Why do we blackout? Not how, WHY.

[strauss]

Perhaps this has been discussed previously, but I could not find anything.

My question is, why do we blackout when we get low on 02?

It seems to me that it is not an effective defense mechanism against permanent damage, ie death/brain injury.

Thoughts?

 

[Jorg]

It's a very good defense mechanism. By blacking out you put your whole system on standby preserving energy for only your basic functions like your brain and heart. The problem with freediving and BO is that you're in the water, and if you BO and you don't have a good buddy you will drown...

 

[Erik]

Maybe it's not a defense mechanism? Maybe it's just the inevitable result of the situation?
Erik Y.

 

[lungfish]

HI,
I think the issue also involves C02 build up, of course. But the black out is a "last ditch" defense mechanism and, depending on conditions, can be an effective one. Like drownings in cold water where metabolism slows and recovery is possible...it is the best the body can come up with when the mind is helpless.
Lungfish

 

[Walrus]

I think it is somewhat of a defensive mechanism. The brain shuts down before the heart does. If the heart were to run out of oxygen and stop beating you are then clinicly dead. If you have stopped breathing but the heart is still beating you still have time to be revived. Your body has 4-5 litres of blood with oxygen in it circulating to the brain. On land you also still absorb a small amount of oxygen thru the skin. If the heart has stopped you only have the small amount of blood stored around the brain which runs out very quick. You then get very rapid brain damage and the chances of revival disapear within minutes. I have heard of some odd medical cases where people have stopped breathing for hours (but their heart was still beating) and they were still revived.

Wal

 

[Panos Lianos]

I think it is somewhat of a defensive mechanism. The brain shuts down before the heart does.

Your brain doesn't shut down (in the sense that it doesn't stop working), it just orders your BODY to shut down. People have had dreams while they had a BO. So think of it as a "sleep" mode. Your body is trying to preserve what ever O2 is left, for vital organs.

 

[efattah]

I have read an entire book about brain hypoxia. From everything I read, it does not appear to be a defense mechanism at all. It is simply a matter of energy failure.

The neurons in the brain require certain electrical gradients in order to do their job as neurons. Maintaining the electrical gradient requires energy. The energy comes from ATP (adenosine triphosphate), which stores lots of energy in the phosphate chemical bond. The brain gets ATP from several sources, including:
1. Stored ATP in the brain (very small, enough for 3 second of consciousness)
2. Stored creatine phosphate in the brain (which can generate ATP, enough for 3 seconds of consciousness)
3. Burning O2 with glucose or ketone, generating ATP (requires oxygen supplied from the blood, as well as either glucose or ketone, also from the blood)
4. O2 stored in neuroglobin (very small --just discovered recently)

If #3 is cut off (from either holding your breath too long, or being strangled or choked), then you have enough energy from #1 and #2 to maintain consciousness for 6-7 seconds. This is why in the old days when people had their heads cut off (guillotine), they remained conscious for 6-7 seconds, and responded if you called their name by looking you in the eye (even after their head was cut off).

Once all these energy sources have been exhausted, there is no more ATP available. Burning glucose anaerobically (without O2) creates lactic acid very fast, and generates very little energy, enough for maybe 1 second of consciousness.

Once no energy is available, the neurons lose their electrical gradient and 'depolarize'. The process creates a chain reaction called 'anoxic depolarization'. When neurons no longer function, consciousness cannot be maintained, because the brain simply isn't working properly anymore.

The moment when blackout occurs depends on many variables, including blood pressure, hydration, brain creatine stores, carotid artery width, and so on. Because the moment of blackout can vary dramatically (either very early or very late), it seems even more likely that it isn't a defence mechanism. If it were a defense mechanism, then it would not be possible to develop tolerance to brain hypoxia and delay the blackout. If it were a defence mechanism, why not just black out nice and early to 'save' the organism?

 

[Walrus]

Hi Panos,
parts of your brain do shutdown, including the most complex processes ie vision and conscious thought. I guess you could compare it to sleep mode, or a coma state. Either way there is far less brain activity and energy used in this state.

 

[Skywalker]

Hi Eric

I would mostly agree with you. I think as well it is compareable to closing the fuel line to the engine or the tank just running out.

Robert

 

[Jorg]

If it were a defense mechanism, then it would not be possible to develop tolerance to brain hypoxia and delay the blackout. If it were a defence mechanism, why not just black out nice and early to 'save' the organism?

Maybe when someone has a blackout it IS early? who knows where and when the real damage begins?

To develop tolerance to something and have a defense mechanism in it is for sure possible. For example, I did a lot of testing with muscle failure and powerlifting. Some years back my maximum bench press for 2 reps was 160 kilo. When I tried 180 kilo's for 1 rep I experienced total muscle failure. I wasn't even able to try and hold on, my muscles would just quit on me just to have a defense mechanism to protect muscles, tendons, etc. Few years later my bench press was up to 185 for 2 reps and with a 205 kilo bench press I experenced the same. Muscles and tendons did get a much bigger tolerance, but they still had a defense mechanism so that nothing would rip apart. The same with other sports and many examples to give. So why wouldn't this apply to hypoxia tolerance?

 

[Skywalker]

I do not think that eric was reffering to damage or when brain damage begins. The fact that the brain shuts down bc of a low oxygenlevel does not mean that the damage starts to beginn at the same time. I think we need a little comparison here. If I talk about defense mechanism I talk about things like the breathing reflex that tells you your body would actualy like more oxygen again rofl that is a defense mechanism that a freediver can ignore an prospone till the moment he blacks out.
Regarding the black out itself that is for me no defense mechanism as it is nothing you can influence by will. You can maybe train that hard that the bo will come later and later but it is nothing you can ignore and prospone in an actual attempt when it is coming.

Robert

 

[Ricochet]

I always interpreted it like this:

Saying the brain on low O2 blacks out to save oxygen is like saying a car without fuel goes off to save gas.

 

[naiad]

I don't know if blackout is a protective mechanism, but surely LMC is not. Moving around uses more O2, and it can also prevent an efficient return to the surface if the person is conscious.

 

[perow1]

Why cannot LMC be a protective mechanism? Does not LMC occur when your muscles are "low on oxygen" and when you try to move your muscles wont obey and you make silly movements. If you are able to lie down and relax until blackout will the LMC occur?

 

[naiad]

LMC can occur even if the person is not trying to move. The movements are involuntary. Sometimes an LMC occurs in someone who is relaxed. They suddenly start shaking uncontrollably.

I think the loss of control is in the brain and nervous system, not in the muscles.

 

[perow1]

Ok, thank you. Might just be that I have not experienced it that way.

 

[jome]

That's some impressive benching Jörg! Probably not that beneficial to diving though

 

[Jorg]

Yep the good old days... I think that record is from 3 years ago and I definitly are doing it slower these days. But I got some other personal bests that I'm pretty proud of.

LMC; If I look at the shaking and dancing of a LMC I find this more disturbing then blacking out. But are these movements created by no oxygen in certain muscles or by the brains which aren't function 100%? If it's the last option then it's definitly better to blackout then to have a LMC!

 

[strauss]

Ok, I think I followed most of that. However, I cannot seem to form a definitive answer - brain failure I guess.

Thanks guys.

 

[Panos Lianos]

If it were a defense mechanism, then it would not be possible to develop tolerance to brain hypoxia and delay the blackout.

Why would it not be possible?