Air. It’s the greatest invention since, and possibly even surpassing, sliced bread.
We, like practically every other living being on planet Earth, require air to survive. More specifically, we need the oxygen, which is only a relatively small component of air. (The majority of air is made of nitrogen; only about 21% of the air that we pull into our lungs is pure oxygen.)
And our bodies are very good at telling us when we need more air. For easy proof, just try holding your breath for a few minutes. Not even a minute in, and you’re likely to feel some discomfort.
The other day, however, I had to change the batteries in my carbon monoxide detector. This got me thinking:
“If we’re so good at knowing when we’re running out of air… why do we need a carbon monoxide detector? Shouldn’t I wake up gasping if I’m low on oxygen?”
How can our bodies tell, within a few seconds, when we’re low on air — but we can’t tell when we’re dying from carbon monoxide?
Our Bodies Are Backwards When Monitoring Oxygen
As it turns out, the human body can detect oxygen levels in the blood, but most of our breathing decisions are instead based on the level of a different molecule.
Instead of making decisions based directly off the oxygen level, the body focuses on detecting two components of our blood:
- How much carbon dioxide (CO2) is present;
- What the pH level (acidity level) of our blood is.
Remember, carbon dioxide, or CO2, is created as the byproduct of respiration. We pull oxygen into our cells, use it for energy, and carbon dioxide is produced as a waste product that must be discarded. We discard it by pushing it back out into the air through our lungs, exchanging it for fresh oxygen.
When carbon dioxide levels rise too high in our body, this signals that we aren’t pushing it out to exchange for fresh oxygen — and we begin breathing faster.
Our bodies are designed to remain in balance, and are pretty good at adjusting to maintain this balance.
Similarly, carbon dioxide is acidic; the more of it is in our bloodstream, the lower the pH level of our blood. Our pH receptors send an alert signal when the pH of our blood falls too low, also triggering us to breathe faster.
This is a great example of a negative feedback loop, where too much of something starts a chain of reactions that remove the original stimulus. In this case:
- We work hard, using more oxygen.
- The oxygen is converted to carbon dioxide, which enters the blood and makes our blood pH level drop.
- Sensory receptors detect the increased levels of carbon dioxide and the lowered pH, and send signals to our brain to increase our breathing rate.
- We breathe faster and more deeply, expelling more carbon dioxide and taking in more oxygen.
- The lower levels of carbon dioxide make our blood pH rise back up to normal.
Generally, this system works pretty well. You can test it out for yourself — go outside and jog around the block. Even if you try to focus on breathing slowly and steadily, you’ll soon find your breathing rate increasing as your body adjusts to the increased production of carbon dioxide. Our bodies are designed to remain in balance, and are pretty good at adjusting to maintain this balance.
But if this is the case, what’s the deal with carbon monoxide? Why do we need a machine to detect when we’re suffocating on this gas?
Carbon Monoxide: A Wolf In Sheep’s Clothing
Carbon monoxide is made of a carbon atom attached to a single oxygen atom (CO), instead of being attached to two oxygen atoms like its cousin, CO2. This means that carbon monoxide looks very similar to another molecule — oxygen, O2.
Carbon monoxide acts a lot like oxygen, as well — it can be absorbed by our red blood cells, just like oxygen.
But here’s where things start to go wrong. Carbon monoxide can’t be used by our cells to produce energy, like they can use oxygen. Instead, the carbon monoxide remains stuck to our red blood cells; they’re unable to discharge it. The more carbon monoxide we take in, the less oxygen our blood can pick up and carry around — leading to suffocation.
So why doesn’t our body react? Even though we aren’t getting any oxygen, our cells aren’t producing any carbon dioxide — so the pH of our blood remains steady and no alarms go off. And as we run out of oxygen, we experience symptoms like confusion, weakness, and loss of consciousness — all of which further incapacitate us and prevent us from reacting.
Carbon monoxide looks like oxygen, so it doesn’t set off alarms — and it interrupts the entire respiration cycle, so we can’t rely on increasing levels of waste products (CO2) to make us breathe faster.
But What About Oxygen Sensing?
Earlier, I did mention that our bodies are capable of detecting oxygen levels — but they don’t use it as the primary driver of how rapidly we breathe.
Instead, when our cells detect low levels of oxygen, it triggers a physiological response. Cells that sense low levels of oxygen kick off a signal cascade that encourages the production of extra red blood cells, so that we’ll have more blood cells available to take in oxygen from the lungs and carry it to the tissues where it’s needed.
This is why many pro athletes do their training at higher altitudes; the reduced oxygen levels trigger an increased production of red blood cells — which can help them deliver more oxygen to muscle in competition to provide an advantage.
There’s one other place where cells sense oxygen, which is of special importance to doctors and researchers: tumors. As a tumor grows, it needs to get oxygen delivered to its insides — and it sends out signals to promote growth of new blood vessels, to deliver oxygen-rich blood to fuel its continued growth. Some anti-cancer drugs focus on this signal, preventing new blood vessels from growing in order to “starve” the tumor.
Our bodies are capable of sensing oxygen — but even though this is the molecule we need in the air, our breathing rate is determined instead by the level of carbon dioxide we generate.
Carbon monoxide takes the place of oxygen; it doesn’t lead to excess carbon dioxide production, so we don’t feel like we’re being choked, even as we’re suffocating. This is why it’s vitally important to have a carbon monoxide detector, and to be aware of the causes and symptoms behind carbon monoxide poisoning. Until our body evolves new detection mechanisms, a carbon monoxide detector is the best way to compensate for this vulnerability in our method of detecting suffocation!
Sam Westreich holds his PhD in genetics, focusing on methods for studying the gut-associated microbiome. He currently works at a bioinformatics-focused startup in Silicon Valley. Follow on Medium, or on Twitter at @swestreich.
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