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.
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!