Lack of oxygen during a free ascent?

[copter53]

My question is "why" is it possible for you to go up from say 100' without breathing in? Doesn't the oxygen in your lungs get used up by your body and then you need to breath in after a certain time to get more oxygen?

Or is it that as the air expands your body is able to absorb more oxygen from the air that is already in your lungs?

 

[dumpsterDiver]

Take a big hreath at 100 feet. hold it in and swim gently in a horizontal position. See how long/far you can do that. You will have MORE oxygen in your lungs when doing this compared to when you are swimming or floating up.

I think many people put way too much reliance on the idea that they can just swim up. How far can you walk after fully exhalling..... In a true emergency you may be leaving the bottom with empty lungs not full ones..

 

[TSandM]

Well, part of the answer is that the major drive to breathe is carbon dioxide buildup, and not low oxygen. Low O2 levels WILL provide what we call "hypoxic drive", but it's a fairly weak response compared with the intensity of the need to keep your CO2 normal. Given the amount of oxygen that is in compressed gas breathed at depth, and the rate at which we normally metabolize oxygen, someone not exerting himself very much actually has quite a bit of time before the oxygen level will drop enough to make him short of breath.

You DO continue to absorb oxygen from the air that is in your lungs, but remember -- at depth, the partial pressure of oxygen is quite high, and as you ascend and the gas expands, you have to vent some of it, leaving fewer oxygen molecules in your lungs to be absorbed. This is the source of the phenomenon of "shallow water blackout", where freedivers, who spend long periods underwater on a single breath-hold, drop their oxygen levels on ascent to where they can't stay conscious.

So the bottom line is that it IS possible to ascent on a single breath from 100 feet, but there are significant hazards to doing so, including lung barotrauma, hypoxia and drowning. MUCH better not to need to do it in the first place!

 

[knotical]

At depths, the partial pressure of oxygen (and nitrogen) in your lungs is greater, so there are more molecules of oxygen available to take into your bloodstream.
At the surface, we only use a portion (perhaps one-fourth) of the oxygen in our lungs on each breath
As long as your blood is flowing and sufficient oxygen is available in your lungs, your body will continue to take in oxygen.

edit: TSandM's more authoritative answer arrived before mine by some fraction of a minute.

 

[Peter Guy]

Copter -- what everyone else said -- but perhaps a little different.

At 100 Feet, you are at 4 ATAs (remember -- 33 feet per ATA + 1 [surface]) -- so the air (gas) you breathe in at 100 feet is 4 times denser than what you breathe at the surface. Another way of saying that is that you have 4 times the number of O2 (oxygen) molecules in your lungs than you would at the surface (lung volume won't change that much, it is the density of the air IN the lungs that changes with depth/pressure). So every breath you take at 100 feet, you "waste" Three out of Four O2 molecules since the blood only "wants" the number of O2 molecules it would have at the surface. (Note, our blood is actually pretty inefficient at taking in O2 -- at the surface, we inhale air that is about 21% O2 and we exhale air which is about 17% O2 so even at the surface we are "wasting" most of the O2 we inhale.)

BUT, as TSANDM wrote, what really makes us want to breathe is NOT the lack of O2, but the CO2 that we create and the blood wants to off load into our lungs. IF we don't exhale the CO2, we will have a significant desire to do so.

Well, the physics of going from 100 Feet to the surface allow us to exhale as we ascend which allows the blood to off load the CO2. At the same time, since we started with 4 times the normal number of O2 molecules, the blood stream is content to just keep getting the O2 it wants/needs. As long as one exhales the "excess volume" and CO2 as you ascend -- "excess volume" is the amount of air in the lungs that expands as you ascend since your lungs are only so big -- you'll still have plenty of O2 molecules to keep the Hemoglobin in the blood saturated and there won't be any desire, or need, to breathe since the CO2 is being exhaled on the way up.

Disclaimer -- I have no special training in human physiology or physics nor did I stay in a Holiday Inn Express last night so any errors in this statement are to be corrected by those much more knowledgable than I.

 

[Akimbo]

This is a fascinating subject and is a shame it is not often taught in Scuba 101 anymore (geezer curmudgeon alert). The human body essentially exhibits one symptom of low oxygen in breathing gas, loss of consciousness. Carbon dioxide is our primary respiratory stimulant.

When a person holding their breath feels they are completely out of oxygen, the reality is they still have plenty -- or more accurately, how would they know? CO2 is what tells the brain "Breathe NOW or you're going to die!!!!!". CO2 concentration building in your lungs on a free ascent is reduced because a significant volume is being exhaled -- like 50% from 33', 75% from 66', etc. Therefore, the CO2 concentration is lower than if you were holding your breath for the same period at the same workload.

Assuming the diver is on air, there is 21% O2. The body cares about partial pressure of oxygen, not percentage by volume. As a result, we have 0.42 ATA (Atmospheres Absolute) of oxygen at 33' instead of 0.21 ATA on the surface. Most people will begin to black out below 14% or 0.14 ATA oxygen. However, most people start to experience CO2 symptoms in the 1-3% range. Near enough, O2 is converted to CO2 at the same rate. Your are fine with 18% O2 (21%-3% that has been converted), but you are in distress when CO2 is at 3% (3% of O2 converted to CO2). You can see how most Scuba divers black out from high CO2, not low O2.

Unfortunately for divers, that strong incentive to breath coupled with several other symptoms of high CO2 conspire to induce panic. Panic not only convinces us to do dirt-stupid things when time is of the essence, it accelerates CO2 production! The proverbial downward spiral.

That is why excessive hyperventilation causes people to stop breathing and black out, at least until CO2 builds back up to stimulate breathing again. Hyperventilation is when the lungs exchange air at a rate that reduces the CO2 below a minimum threshold. In simple terms (because I am a simple guy), one part of the brain says "Low CO2, I don't need to breath for a while" while another part say "HIGH ALERT! I've stopped breathing. I better shut everything down to conserve oxygen". Apparently our evolutionary ancestors didn't dive much.

One of the first things they teach in oxygen rebreather training is to purge the air out of the system. A rebreather will remove the CO2 as your breathe, but the volume reduction in the breathing bags is too small to notice. If you didn't purge, pretty soon there is nothing left except a too little oxygen and lots of Nitrogen (and the small amounts of other stuff in air and respiration). Lights out, you have left the gene pool; but there are no other symptoms.

A similar situation happens all too often on saturation diving systems. A breathing mix is about 1% oxygen and 99% helium at 1000' (around 1½% at 600') in order to maintain a partial pressure of oxygen of about 0.3 atmospheres. Once in a while the deep mix is not fully purged from the lines and somebody will take a breath at the surface on a pre-dive to test a BIBs mask (emergency breathing mask) or the diver's hat. Their lights go out immediately. The next thing they know they are flat on the deck plates wondering how they got there and where that goose-egg came from. NEVER strap a BIBs mask on when testing or you may not get to the waking up part.

It can also happen if you go into a chamber, usually the bell, without checking oxygen levels. It may still be full of deep mix or may have a slow leak in a deep mix or pure helium system that displaced the oxygen. The same kinds of things happen industrially all the time, except a lot more people die from it.

Edit: I was working on this reply and had not seen Peter Guy's reply before posting. Sorry for any redundancy.

 

[Akimbo]

Take a big hreath at 100 feet. hold it in and swim gently in a horizontal position. See how long/far you can do that. You will have MORE oxygen in your lungs when doing this compared to when you are swimming or floating up. ...

Lack of oxygen is not the issue so your test is not a valid analogy. It is about carbon dioxide.

…I think many people put way too much reliance on the idea that they can just swim up. How far can you walk after fully exhalling..... In a true emergency you may be leaving the bottom with empty lungs not full ones..

Walking is also not a valid analogy because you have less oxygen to start with and you are not reducing carbon dioxide. Even with half full lungs, it does not take much decrease in depth to fill them back up and a whole lot more.

 

[Akimbo]

... So the bottom line is that it IS possible to ascent on a single breath from 100 feet, but there are significant hazards to doing so, including lung barotrauma, hypoxia and drowning...

Sounds like the same risks as recreation diving when you don't know what you are doing. BTW, you forgot decompression sickness and getting run over by a passing tanker.

Seriously, the whole idea is to experience and practice free ascents so you are far less likely to screw up and give yourself a barotrauma when there is a real emergency. You will discover your limits, so you won't suffer hypoxia and drown. If you overestimate your limits during practice, put your regulator back in your mouth and try shallower next time.

...So the bottom line is that it IS possible to ascent on a single breath from 100 feet, but there are significant hazards to doing so, including lung barotrauma, hypoxia and drowning. MUCH better not to need to do it in the first place!

I concur that it is always better when nothing goes wrong. Unfortunately, that plan doesn't always work out so well. Yes, it IS possible to ascent on a single breath from 100 feet, in fact most people are amazed how easy it is after working up to 100' or more. I sure was. You have 4x the oxygen in your lungs at 99'. Making a 99' free ascent isn't quite as easy as swimming 99' on four breaths, but not much harder.

Every piece of equipment adds some level of complexity and failure potential. Every procedure that depends on another person requires them to recognize the problem and properly respond. More often than not, accidents happen when more than one thing goes wrong. The more systems and procedures involved, the higher the probability systems will fail and a problem will be misjudged, misdiagnosed, and/or improperly responded to by yourself and others.

Say you were in a 4' swimming pool and your air suddenly stopped. Would you grab another regulator or just stand up? If you went for that regulator and it didn't work, would you look for your buddy or just stand up? Now take it a little deeper, say 10' where you had to swim up. Then deeper and deeper in small increments. The whole idea of experiencing and practicing free ascents is so they are not much more stress-inducing than just standing up.

There are two important values here. One is instinctively knowing that you will make it to the surface even when everything turns to crap. The big payoff is that simple knowledge allows you to realize there is no reason to panic. IMHO, panic is far more dangerous than stupid and stupid is more dangerous than unlucky. The surface is an extremely reliable, available, and easy to operate backup air system... and the price is right. I see no reason not to use it.

 

[copter53]

Sounds like the same risks as recreation diving when you don’t know what you are doing. BTW, you forgot decompression sickness and getting run over by a passing tanker.

Seriously, the whole idea is to experience and practice free ascents so you are far less likely to screw up and give yourself a barotrauma when there is a real emergency. You will discover your limits, so you won’t suffer hypoxia and drown. If you overestimate your limits during practice, put your regulator back in your mouth and try shallower next time.



I concur that it is always better when nothing goes wrong. Unfortunately, that plan doesn’t always work out so well. Yes, it IS possible to ascent on a single breath from 100 feet, in fact most people are amazed how easy it is after working up to 100' or more. I sure was. You have 4x the oxygen in your lungs at 99’. Making a 99’ free ascent isn’t quite as easy as swimming 99’ on four breaths, but not much harder.

Every piece of equipment adds some level of complexity and failure potential. Every procedure that depends on another person requires them to recognize the problem and properly respond. More often than not, accidents happen when more than one thing goes wrong. The more systems and procedures involved, the higher the probability systems will fail and a problem will be misjudged, misdiagnosed, and/or improperly responded to by yourself and others.

Say you were in a 4' swimming pool and your air suddenly stopped. Would you grab another regulator or just stand up? If you went for that regulator and it didn’t work, would you look for your buddy or just stand up? Now take it a little deeper, say 10’ where you had to swim up. Then deeper and deeper in small increments. The whole idea of experiencing and practicing free ascents is so they are not much more stress-inducing than just standing up.

There are two important values here. One is instinctively knowing that you will make it to the surface even when everything turns to crap. The big payoff is that simple knowledge allows you to realize there is no reason to panic. IMHO, panic is far more dangerous than stupid and stupid is more dangerous than unlucky. The surface is an extremely reliable, available, and easy to operate backup air system… and the price is right. I see no reason not to use it.

What increments would you recommend I start with and build up to for practice?

 

[copter53]

As I go up should I be merely saying "ahhh" or should I be yelling "AAHHHHHH"?