5 ms·

Humans detect drowning by elevated CO2 in the blood. This means people walking into downed weather balloons have no warning that they're running low on oxygen b

Humans detect drowning by elevated CO2 in the blood. This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying. IIRC most aquatic mammals detect low oxygen directly instead, allowing them to dive for over an hour without panicking. CO2 builds up in the blood, but they can feel their oxygen levels directly.

vacri 9y ago
Humans can detect both low CO2 and low O2, it's just that the CO2 'drive' is both stronger and happens faster.
suneilp 9y ago
Since oxygen plays a rather vital role in our functioning, I'm curious why we can't detect low oxygen levels. Or perhaps we do detect them but only when it's dangerously low. How has it been confirmed that we detect CO2 levels instead of low oxygen?
- deleted 9y ago
  [deleted]
- hwillis 9y ago
  Directly: https://en.m.wikipedia.org/wiki/Inert_gas_asphyxiation https://en.m.wikipedia.org/wiki/Inert_gas_asphyxiation You are totally unaware of your brain shutting down as the oxygen level decreases. People lose the ability to do arithmetic or simple puzzles including blocks in holes, but by the time that happens youre too far gone to recognize what that means.
- autokad 9y ago
  perhaps its because we and our ancestors do not have the environmental stress to have that as one of our optimizations
- sn9 9y ago
  As CO2 concentrations rise, it raises levels of carbonic acid in the blood (something CO2 does in aqueous environments; same thing that's bleaching corals in the sea). Our body tracks blood pH and when it dips too low from high CO2, that's when the body knows it needs to breathe. (The above is just going off my memory of my physiology class from years ago. Apologies if I misremembered something.)
- aquaticor 9y ago
  Just a guess, but CO2 levels in our body are much lower which would mean small changes are easier to detect. Also just a guess, when we were all just fish making a first few steps into land, atmosphere O2 concentration was lower, CO2 higher, so maybe we evolved to detect what was more abundant then.
kqr 9y ago
> This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying. It also means that by hyperventilating, you can reducessubsequent feelings of asphyxiation, because hyperventilation (rapid, shallow breaths) purges CO2 from the blood. However, it is somewhat inefficient at building up oxygen (compared to slow, deep breaths) so it's easy to accidentally asphyxiate yourself that way.
aquaticor 9y ago
A few observations from my freediving days (I've trained competitively for a few years). The urge to breathe you get when you're holding your breath long enough is from elevated CO2 in your blood and the subsequent change in PH (as a comment here already stated). That urge is actually your diaphragm contracting involuntarily, but you can learn to endure that feeling which is the key for apnea attempts. But besides that, there are no almost no other indications before you pass out. Black-outs are quite common in freediving competitions because of that fact (freedivers just ignore the breathing reflex and nothing else tells them when to stop). O2 saturation may fall 20-40% during your breath hold. You generally won't feel anything until it's too late. The problem is that you won't feel the difference between 99% and 65% saturation and you may black-out at 64% (those numbers are not the same for everyone). The signs you can detect before blacking out are blurred vision, tunnel vision, fingers tingling, metallic taste in your mouth... But, you have only a very small window of time between those symptoms starting and blacking out (literally a second or two if you're lucky). Interestingly, the window of time depends on the discipline - the more physically engaged you are the window is bigger. The static apnea (just floating face down in water) gives you almost no warning at all while at longer dynamic apnea attempts you have a few seconds when you can "detect" something is funky with your body.

vacri 9y ago

Humans can detect both low CO2 and low O2, it's just that the CO2 'drive' is both stronger and happens faster.

suneilp 9y ago

Since oxygen plays a rather vital role in our functioning, I'm curious why we can't detect low oxygen levels. Or perhaps we do detect them but only when it's dangerously low. How has it been confirmed that we detect CO2 levels instead of low oxygen?

deleted 9y ago
[deleted]
hwillis 9y ago
Directly: https://en.m.wikipedia.org/wiki/Inert_gas_asphyxiation https://en.m.wikipedia.org/wiki/Inert_gas_asphyxiation You are totally unaware of your brain shutting down as the oxygen level decreases. People lose the ability to do arithmetic or simple puzzles including blocks in holes, but by the time that happens youre too far gone to recognize what that means.
autokad 9y ago
perhaps its because we and our ancestors do not have the environmental stress to have that as one of our optimizations
sn9 9y ago
As CO2 concentrations rise, it raises levels of carbonic acid in the blood (something CO2 does in aqueous environments; same thing that's bleaching corals in the sea). Our body tracks blood pH and when it dips too low from high CO2, that's when the body knows it needs to breathe. (The above is just going off my memory of my physiology class from years ago. Apologies if I misremembered something.)
aquaticor 9y ago
Just a guess, but CO2 levels in our body are much lower which would mean small changes are easier to detect. Also just a guess, when we were all just fish making a first few steps into land, atmosphere O2 concentration was lower, CO2 higher, so maybe we evolved to detect what was more abundant then.

kqr 9y ago

> This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying. It also means that by hyperventilating, you can reducessubsequent feelings of asphyxiation, because hyperventilation (rapid, shallow breaths) purges CO2 from the blood. However, it is somewhat inefficient at building up oxygen (compared to slow, deep breaths) so it's easy to accidentally asphyxiate yourself that way.

aquaticor 9y ago

A few observations from my freediving days (I've trained competitively for a few years). The urge to breathe you get when you're holding your breath long enough is from elevated CO2 in your blood and the subsequent change in PH (as a comment here already stated). That urge is actually your diaphragm contracting involuntarily, but you can learn to endure that feeling which is the key for apnea attempts. But besides that, there are no almost no other indications before you pass out. Black-outs are quite common in freediving competitions because of that fact (freedivers just ignore the breathing reflex and nothing else tells them when to stop). O2 saturation may fall 20-40% during your breath hold. You generally won't feel anything until it's too late. The problem is that you won't feel the difference between 99% and 65% saturation and you may black-out at 64% (those numbers are not the same for everyone). The signs you can detect before blacking out are blurred vision, tunnel vision, fingers tingling, metallic taste in your mouth... But, you have only a very small window of time between those symptoms starting and blacking out (literally a second or two if you're lucky). Interestingly, the window of time depends on the discipline - the more physically engaged you are the window is bigger. The static apnea (just floating face down in water) gives you almost no warning at all while at longer dynamic apnea attempts you have a few seconds when you can "detect" something is funky with your body.