Part of the show Naked Science Q&A and Polonium Poisoning
Bob in California asked:
What happens physiologically when the body is cut off from oxygen?
The body has an incredibly high metabolic rate, so you're burning off oxygen really, really fast. The brain has the highest metabolic rate and in fact the retina is the part of the brain with the highest metabolic rate. So that's why when you stand up out of a hot bath very quickly you get a very brief dip in blood pressure, because all the blood is in your legs. The result of that is the retina gets slightly less blood for a fraction of a second and that's why you see those funny lights and you feel a bit woozy. But then as your blood pressure comes back up again the brain gets a good supply back. That's evidence of how oxygen dependent your brain is. If you cut off the supply for a fraction of a second, the reduced blood flow just makes you lose consciousness. Tissues like the brain are incredibly sensitive to just a minor interruption in their oxygen supply. And that's why you faint. When I was at medical school we did an interesting experiment where they simulated going to the top of a mountain. So I said "oh I'll be the subject". I was laying on a table and breathing out of a bag which was 10% of oxygen instead of 20% you normally find in a room. They were trying to see if my blood would get slightly less oxygen dissolved in the blood stream. But it didn't actually work very well. I breathed this bag for about 20 minutes and it didn't actually reduce my oxygen capacity terribly much. If you do go mountaineering what you do to compensate is you just breathe more. So you pack more breaths in and your haemoglobin's very good at grabbing oxygen out of the lung and putting it in the blood stream. A red blood cell takes about 0.8 seconds to go through the tiny blood vessel through each of the air sacks in the lungs. But it only takes about 0.3 seconds for the oxygen to get into it. So there's a safety factor of at least threefold. It's got three times longer to pick up oxygen that it needs, so if you breath a bit harder and shove more oxygen into the lung, you can pick it up. This is also why you've got to be careful breathing helium balloons, because there's no oxygen in that helium. I did it a while ago and it made my head go incredibly dizzy.