The details of CO2 transport and CO2 tolerance for freedivers

The details of CO2 transport and CO2 tolerance for freedivers

CO2 tolerance is complicated, hence this post is complicated. Hopefully it is less complicated to read than it was to write. Just in case it gives you a headache, here are the highlights:

  • CO2 is transported in three ways: bound to hemoglobin, as bicarbonate ion, and dissolved in the blood stream.
  • Red blood cells are crucial for generating bicarbonate ion, and for facilitating CO2  bound to hemoglobin.
  • Only dissolved CO2 can diffuse to the central chemoreceptors. 
  • Physical CO2 tolerance may be governed by blood volume and quality, which is commonly overlooked in CO2 tolerance training.
  • Endurance athletes commonly have the highest CO2 tolerance and storage (in comparison to short and mid distance sprinters).
  • Mental CO2 tolerance focuses on desensitization of the central chemoreceptors.

Influencing your breathing rate to an extreme extent means either hyperventilating or holding your breath. If you hyperventilate your skin can start to tingle. If you hold your breath you can get contractions and tunnel vision.

Oh, and did I mention you can pass out from both?

So how do freedivers, who hold their breath as a leisurely past-time activity, deal with the buildup of CO2? What happens in their body that enables them to tolerate more CO2?
The Mitochondria, the energy factories in your cells, produce CO2 as a result of aerobic metabolism. The CO2 diffuses out of the cell and is transported to the lungs where you can breathe it out. There are a few different ways that CO2 is transported, and thus temporarily stored.

The following three paragraphs lean heavily on this article, and references therein.

In the red blood cells: CO2 bound to hemoglobin

CO2 can bind to hemoglobin in the red blood cells. Hemoglobin with bound CO2 is called carbamino-hemoglobin. There is no competition between O2 and CO2 bound to hemoglobin (unlike O2 and CO, which do compete).

Interestingly, CO2 binds easily to hemoglobin that has already released its oxygen. This makes hemoglobin a very effective transporter of CO2 from cells that require O2 and produce CO2. The effect exists the other way around too, once the hemoglobin encounters oxygen in our lung tissue it releases its CO2 and binds to O2 again, repeating the cycle. This effect is called the Haldane effect.

With help from red blood cells: CO2 as bicarbonate

Water can combine with CO2 to form carbonic acid: H2CO3. This H2CO3 easily dissociates into HCO3 (bicarbonate) and H+. In blood plasma this reaction is very slow, but our red blood cells carry an enzyme called carbonic anhydrase that speeds up the reaction. So the CO2 that moves into a red blood cell and is not bound to hemoglobin is converted by carbonic anhydrase into carbonic acid and then bicarbonate. Most of the CO2 in the body occurs as bicarbonate.

The H+ stays within the red blood cell and the HCO3 moves into the blood plasma. HCO3 is slightly basic, and our blood is protected from intense acidity for two reasons, 1: the red blood cell’s buffering capacity for H+ from carbonic acid, and 2: HCO3 acts as a buffer for H+ from other tissues such as the muscles.

CO2 dissolved in blood

As opposed to O2, CO2 dissolves readily in the blood plasma. In the blood plasma, CO2 can be dissolved as a gas and can be transported to the lungs where some of it is expelled. Because CO2 is so easy to dissolve in blood, the concentration of dissolved CO2 actually does not change much when blood is transported through the lung tissue. During normal breathing, only about 15% of dissolved CO2 is removed in the lungs.

CO2 freediving
CO2 is transported in three ways: bound to Hb, as bicarbonate, and dissolved as a gas. The dissolved can cross the blood brain barrier and end up in the cerebrospinal fluid, where the central chemoreceptors may trigger contractions. The initial systemic increase of CO2 may help induce the dive reflex.

Relative abundances

Most of the CO2 in the body is stored as bicarbonate ion. In arterial blood this is up to 90%. However, it is important to realize that this is not all expelled in the lungs. In veinous blood, only 60% of the CO2  is contained in bicarbonate ion. This doesn’t mean that there is less CO2 in veinous blood than in arterial blood. On the contrary! it means that a significant portion of the CO2 that is expelled in the lungs is actually bound in hemoglobin and transported as dissolved CO2.

‘Physical’ tolerance to CO2

CO2 tolerance is not just a mental game. At Freedive Wire we have been digging into the mechanisms of CO2 tolerance and contractions for quite some time.

In an article we published on Freedive Wire in September 2018, Luca speculated that it is the dissolved CO2 that ends up triggering contractions. Long story short, neither HCO3 nor H+ can be transported to the central chemoreceptors, but dissolved CO2 can.

Is the real trick to better CO2 tolerance to limit the amount of dissolved CO2? If so, maybe we should train CO2 tolerance by increasing blood volume and quality. After all, more red blood cells will allow more CO2 to bind to hemoglobin and more blood plasma means a bigger volume to dissolve HCO3 in.

Perhaps it is not a coincidence that endurance athletes, who generally have a higher concentration of red blood cells, also have the highest CO2 storage capabilities. Elite freedivers commonly have a high amount of red blood cells too, as a result of partly desaturating the blood of oxygen on a regular basis.

Unfortunately, divers that only get in the water every now and then won’t be able to reap the benefits of high(er) counts of red blood cells and blood volume without specific training.

Don’t despair though: 35 minutes of exercise (cardio and/or power) 3 times per week already has a positive effect on blood volume and quality. Add 20 minutes of sauna to that after the session and the effect will be even larger.

’Mental’ tolerance to CO2

Another aspect of CO2 tolerance is desensitization. After a few breath holds, your body might have come to expect a high CO2 concentration and the alarm bells won’t jingle quite as fast. If you expose yourself to high CO2 concentrations regularly for a long period, you might be able to postpone those alarm bells on every dive.

On land, freedivers mostly train this type of tolerance is with CO2 tables. Using CO2 or tables or slow breathing you can expose yourself to high CO2 concentrations for as long as you want. You don’t even have to move while training. A study from 2000 further confirmed that slow breathing and yoga independently increased CO2 tolerance during hypoxia and hypercapnia. Other methods that increase our ‘mental’ tolerance to CO2 are Buteyko breathing and the use of training masks.

How do you approach CO2 tolerance? Let us know in the comments!


Jaap is a geologist by trade and a freediver by passion. Jaap wrote the book Longer and Deeper in 2018. His book teaches how to train for freediving and spearfishing on land.

This Post Has 15 Comments

  1. Simo Kurra

    Good stuff! One question: how long does it take for bicarbonate levels to return to normal? I “feel” this buffering effect (or what I’ve taken to be it) after at least 24h, even several days. Concretely, afyer doing statics few days in a row, contractions are clearly “mellower” on the next day.

    Not directly related to this, but a recent discovery for me was how narcotic CO2 can be. The strange feelings at the end of a long static that I used to thing were due to lack of O2, are actually CO2. Doing “no breathup” statics with a gulp of pure O2 allowed me to push a static with 99% SaO2 to over 8 minutes, but I was narcotic to the point of hallucination and actually had trouble snapping out of it enough to start breathing again (an obvious warning to anyone about to try this: it’s no joke, can get very dangerous). I’d assume this effect is further amplified by depth?

    1. Jaap

      Hi Simo, the excess CO2 should be vented off within a few minutes after a good sprint or a regular dive, and that is likely when bicarbonate levels are back to normal. You are noticing some adaptation from doing a long static like that but it might be related to something other than bicarbonate. It might take longer to recover from a maximum static but it shouldn’t be more than a day.

      CO2 in high levels can be dangerous! It can actually induce a full-on blackout. Depth causes CO2 to dissolve in the blood more rapidly, but of course, there is also less blood flow in the extremities where we produce quite a bit. Those two processes may partly counteract each other.

      1. Raquel Valair

        Hi there. I have been doing most of my weight training sessions on breath hold for over a year now. I started with sets of 5, then have since graduated to sets as high as 70 on a single breath. I do this to increase EPO, RBCs, hemoglobin and myoglobin counts. As this would not only help with swimming, but also help with overall health and wellness. Prevention of pathologies beyond average. I ended up losing most, if not all of my body fat. Have visibility and physically very strong muscles. I rarely get out of breath. My heart rate stays low, even during the workout sessions. My blood pressure is usually on the LOW end of normal. I believe breath hold weight training causes much better adaptations to low oxygen environments, especially when done at or above 10,000 feet above sea level. Allot of other folks say not to do it but I think it builds better physical toughness. What are your thoughts? Do other swimmers and divers train this way? I’ve been land locked in Portland since the pandemic though.

        1. Luca Malaguti

          Hey Raquel,

          This is awesome, good on you for applying this to your training. Are you a professional athlete?

          There’s no doubt this type of training is good for “increasing your body’s affinity to lower PP02 levels”. So, you’re body adapts better at altitude both hypoxic and hypercapnic.

          Keep It up.


  2. Roman Leija

    Hey man, thank you for writing the article, I’d appreacite the read and the hard work you put in. But of course I several quesitons:
    You mention the Haladine effect but, which of the three ways that CO2 is transported affects the Bohr effect?
    Does the binding of CO2 to hemoglobin within the red blood cells or CO2 dissolved as a gas change the pH of the blood?

    1. Jaap

      Thanks Roman, glad you liked it. Great questions. It pays to remember that anything that affects pH affects the affinity of hemoglobin for oxygen, not only reactions associated with CO2. Although Bohr initially plotted the dissociation curves for hemoglobin against CO2 concentration, there are other reactions at play that influence the pH of the blood as well.

      In the alveoli the Bohr effect allows HCO3- to combine with the H+ within red blood cells so that we can remove CO2. In the bloodstream, H+ from either from H+ released from working muscle, or from the formation of bicarbonate (without anhydrase, so in the plasma), will facilitate the release of oxygen from hemoglobin.

      If you check the first link in the article you will be able to read it in more detail.

      1. Roman Leija

        Hey Jaap, thanks for the reply. I’ll definitely be checking out that article as the days come by, thanks for referencing it.
        I just have one more question, what types of contractions were you referencing when you were speaking on how the dissolved CO2 signals the chemoreceptors to produce contractions?

        1. Jaap

          Hi Roman, I’m just talking about contractions of the diaphragm in general. In the scientific literature there is no distinction between contractions (CO2 or pressure contraction as divers like to call them). There are simply different triggers for the same contraction.

          1. Roman Leija

            Got it. Thanks again for the reply Jaap! I’ll definitely being checking out your other articles.

  3. Connor Davis

    Great Stuff, thanks!

    It will take me a while(and several re-readings) to digest this, but a couple of observational questions.

    Dehydration drops blood volume, probably quite a bit. Diving tends to dehydrate the body even when it doesn’t feel like it. Does that explain(maybe partly) the tendency for divers to see reduced performance and higher tendency towards blackout when dehydrated, often late in the diving session?

    1. Jaap

      Hi Connor, good question. We definitely might become more prone to contractions with less blood volume (it is the plasma that is reduced during dehydration so less volume to dissolve CO2 in), and that could cause us to burn O2 faster. However for some people the desensitization to CO2 over a session may outpace that effect.

  4. prashant

    fantastic article, like the format and information, would be nice if you could set the clickable links on this page to open up on a separate tab, so that we don’t have to flip back and forth to read the article

    1. Jaap

      Thanks for the note and we will consider that for following articles!

  5. freediver jk

    I have a question 1. what’s reason that “Add 20 minutes of sauna to that after the session and the effect will be even larger”

    1. Jaap

      Hi jk, there was a study (Scoon et al., 2007 Journal of Science and Medicine in Sport) on the effect of sauna use on endurance athletes and it concluded there was a positive effect related to an increase in blood volume/quality.

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