holding is extensively practiced in our daily lives. It is commonly thought that whenever we hold our breath, the drive to regain our breath is a result of oxygen deprivation. Well, that’s not really the case.

Oxygen vs Carbon dioxide:

Indeed, various studies have shown that it is the buildup of CO2 in the blood (rather than the deprivation of oxygen) that primarily governs our urge to breathe. Thus, although it may feel that you are running out of air when underwater, it is in fact the accumulation of CO2 that gives you that sensation of breathlessness.

But why? How can too much air in our body provoke us to breathe? Let’s briefly dive into the science.


Chemoreceptors are sensory receptors that control the process of breathing.They do this by detecting changes in the partial pressure of CO2 (pCO2) and O2 (pO2) in the body, and responding to changes in blood pH. These receptors are classified as central chemoreceptors (located in the brain stem) and peripheral chemoreceptors (located in the carotid body). It is important to note that while peripheral chemoreceptors respond to changes in both pCO2 and pO2, central chemoreceptors only respond to changes in pCO2. Moreover, the strictly CO2-sensitive central chemoreceptors are the main respiratory triggers, which explains the importance of CO2.

What Really Happens in the Brain Stem:

At this point, we’ve established that CO2 is the main respiratory trigger. Let’s take a look at how this gas does its job:

When blood reaches the brain, CO2 in the blood diffuses across the blood-brain barrier, forming H+ ions in the cerebrospinal fluid. These hydrogen ions trigger a decrease in blood pH, in turn stimulating the central chemoreceptors and provoking us to take a breath. This is often the case when breath-holding.

So, you may be wondering: Can I just exhale as much as I can (hyperventilate) to get rid of the CO2 before going underwater? Will I be able to hold my breath like a professional diver?

Do NOT Hyperventilate:

This is where breath-holding really gets dangerous. While searching for ways to extend breath-holding time, you might have come across hyperventilation—the process of ventilating excessively before holding your breath. This technique often works because CO2 levels in the blood decrease significantly, meaning that the main respiratory trigger is temporarily shut off (your brain is no longer signaled to breathe because CO2 levels are relatively low).

However, hyperventilating is fatal, as your brain can resist the urge to breathe even as it starves from oxygen. (Remember! Although it is primarily CO2 that triggers the urge to breathe, the main purpose of breathing is to supply oxygen-rich blood to our vital organs). This may lead to a blackout, whereby the breathing reflex unconsciously kicks in once again when underwater, and water is inhaled into the lungs. If the person is not removed from the water immediately, permanent brain damage may be inflicted, leading to death.

All in all, do not hyperventilate when holding your breath underwater.

Don’t Get Fooled, Oxygen is Still Important:

After all this talk about CO2, it is important not to overlook the role (although minimal in comparison to that of carbon dioxide) that oxygen plays in triggering our urge to breathe. If you remember, peripheral chemoreceptors are responsive to changes in both pCO2 and pO2, meaning that the latter undoubtedly has a role in provoking a desire to breathe. But where do we see this role?

When oxygen levels in the lung alveoli and the vital organs decrease below a certain threshold (this threshold varies between people), the effect of pO2 overrides that of pCO2, leading to an urge to breathe even if CO2 levels are low.

For instance, when you exhale CO2 amidst the process of holding your breath, the urge to breathe is only temporarily delayed, and eventually kicks back in. This is a prime example of oxygen overriding carbon dioxide as a breathing trigger.

The role of oxygen is also displayed after hyperventilating. Sometimes, despite disposing of CO2 before breath-holding, the urge to breathe still kicks in due to the decrease in oxygen levels. This is sort of a warning signal from the brain, reminding you to breathe before it starves from oxygen. However, this is not always the case in many people, which emphasizes the danger associated with hyperventilating.


Just like basically every rule in science, the idea that CO2 is the primary driver for breathing has its exceptions:

  • Patients with Chronic Obstructive Pulmonary Disease (COPD): In people with respiratory problems, the role of oxygen as a trigger for breathing overrides that of CO2. This is because the brain’s main focus is to maintain oxygen saturation levels above a certain threshold. Hence, whenever oxygen saturation is within a certain range (typically between 88-92%), peripheral chemoreceptors stimulate a breathing reflex. Understanding this is extremely important when treating COPD patients with a ventilator; setting the oxygen saturation to 100% would switch off the patient’s breathing triggers, leading to hypoventilation (shallow breathing), which can be fatal. (Since oxygen saturation is above the threshold, the peripheral chemoreceptors won’t signal the brain to breathe.)

  • People living in high altitudes: Due to the considerably lower pO2 in the air at higher altitudes, people adapt to become more sensitive to decreases in blood oxygen saturation- this is called altitude acclimatization, and generally takes 1-3 days to occur in the body. This is a very useful adaptation that makes sure oxygen levels remain within a healthy range, since people at high altitudes have a limited exposure to oxygen in the atmosphere.

All in all, it is important to understand the science behind breathing and how it applies to our daily lives. It is truly fascinating how intricately our body is designed, and how it can adapt to new circumstances in order to make sure we are breathing as efficiently as possible. Moreover, it is essential to recognize the significant role played by CO2 in the breathing process; the “waste gas” that you learn all about in science class is not so much of a waste anymore, huh? Finally, it is more important to make use of this knowledge whenever we engage in a breath-holding activity—specifically when underwater. The last thing we want is to black out in a pool after a friend tells us to hyperventilate before a breath-holding competition!


Bernardi, L., et al. “Hypoxic Ventilatory Response in Successful Extreme Altitude Climbers.” European Respiratory Society, European Respiratory Society, 1 Jan. 2006,,reach%20extreme%20altitude%20without%20oxygen.&text=A%20less%20sensitive%20hypoxic%20response,extreme%20hypoxia%20at%20the%20summit.

Kiran Rajneesh. “What Happens When You Hold Your Breath?” Ohio State Medical Center, 15 Sept. 2020,

Knott, Dr Laurence. “Use of Oxygen Therapy in COPD. Advantages and Information.”, 28 July 2020,,2%20without%20worsening%20the%20acidosis.

Patel, Shivani. “Physiology, Carbon Dioxide Retention.” StatPearls [Internet]., U.S. National Library of Medicine, 4 Jan. 2022,

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