South Africa lost one of its best underwater hockey players at the weekend when Morkel Swart drowned while practising to hold his breath under water....
He did this while using weights and wearing a mask with a stopwatch.
"Morkel could easily hold his breath for four-and-a-half minutes," Schmidt said....
According to the stopwatch, Swart had been underwater for eight minutes.
from "Top Sportsman Drowns," News24, February 17, 2004
xmatt aboves gives a succint, plausible explanation as to why you cannot sever your soul from your carcass by sitting there holding your breath. Still, whenever discussing stupid human tricks, never say never. Just add water....
Mr. Morkel Swart died an unfortunate death. His name lends itself to all kinds of snorkel silliness, and dying preparing for underwater hockey, well, some stuff you cannot make up. But this is not going to be one of those kind of nodes.
Mr. Swart died doing something dangerous. Hyperventilating in order to prolong your voluntary breathholding spell while held underwater can kill you! Oooooh....Stll, I love doing dangerous stupid things if they give me sheer stupid joy. I am old enough to know better. So are you.
If I could figure out a way to hold a bottle of Calvados in one hand, a good cigar in the other while flying 95 miles an hour on Peekamoose Road on an RS100 with my wife clinging to me, laughing maniacally, I would do so. So this is not going to be one of those serious self-righteous better-not-try-this-at-home nodes either.
No, this is another physiology node. You have been warned.
When I was younger (and every bit as stupid as I am now about these sorts of things), I would spend lazy August afternoons alone in a pool trying to see how long I could hold my breath. I figured that if I breathed heavily for a minute or two before holding my breath, I would be able to store up enough oxygen to break my personal best of 2 minutes and 37 seconds.
I breathed and breathed until my fingers tingled and my vision started to fade. Down I went. 2 minutes and 41seconds later I erupted from the pool gasping for air, proud as could be. So I did break my record; not until years later, however, did I learn why.
The urge to breathe normally comes from a build-up of carbon dioxide, not a lack of oxygen. The partial pressure of carbon dioxide in the arterial blood is about 40 torr; the partial pressure of atmospheric carbon dioxide is close to zero. Hold your breath, the arterial carbon dioxide goes up. Breathe faster, take deeper breaths (or some combination of the two), and you will lower your arterial carbon dioxide.
Just about all of the oxygen carried in your blood is carried by hemoglobin. The oxygen saturation is the percentage of hemoglobin oxygen receptor sites occupied by oxygen over the total number of sites available. A gram of hemoglobin has millions of sites available.
Humans are phenomenally efficient at extracting oxygen from the air. I am just sitting here in front of the monitor, listening to a siren, and without any effort on my part manage to fill my hemoglobin receptors without trying. With one pass through the lungs, virtually all the hemoglobin sites grab oxygen, giving an oxygen saturation of 100%. If you hyperventilate, it's still going to be 100%. If I am exercising, 100%. (And if I am listening to Barry Manilow sing Maureen McGovern's greatest hits? Still 100%.)
When I hyperventilate, however, I can lower my arterial carbon dioxide by quite a bit; when I do this, I lose the drive to breathe. When I lose the drive to breathe, I can hold my breath longer. The longer I hold my breath, the lower my oxygen gets.
If the oxygen saturation gets low enough, I will pass out, and as xmatt notes above, I will start breathing again.
Unless, of course, I am underwater. If I pass out underwater, I will likely drown. And people do. Even very skilled people like Morkel Swart.
I am not going to tell you not to do this. That's none of my business.
If the last thought in you is "Wow, I should never have stored so much oxygen, now I'm drowning," well, that is my business.
What you should be thinking is "Wow, I suppressed the urge to breathe long enough to use up too much oxygen, and now I'm going to drown. Thanks to E2, I am dying foolishly but not ignorantly."
There's a difference. | 
