So, while most people wander around breathing air without every really even noticing it (entitled bastards), once you get into free diving, seemingly stupid questions all the sudden pop into your head. Why do I breathe? What makes me feel like I need to? Why can’t I only breathe like once every few minutes? When your hobby entails not breathing for extended periods of time, these questions go from the ramblings of the stoned or stupid to important questions that need answers. While I don’t have all the answers, I’ll try to do my best to shed some light on these topics.

First and foremost is the thing that virtually every moderately experienced free diver has learned, but most beginners and lay-people are ignorant of: You actually have no real way to directly sense how much oxygen you have in your blood. If the percent oxygen saturation (commonly SpO2) is between 95 and 100, you’re a perfectly normal, healthy, breathing human being. Congratulations. If it falls to 80 or so, you’re either not super healthy, or you’re not breathing for a while, but you won’t actually feel any difference at all. If it falls below about 65, your lips might start changing color, but aside from that, you’re probably still the same, charming individual your mother loves so much (although she might wish you helped out around the house a bit more, you’re 25 and jobless, her patience can only last so long, you know), but you might feel a bit of tingling in your extremities. Nothing major or unpleasant, but it’s there. Now, if you fall down to about 50%, you’re probably getting a little stupid, but don’t worry, you won’t care, because you feel amazing. Everything is groovy. You probably don’t remember what you were doing before hand, and if you still remember air, you remember it like pokemon. Sure, you might have had fun with it back in the day, but you’ve moved on. It’s just not important to you anymore. You’re also tingling like a motherfucker right now, and it feels like you’re swimming in 7-Up. The bubbles are awesome. If you drop below 40, it’s nap time.

Now, you may be saying, “This guy’s an idiot. That’s not how I feel at all when I’m holding my breath. I feel like shit after a minute and a half.” And that’s true, because what I described above only deals with ONE vital gas. There’s one more gas that’s pretty damned important, and it’s not flatus. Carbon Dioxide. That’s right, that same gas that’s slowly turning the planet into a Hot Pocket is making you feel like you’re about to die just because you haven’t opened your cake hole for a while. This is because our Number 1 mechanism for deciding when we need to breathe is CO2 concentration in the blood. This might seem a little weird, because if you remember your 8th grade biology class, you know that you need the little Oxygen guy to break down the big, mean Glucose, so you can make the sad ADPs into happy ATPs, then your cells poop out CO2 into your blood stream. Why measure the waste when you could measure the IMPORTANT things. I might not have the perfect answer to this question, but I do know that it all boils down to evolutionary biology and the fact that about a billion years ago, your great^90871899678923 grandfather, who was a fish-like dude, got all the ladies because he could monitor his CO2. Now, we all use that same mechanism old fish-gramps happened to luck into, simply because we have it, and it seems to work pretty well.

So, since your cells are constantly pooping CO2 into your blood stream, it accumulates whenever you stop breathing. What are the symptoms you ask? Or maybe you don’t, I don’t care, I’m telling you anyway. The very first symptom of high CO2 concentration in your blood is a subtle suggestion that you should probably breathe a bit more than you are right now. As the concentration builds a bit higher, that suggestion turns into an urge. For most of the general populace, this will come between 45 seconds and about 1:15. This is the first suggestion that you just might die any time now if you don’t breathe. It’s a lie. Next is an increase in core temperature, which usually accompanies vasodilation. You very well may start sweating when you hit this point. There’s a reason people like to do statics with a fan on or in the pool. Next comes the thing that most beginners don’t even know exists, but most intermediates hate: contractions. This is quite simply your body telling your brain that it’s an idiot and trying to breathe without permission. Now, this will go on for a really long time. Ridiculously high concentrations of CO2 can be tolerated by the human body before any real problems occur. The next real level that you can get to with hypercarbia is either acidosis, which is when the levels of CO2 get so high that it actually lowers the pH of your blood enough to irritate your blood vessels, or clear to the point of passing out, which only occurs when CO2 is actually displacing oxygen in the air you’re breathing.

Now, there are two situations that aren’t covered in the above, Hyperoxia and Hypocarbia, or too much oxygen and too little CO2. Hyperoxia is pretty simple. It basically doesn’t happen, and when it does, unless it’s happening for days, it’s not much of an issue. You can breathe pure O2, and as long as you’re at 1 atmosphere pressure, your lungs might get a little irritated, but for the most part, you’ll just end up with about the same O2 levels that you normally have and go on about your day. After a few days of it, or in higher pressures, it can be dangerous, even fatal. Hypocarbia, on the other hand, is very easy to attain via hyperventilation, and can therefore be very dangerous. Hypocarbia, also called hypocapnia, by definition is simply abnormally low levels of carbon dioxide in the blood. Now, you may initially think that since we naturally breathe to take in oxygen and get rid of carbon dioxide, how can it be bad to have LOW levels of CO2? Well, remember, you use CO2 to tell you when to breathe, but that’s not all it does. CO2 is also an acid, which means when the levels are too low, your blood may become alkaline, in the opposite problem of acidosis, called alkalosis, which can cause involuntary muscle spasms and twitching, plus a whole bunch of even less pleasant stuff. However, that’s only on the extreme. There are a few more important issues that arise much more often in free divers. Hypocapnia causes general vasoconstriction, not like the mammalian dive response which only causes peripheral vasoconstriction. This means that when you have low CO2, even if you have really high O2 levels, the blood vessels in your brain can potentially constrict enough that you will suffer symptoms of hypoxia and may even black out. This is why people who have panic attacks can pass out, simply by breathing too fast. Another side effect of low CO2 levels is anxiety, which is again, why panic attacks tend to build, and why telling someone who’s panicking to breathe is silly and wrong. This is also why it’s very dangerous to hyperventilate before a dive, but not the only reason. If you drop your CO2 levels especially low, it delays your urge to breathe. If you drop it low enough, then your urge to breathe can be delayed so long that your O2 level drops below the conscious threshold and you can pass out, without ever feeling any discomfort at all. And that, folks, is what kills you while you’re diving if there’s no one there to recover you. You might think you’re diving safe, because you’re never pushing until you feel much of a need to breathe, but if you’re not doing a safe breath-up, you may be right at the edge of what your O2 levels can support and never know you’ve got a problem.

There is one more minor player that tells you that you need to breathe in or out, and that’s stretch receptors in your chest. These are very simple. Breathe all the way in. Now breathe in a bit more. Come on, don’t slack off, you can hold more air than that. Ok, feeling pretty full about now? You’d like to let out a little air, right? Well, there’s a stretch receptor response. Now for the other end. Exhale EVERYTHING. Like you mean it. Now hold it. Probably feeling a pretty strong urge to breathe. You’ll probably notice that if you actually do a static breathhold, you’ll feel miserable a lot sooner, way more than the simple math of having less air in your lungs would predict. This is a stretch receptor response. These become very important if you’re diving to 100+ feet, because a full lung of air up here where we breathe in is probably just about as small of volume as you can purposefully make your lungs once you take it past 100 feet. It’s a little uncomfortable for most.

So what can we pull from all this? Well, we can pull all the guidelines that are taught in freediving classes! This knowledge allows us to push our bodies farther while still leaving a margin of safety. Knowing the difference between a hypoxic and hypercapnic symptom can be the difference between making it to the surface with plenty of time to spare, and not making it there at all.

And now you’re breathing consciously. You’re welcome for that.

Dive safe.