Carbonic Acid as a Mechanism for Modulating Breathe
The reason why we breath, or respire, is because our bodies need oxygen. More importantly than this, however, is our need to exhale carbon dioxide, and in this article I share examples that help to reinforce this intuition.
Why We Breathe
We need oxygen so that our mitochondria can create ATP, the energy currency our body uses at a cellular level to survive. We also need to expel carbon dioxide so that our blood’s pH levels don’t become too low; carbon dioxide is the anhydride that dissociates into carbonic acid, which increases the acidity and lowers the pH of blood. If our systemic pH is out of equilibrium, then the biochemical reactions that support homeostasis are thrown out of whack.
Interestingly enough, our bodies have evolved to use carbon dioxide levels, rather than oxygen levels, as the main mechanism and impulse for breathing. Our bodies still respond to low oxygen levels, albeit at more extreme and therefore peripheral thresholds.
Low Oxygen, High Carbon Dioxide
When you hold your breath underwater, your body experiences a build-up of carbon dioxide. Even if you keep blowing underwater bubbles, eventually you have no more air in your lungs to use as a medium to exhale your CO2. But how do we know that the discomfort of holding your breath — and the relief when you gulp in air — is primarily the result of an instinct to exhale carbon dioxide, and not to inhale oxygen?
We look to carbon monoxide poisoning.
Low Oxygen, Regular Carbon Dioxide
Carbon monoxide poisoning is the result of how carbon monoxide has a higher binding affinity than oxygen to the hemoglobin in your red blood cells. When oxygen can’t bond to your blood cells because the hemoglobin is already “occupied,” that oxygen can’t be carried to the rest of your body.
You’re still taking breathes at a normal rate and exhaling carbon dioxide at a normal rate, but the oxygen you’re breathing in isn’t reaching your body.
There’s an infamous Reddit post where the original poster (OP) thought his landlord was stalking him for weeks, but in his follow-up he reveals that he was a victim of carbon monoxide poisoning.
Despite the OP’s low blood and brain oxygen levels, his impulse wasn’t to seek more oxygen because they were at sufficient levels for survival, and because his carbon dioxide levels were fine.
High Oxygen, Regular Carbon Dioxide
Oxygen toxicity, or hyperoxia, is when the body has too much oxygen and tissues and organs get oxidized. This can happen without your body noticing, until you start experiencing symptoms such as seizures or coughing.
The rate of exhaling carbon dioxide is normal, so the body doesn’t have an instinct to breath more or do anything to mitigate the hyperoxia.
Regular/Low Oxygen, High Carbon Dioxide
The only example of high levels of carbon dioxide I could find was hypercapnia, which typically happens with hypoxia (low oxygen levels). That means there’s an overall breathing problem that is affecting both oxygen and carbon dioxide levels. Hypercapnia typically tends to resolve itself because “the body can often balance carbon dioxide levels in the bloodstream and correct the symptoms by itself.” (source)
Conclusion
I hope these examples help “intuit” using carbon dioxide over oxygen as the main mechanism for regulating breathing, despite how counterintuitive it may seem!
The takeaway is that as long as you can keep exhaling, you can prolong your survival instinct to breathe.
Great Explanations from Other Places
