Two weeks ago I described the brain as a hungry organ, consuming 25% of your oxygen. Beyond this, the brain is a fragile organ, suffering irreversible damage after 3 to 4 minutes of anoxia. With so much at stake, there ought to be a physiological mechanism, as powerful as pain itself, to regulate breathing and keep the brain properly oxygenated. Indeed there is. How long can you hold your breath? A minute perhaps, if you are sitting quietly in a chair, and you are an average adult. But as you pass the 45 second mark, desperation grows, until you finally gasp for air in a near panic. Evolution will not allow you to deprive your brain of oxygen for very long, any more than you can leave your hand in a fire. Sadly, this mechanism has been exploited by torturers around the world and throughout time, from the ducking stool of the 17th century, to waterboarding in the United States, as promulgated by Dick Cheney, a man who's highest office should have been dog catcher.
As so often happens, technology has created some new situations that evolution could not possibly anticipate. There is an environment where the brain can run out of oxygen without realizing it. There is no panic, and no desperate gasps for air; you simply lose consciousness and die. I alluded to this in the previous article. In World War I, a fighter pilot could rise from sea level to 18,000 feet in a matter of minutes. At that altitude, the air, and thus the oxygen in the air, is 50% thinner than the air on the ground. Casual breathing will not bring enough oxygen into the blood stream to support normal metabolic activities. As hypoxia sets in, the pilot should be gasping for breath, but he's not. Five minutes later he loses consciousness, and his plane, with engines still running at low RPM, slowly falls to earth. The rudder is probably a bit off of true, a little to the left or the right, hence the plane gently turns as it descends, tracing a spiral in the sky. (Technically it's a helix, though it is commonly referred to as a spiral.) This is where we get phrases like "spiraling down", and "spiraling out of control", and "death spiral". At lower altitudes the pilot might revive and regain control of his aircraft, or he might not. Anticipating this disaster, pilots of the day were given the following advice.
"Check your fingernails from time to time. If they are blue, you aren't getting enough oxygen. Take long deep breaths and descend to a lower altitude. Your life may depend on it."
Why does the ventilation control mechanism fail in this case? Why is the pilot unaware of his hypoxia? To answer this question we need to understand the mechanism at a biochemical level.
Since the dawn of lungs, a deficiency of oxygen was always accompanied by an excess of carbon dioxide. If gas exchange is not sufficient to meet metabolic needs, because respiration is suspended or because the animal is exercising vigorously, then carbon dioxide builds up in the blood as oxygen is consumed. Even if you are breathing freely in a confined space, ambient CO2 rises as O2 depletes. Thus evolution has a choice - measure oxygen or measure carbon dioxide, and as it turns out, the latter is easier. CO2 joins water in the blood to form carbonic acid, H2CO3. This lowers the pH of the blood. Remember that blood pH is held in the narrow range of 7.35 to 7.45. As blood becomes more acidic, the brain cries out for air. Resume breathing (if you were holding your breath), or breathe faster and deeper, and do so now! Get that blood pH back where it belongs.
At high altitudes, carbon dioxide diffuses freely from the lungs and disperses into the air, but oxygen does not saturate the blood as it would on the ground. Without a build-up of CO2, blood does not turn acidic, and alarm bells do not ring. You breathe normally, and think you are getting enough oxygen, but you're not. To be fair, other symptoms may arise, such as dizziness, muddled thinking, or fuzzy vision, but you might not notice these until it is too late. In summary, oxygen regulation does not work in this new, technological environment.
Fast forward to the 21st century, and most modern cockpits are closed and pressurized, so that hypoxia is not a concern. However, there are still some private planes that are not pressurized, and these pilots are encouraged to carry and use oxygen during any flight above 12,500 feet. Watch your altimeter (more accurate than fingernails), and flip on the oxygen when needed.
back on the ground, there is yet another way to circumvent the breathing reflex, though it is not recommended. Consider free diving, using only the air in your lungs (no scuba gear etc). After a minute or so under water, alarm bells ring, and you must return to the surface for air. You can extend the length of your dive by hyperventilating first, but this can be a deadly mistake. Deep and rapid breathing does not increase oxygen levels in the blood, but it does clear the blood of carbon dioxide. The dive begins with a CO2 deficit, whence the blood is abnormally alkaline. As you swim about under water, O2 falls, and CO2 rises, but by the time rising CO2 pulls blood pH back down through the normal range, and into an acidic region where alarm bells might ring, oxygen is already too low to support higher brain functions. Unaware of the danger, you might swim about in a relaxed state, easily holding your breath, until you lose consciousness just a few feet under water. This is called a shallow water blackout. If a companion does not recognize the condition and pull you to the surface, you could drown. If you practice free diving, or underwater swimming, don't hyperventilate before a dive, and never dive alone.