Air pressure confusion.

[mjj]

Hello, first post in this sub forum, but I'm hoping one of you can explain something to me as I can't get my head around it. Excuse the lack of exact facts, the numbers are irrelevant anyway, it's the science part I am interested in.

As you know the further you dive, the more air you need to inflate your lungs due to the pressure around you.

This I know.

The bit I don't get is, why? How is there pressure on your lungs? I know the pressure comes from the water around you, but it's not like your ribs are pushing your lungs. Why does your body not protect lungs in the same way that a submarine protects its occupants?

I.e. Submarine = body

Occupant = lungs

I've been searching the internet for answers and have come up with nothing. Please don't quote the balloon with air in, this makes perfect sense to me ask the balloon is in direct contact with the water.

Thank you in advance, and apologies if this makes you want to headbutt the computer for its stupidity. :-)

[NomadJoe]

Not an unreasonable question, actually. One I get from dive students from time to time.

Simply put, your lungs act as a flexible container, the walls of a submarine do not. And no direct contact is needed. The pressure changes effects every cell of your body, as there is nothing rigid to hold back the pressure. Ribs only surround the lungs, but your breathing continually equalizes them with surrounding pressure. Does that help? You may also want to read up on Boyle's Law. http://en.wikipedia.org/wiki/Boyle

Edited December 5, 2013 by NomadJoe

[ktm jeff]

As NomadJoe says , pressure is applied all over the body , your body is "squishy/flexible" and yes your body does reduce in volume , but not as much as the balloon !.The pressure does press on the lungs , but your fresh breath and the air distributed throughout your organs/cells/vessels etc stop you being squashed too much , but only up to so much pressure. Your rib cage will "bend closed" slightly as it is a flexible open cage . If you dive to 40 Meters you are at 5 Bars pressure (73.5 PSI). Your amazing body can handle this , and more , but too much and you "implode" - Squashed into pieces !.Your vocal chords are also effected by the pressure and talking in an air space under water , such as a ship wreck ,will result in you sounding high pitched like the cartoon "Pinky & Perky". Welcome to TV - some very useful info on here as there seems to be an expert on nearly every subject except women !!!!Then again would they be as wonderfull if we understood them. Or us/them.

[mjj]

Thanks for the replies. I think I've got it now. Kind of........

Where I was going wrong, I think, is that I was looking at pressure working only externally. But from what you have written it works throughout your body.

To clear things up a bit more, how is breathing under water different from breathing with someone sitting on your chest in normal air pressure? I'm guessing that the sitting is just external pressure that dissipates through your bone and muscle, but the pressure under water works in the same way that gravity does and effects all your internal cells..................am I right or have I misses it completely?? Lol.

[ktm jeff]

Your on the right track.When we are "at surface" or sea level , we are subjected to gravity , as you say , but also pressure. At this (sea) level we have 1 ATM pressure acting upon us (internally also , remember).1 ATM is 14-7 PSI.The atmosphere is thinner at altitude.When you breath , under-water , the air you breath in is now under pressure (compressed) so, although your right in that there is pressure "spread evenly "over and in your body , the compressed air you breath in , helps blow-up / expand and fill your lungs (like a turbo-charger). This means it only takes a little more effort , on your behalf , to breath in. Remember the air in the balloon , that air is under pressure under water but as no more air has been added to the balloon , its volume has , accordingly , been reduced. If you took a ONE breath (say 4 Liters)of air from your scuba gear at 40 Meters (not forgetting that this air is compressed) , and held it till you surfaced , your lungs would have expanded to 20 Liters.Hope this helps . BTW are you planning to take up scuba ?

[mjj]

Thanks again Nigel, I get it now. :-)

This all came about as I have befriended a diving instructor who is Spanish. While we were chatting about me doing a course his explanations were good, but on this point his English skills were a bit wanting leaving me a bit confused on the 'why' part.

[VocalNeal]

I suppose another way to look at it would be that at sea level you have 14.7 psi pressing against the outside of your body and 14.7 psi pressing on the inside of your lungs. The muscles in your chest only have to over come the difference between outside and inside area. If you go down 33 ft the pressure will be 29.4 psi, so double, and your muscles don't have the strength to overcome the difference.

Snorkels are short for a reason much more than snorkel depth you can't breath.

What the 2 stage regulator in a Scuba kit does is to equalize the air pressure in the mouth piece to the same pressure as the surrounding water. ( technically just tad less pressure. The difference is so small that simply turning the mouth piece upside down can cause the second stage regulator to "free flow') When you suck on the mouth piece you use about the same amount of muscle power as you do at the surface, as the pressure difference between the outside of your body and the inside of your lungs is about the same. The volume of air in your lungs is the same but the pressure is higher.

As has been said when you rise in the water the pressure on the outside becomes less as does the balancing pressure in your lungs , so the air volume increases. Which is why you will be taught to always breath out slightly when ascending.

Edited December 6, 2013 by VocalNeal

[12DrinkMore]

1. Why does your body not protect lungs in the same way that a submarine protects its occupants?

Nomadjoe answered the question. Your body is not rigid but flexible (like a balloon, sorry, but that is pretty much the effect). As you breathe from the regulator you inhale air at the ambient pressure due to the design of the regulator. This is why it should always be easy to breathe from the regulator.

2. To clear things up a bit more, how is breathing under water different from breathing with someone sitting on your chest in normal air pressure?

If you were not breathing air under water from a regulator the effect would be pretty much the same. You would not be able to inhale. You can try this in a pool with a snorkel. At a very slight depth your diaphragm is too weak to suck air into your lungs.

Bullshit information above is.

1, "air distributed throughout your organs/cells/vessels". This is nonsense oxygen is distributed by combining with hemoglobin and also a smaller amount dissolving into the blood plasma. Nitrogen is also distributed by dissolving into the blood plasma. The is no "air" being circulated around your body. You would be suffering from decompression sickness if that was the case.

2. "implode" - Squashed into pieces". Utter nonsense. The record for free diving is over 200 meters, where the lungs will be compressed down a ridiculously small 5% of the surface volume.

3. "talking in an air space under water , such as a ship wreck ,will result in you sounding high pitched like the cartoon". More utter drivel. Due to the increased density of the air in the air-pocket, which slows down the velocity of sound, your voice will sound deeper.

4. "the compressed air you breath in , helps blow-up / expand to fill your lungs (like a turbo-charger)" Nonsense. If the regulator operated in that way there would vast numbers of seriously injured divers with lung over-expansion injuries.

5. "held it till you surfaced , your lungs would have expanded to 20 Liters." Before that happened you would have suffered a sever lung overexpansion injury, quite possibly a pneumothorax and an arterial gas embolism. You would probably be dead. Never hold your breath when on Scuba.

6. "technically just tad less pressure." When you inhale you reduce the pressure in the regulator slightly, causing the diaphragm to open the valve. When you breathe out there is a slight positive pressure. When you are between breaths the air in the regulator is at ambient pressure. I have no idea where this "technically lower tad" would come from.

7. "simply turning the mouth piece upside down can cause the second stage regulator to "free flow'." If you turn the regulator over slowly it will not free flow. It will go into free flow because the air inside the regulator in trying to escape upwards, sucks in the diaphragm which opens the valve causing more air to escape. This is a positive reinforcement and can often occur on the dive boat if the regulator has been very finely adjust. It might be interesting to try breathing from a regulator underwater whilst looking up. It becomes very hard to breath because the air in the regulator is exerting an upward pressure on the diaphragm which increases the force required to open it on inhalation.

8. "Which is why you will be taught to always breath out slightly when ascending". You will be taught to breathe normally and not hold your breath.

"there seems to be an expert on nearly every subject". Yeah, right.

[Shurup]

I suppose another way to look at it would be that at sea level you have 14.7 psi pressing against the outside of your body and 14.7 psi pressing on the inside of your lungs. The muscles in your chest only have to over come the difference between outside and inside area. If you go down 33 ft the pressure will be 29.4 psi, so double, and your muscles don't have the strength to overcome the difference.

Snorkels are short for a reason much more than snorkel depth you can't breath.

What the 2 stage regulator in a Scuba kit does is to equalize the air pressure in the mouth piece to the same pressure as the surrounding water. ( technically just tad less pressure. The difference is so small that simply turning the mouth piece upside down can cause the second stage regulator to "free flow') When you suck on the mouth piece you use about the same amount of muscle power as you do at the surface, as the pressure difference between the outside of your body and the inside of your lungs is about the same. The volume of air in your lungs is the same but the pressure is higher.

As has been said when you rise in the water the pressure on the outside becomes less as does the balancing pressure in your lungs , so the air volume increases. Which is why you will be taught to always breath out slightly when ascending.

As VN pointed out, your body doesn't know the difference if the internal/external pressure is the same.

I've been in the sports diving for 7-8 years in my teen years and we normally trained with custom snorkels, I did experiment with longer snorkels and while 1/2 meter of water doesn't sound like a big deal, you can't breath atmospheric pressure air at this depth.