I heard that the body doesn’t detect a lack of oxygen, just an excess of carbon dioxide. Is this true? If someone unwittingly stepped into a room filled with an inert gas and little oxygen, would they suddenly pass out without any warning?
http://www.anxietyselfhelp.com/importan.htm
So it does detect oxygen, and it’s the balance between oxygen and CO2 in the blood that the body also reacts to.
“I heard that the body doesn’t detect a lack of oxygen, just an excess of carbon dioxide. Is this true?”
More or less. That’s why you get a headache when you try to sleep at a high elevation (if you are not acclimatized). Your respiratory drive is inhibited by the low partial pressure of carbon dioxide and you end up not adequately compensating for the low partial pressure of oxygen.
An interesting study of the way this works is in people with central sleep apnea. The muscles controlling breathing fail to operate properly, causing periods of up to mintues where the sleeper fails to breathe. The brain will sense the drop in oxygen levels and increased co2 in the bloodstream and ‘kick-start’ the breathing process again, usually with a cough or a gag.
More information can be found here.
Not a perfect explanation of how the brain detects oxygen/co2, but I would suspect that much of the research in that field could come close to answering your question.
So it does detect oxygen, and it’s the balance between oxygen and CO2 in the blood that the body also reacts to.
It does both, but they act differently. The high CO2 sensor (blood PH detection in the medulla) strongly affects breathing depth and speed. The low O2 sensor makes you go unconscious. The CO2 detection could be viewed as the ‘front line’ sensor because under normal circumstances it’s the first to kick in. You’ll feel respiratory distress (i.e. start gasping and panting) very quickly when there are high CO2 levels. Whereas if there’s low CO2 and low O2, it is true that you’ll probably just faint.
Just to clarify, you are stimulated to breathe by both a low level of oxygen and a high level of carbon dioxide.
If “the body doesn’t detect a lack of oxygen, just an excess of carbon dioxide”, that would mean that if you were deprived of oxygen, but could still exhale your carbon dioxide, you wouldn’t feel short of breath. Not so. In many medical conditions (eg. pneumonia, heart failure …), people feel short short of breath (and thus breathe harder and harder) in exactly that set of circumstances.
Conversely, if your body didn’t detect a rise in carbon dioxide, just a deficiency of oxygen, then if you gave someone 100% oxygen to breathe (and thereby supersaturated their systems with oxygen), they should stop breathing (i.e. their oxygen levels would be so high that they’d have no need to breath). That doesn’t happen to normal people. Why? Even with plenty of oxygen in the blood, a high level of carbon dioxide by itself stimulates breathing. And, the carbon dioxide would indeed begin to rise if your breathing rate began to decline, despite a more than adequate oxygen level.
I believe the reason you get a headache when you sleep at high altitudes is that the low levels of oxygen cause a reflex (and appropriate) dilation of the arteries going to the brain. The subsequent distension of the vessels cause the pain. During sleep, in particular, all of us tend to hypoventilate a bit. This means that during sleep there will be a tendency for oxygen levels to drop further and also for carbon dioxide levels in the blood to rise. And, just as low oxygen causes the brain blood vessels to dilate (and cause a headache) so too does a high carbon dioxide level cause the vessels to dilate (and cause a headache). (As an interesting aside, this means that doctors caring for people with increased pressure in their brain can lower that pressure by lowering the blood carbon dioxide level. The lower CO[sub]2[/sub] level causes constriction of the vessles going to the brain which in turn decreases the pressure there.)
In its pure form, central sleep apnea is due to a central problem with breathing control, i.e. in the brain. On the other hand, obstructive sleep apnea is due to occlusion of the upper airway (i.e. the back of the mouth). In reality, there is often a mixture of both.
I have experienced hypoxia at high altitude in an unpressurized small plane. I had no sensation of difficulty breathing, just a feeling of intoxication, then large pink spots started blooming in front of my eyes. At that point, I noticed the pilot had put on an oxygen mask. We were only at that altitude briefly before descending, so it was not dangerous.
Fear Itself: As the quote below (from the New England Journal of Medicine) states, you are correct - hypoxia does not always lead to breathlessness (dyspnea). Still, it is one of the stimuli for breathing.
KarlGauss
<good explanation snipped>
The point being, I suppose, that the body has evolved its CO2 and O2 sensing to detect and respond to typical situations -exercise, not breathing, sleep - so that perhaps it’s no surprise that the response to non-standard situations, such as rapid transition to low-O2 or high-O2 environment, is a little non-intuitive.
I’d like to point out that the brain is much more sensitive to an increase in CO[sub]2[/sub] than it is to a decrease in O[sub]2[/sub]. This is why, when you wish to hold your breath for a long time (ie while underwater) you can purge your body of much of it’s CO[sub]2[/sub] by taking many short breaths. This is dangerous though, because people have been known to pass out underwater doing this. They become unconscious from lack of oxygen before their body detects an elevation in CO[sub]2[/sub] and makes them come up for air. No cite, just anecdotes from professors.