Breathing physiology... whats best for off-gassing

[The Chairman]

Hmnnn... but it sounds like bona-fide cavers to me!!!

Originally posted by roakey
A huge amount of blame for poor trim I place squarely on the shoulder of instructors and divemasters. Ever go on a led scuba dive where the instructor or divemaster is trying to be a good buoyancy example at the safety stop by hanging vertically, motionless in the water with a smug look of “this is how it’s done!”?

To me they might as well have a huge neon sign hanging around their neck that says: “I have no clue.”

Not only does this demonstrate that they have horrible trim, but a vertical position is the worst position for gas exchange due to the hydrostatic head that’s developed between the top and bottom of your lungs. The alveoli at the bottom are subjected to a greater pressure (about .5 psi) and don’t expand as readily as the ones near the top. In a horizontal position this pressure difference is more than halved, and more alveoli can be used for effective gas exchange.
Roak

Fin-Pivots are a GREAT starting point. If you can't deal with fin pivots then learning how to hover has a far steeper learning curve. Take one step at a time, and become comfortable with your fins on something stationary FIRST!!!

Hovering is next. I like the picking the coins off the bottom of the pool. Think I will try that and then use glue on one (grin).

As for staying still being the "acid test" for trim I sincerely disagree! However, I would rather contend that being still WITH your eyes closed -IS- the acid test. Once you remove that reference, your body can not subconsciously react to maintain your attitude. Use the force Luke...

As for horizontal being "essential" for neutral buoyancy, you are all wet. Cavers need -THAT- degree of control. The average (and above average) OW diver does not! They are not worrying about silt outs and such. They don't want to hit the bottom and they don't want to float to the surface.

Don't get me wrong TRIM IS A GREAT THING TO HAVE ON YOUR SIDE. But asking an OW or AOW to deal with trim while learning the real basics is just plain masochistic. I also agree that backplates and steel tanks GREATLY improve trim without much thought. Still, for MOST OW and AOW divers, their diving attitude (physical not mental) plays a far greater role in how easily they stay at a certain depth. Given a neutral diver trimmed perfectly, a slight incline will make you rise and a slight decline will make you fall.

And Roakey, as for horizontal off-gassing... hmnnn I don't think that's a viable theory. The pressure in the lungs should always be equal to the pressure at the water interface, i.e. the regulator. Just because the water (a liquid) on the outside of your chest may vary in pressure (i.e. has a "head pressure"), the air (a gas) will stay at whatever the pressure is where it comes into contact with water. The extra weight of the air in that system would be beyond negligible, so I can't see a .5 psi variance coming from there. Now, I am not a physicist, nor do I play one on TV, and you could be referring to a system or physical law that I am not familiar with... but how you can get a hydrostatic head in a gaseous system just plum eludes me. I think I can see why that DM was so smug... BTW, this is why it is easier to breathe upside down vertically, less so horizontally and even less so right side up vertically. It is also why snorkels have a maximum useable length! Please elucidate if I have missed something so that I may correct any erroneous thinking.

 

[Uncle Pug]

Originally posted by NetDoc

As for staying still being the "acid test" for trim I sincerely disagree! However, I would rather contend that being still WITH your eyes closed -IS- the acid test. Once you remove that reference, your body can not subconsciously react to maintain your attitude. Use the force Luke...

err..mmm...allow me Roak:
Dear Doc,
Actually we (my dive bud and I) find that hovering is nailed when we lose the reference in mask off drills.... seems you can lose the force with them open.

As for horizontal being "essential" for neutral buoyancy, you are all wet.

Thank you... that is why I don't have to rinse my gear....

And Roakey, as for horizontal off-gassing... hmnnn I don't think that's a viable theory. The pressure in the lungs should always be equal to the pressure at the water interface, i.e. the regulator. Just because the water (a liquid) on the outside of your chest may vary in pressure (i.e. has a "head pressure"), the air (a gas) will stay at whatever the pressure is where it comes into contact with water.

Doc...you should know this...you are made of water...
The water interface is your body....
And if you are verticle there is a pressure differential between your head and your feet.... Roak is right...

 

[roakey]

Originally posted by NetDoc
Fin-Pivots are a GREAT starting point.

No argument here, they're a great starting point, with the emphasis on “starting.”

Originally posted by NetDoc
Don't get me wrong TRIM IS A GREAT THING TO HAVE ON YOUR SIDE. But asking an OW or AOW to deal with trim while learning the real basics is just plain masochistic.

I disagree. Correct trim is a purely mechanical process, getting the right amount of weight in the right place. Trimming a new student correctly does not require any more brain cycles during a dive, so the student can still concentrate on buoyancy, it’s just that they’re horizontal, rather than more or less vertical. Free, at no extra charge, this makes them more efficient and comfortable in the water because it lessens the amount of BC fiddling that’s necessary. Previously, with poor trim their first kicks would drive them upwards, requiring them to vent their BC and then they’d have to put more air into it when they stop swimming.

I’ll move the off-gassing response to one of the technical forums. Give me a day or two to catch up with the rest of my life!

Roak

 

[The Chairman]

Originally posted by Uncle Pug


err..mmm...allow me Roak:
Dear Doc,
Actually we (my dive bud and I) find that hovering is nailed when we lose the reference in mask off drills.... seems you can lose the force with them open.

So, I guess I missed the part in your post where you guys said "mask-off" then? We have gone so far and put electricians tape to obfuscate their vision. You still don't need it for most OW or AOW divers to maintain their attitude or buoyancy.

Originally posted by Uncle Pug

Doc...you should know this...you are made of water...
The water interface is your body....
And if you are verticle there is a pressure differential between your head and your feet.... Roak is right...

You mean YOU have a pressure differential in the air in your lungs? Where the alveoli are??? Air is far less dense (weighs less) than water. The pressure differential over 12 inches is miniscule... I don't own an instrument that could measure it! It would have to be in torr as inches of mercury nor psi would be fine enough. But you feel that the air pressure at the bottom of your lungs is greater by .5 psi than at the top of your lungs???The blood in the alveoli might have an elevated pressure, but surely not the gas in the lungs... Did I read you right? Am I that stupid that I missing something so simple that it is just eluding me?

I admire your loyalty to Roakey! I don't always disagree with him, though we have two very distinct views on diving. I agreed with a lot he had to say, but I still don't get the physics that makes the obvious pressure differential in a fluid system as being the same in a gaseous system. While you might be made of water, you have not learned to breathe it yet. The air in your lungs is a gas and reacts as such.

 

[Uncle Pug]

Originally posted by NetDoc

So, I guess I missed the part in your post where you guys said "mask-off" then?

.

I didn't until the last post.... I was trying to make a point....

You mean YOU have a pressure differential in the air in your lungs?
... Did I read you right? Am I that stupid that I missing something so simple that it is just eluding me?

No... there is no air (gas) pressure differential in your lungs.... but there is a pressure differential in the lung tissure itself...
If you dove a drysuit you would intuitively know what I am talking about....

I admire your loyalty to Roakey!

Hmmmm..... mmmm...

 

[The Chairman]

I just doctor up networks...

However, if I were to hazard a SWAG (Scientific Wild Arse Guess), I would say that the pressure differential you are referring to in the lung tissue, would be from the actual gas pressure within the lungs to the pressure of the gas dissolved within the blood. Your blood pressure would be modified by external pressures of course, but would probably be stable within the twelve-inch distance of your lungs, but the pressure of the gas dissolved within the blood should be constant. Off gassing occurs as the partial pressure of a particular gas in the blood is greater than the partial pressure surrounding the alveoli. So having a lower pressure in the lungs (like being vertical with your head up) would seem to provide the BEST (or fastest) off gassing environment. Some gasses (such as oxygen) are consumed and do not require off gassing.

As for dry suits, they are cool in that they act as a pressure buffer for your body. Being filled with a gas, the pressure inside of that vessel would be the average of the entire interface (or the entire suit). It would eliminate a pressure differential for the area it covered. This would presume that there was -no- contact between the dry suit and the diver, and that he was isolated within the suit itself. Of course that would be the perfect dry suit, which they haven't started making yet. Still, except for the contact areas, this would hold true.

That’s at least what -my- intuition would make of it... your mileage may vary!

 

[roakey]

Originally posted by NetDoc
You mean YOU have a pressure differential in the air in your lungs? Where the alveoli are??? Air is far less dense (weighs less) than water. The pressure differential over 12 inches is miniscule... I don't own an instrument that could measure it!

I admire your loyalty to Roakey! I don't always disagree with him, though we have two very distinct views on diving. I agreed with a lot he had to say, but I still don't get the physics that makes the obvious pressure differential in a fluid system as being the same in a gaseous system. While you might be made of water, you have not learned to breathe it yet. The air in your lungs is a gas and reacts as such.

First off, it's been over a week since Uncle Pug kneeled before me. So much for being loyal.
Seriously, though I’ve never met UP my guess is that he’s a lot like me, I’m loyal only to what I consider right, and if it matches up with what someone else thinks is right, that’s fine. If we ever disagree on anything, I bet we go at each other tooth and nail!

There is no pressure differential of the gas in the lungs, there’s a pressure differential OUTSIDE the lungs, exerted by the water. That’s about .5 psi per foot and you do own a device that can measure pressures that can only be measured in mere inches of water -- your regulator! Sometime hook it up and turn it mouthpiece up and slowly submerge it in water -- with a good performing regulator it’ll start free-flowing before the mouthpiece goes under -- probably about one to two inches of water, which is about .04 to .08 psi.

So let’s run the numbers; please allow me to round heavily so we don’t get stuck in the eight-decimal point zone. Water exerts .5 psi per foot. It’s 2 feet from the bottom of my lungs to my mouth.

So let’s say I’m vertical position, my airway is open and I’m neither inhaling or exhaling. That means the gas pressure in my mouth, and in fact my entire respiratory system is as the ambient pressure of the water at the level of the regulator (as you correctly point out, pressure differential within the gas space can be ignored). Since the bottoms of my lungs are two feet below my mouth, however, there’s a 1psi difference between the gas that’s TRYING to get into the alveoli and the external pressure being exerted on the outside of my body at that depth. 1 psi doesn’t sound like very much, does it? Try breathing through a two-foot snorkel and tell me how it goes. Truth is, your diaphragm can only overcome about a .75 psi deficit, or about 18 inches. To overcome the delta between the external water pressure and the gas pressure in your lungs, you have to rely on all the connective tissue to expand against the outside water pressure. The greater the delta between the internal gas pressure and the external water, the less successful this expansion is. So you get a gradient of effectiveness of alveoli expansion against the external pressure. The greater the pressure delta, the less gas flows in and out of the alveoli. The top of the lungs that are only half a foot below the mouth are expanding against only .25psi of external pressure. However, as you go lower in the lungs, you finally reach a point where there’s virtually no gas exchange due to the alveoli being collapsed by the external pressure.

The lungs and effort of breathing is VERY sensitive, which is why a good regulator’s cracking pressure can be measured in an inch of water or less. Yet by assuming a vertical position you are negating any benefit of a good regulator by imposing a pressure differential that can be measure in feet of water!

Contrast this against being in a horizontal position. Horizontal, with your chin up looking forward puts the regulator virtually at the same depth as the centerline of your lungs! If you’re looking down, technically you have to work to exhale, since your lungs are at a lower pressure gradient than the regulator! This allows all the alveoli to expand and efficiently exchange gas, since the biggest negative pressure gradient you’re going to see from the regulator to the lowest point in your lungs will be measured in mere inches, or about the cracking pressure of the regulator itself.

You’re right that us cave and other technical idiots have good reasons for proper trim. But it’s more than just to avoid silting and stuff like that. It turns out that so-called “technical” techniques just make diving one heck of a lot easier than what’s taught for recreational diving. I DO NOT propose that you teach an OW student all the advanced techniques (note I didn’t call them technical techniques) unless they don’t take any brainpower (as I stated, trim doesn’t take any brainpower). In OW you’re just learning how to come up alive and uninjured. But I see AOW as the perfect time to say to a student “Ok, I want to see all your OW skills (mask flooding, etc.) done hovering five feet above the bottom. How many AOW classes have I seen doing their “narc test” (doing math at depth) while kneeling on the bottom? ALL of them. They should be hovering. After all, this is supposed to be an ADVANCED class! But they’re only following the lead of their instructor, who’s kneeling on the bottom too.

One of my goals of participating on this board is to get the word out that “technical” techniques are not only applicable to technical diving, they VERY applicable to recreational diving as well. Many of our techniques make recreational diving safer and easier, resulting in less stress and better air consumption and a much more enjoyable dive.

Roak

Ps. Good analogy with the dry suit, UP. I kneel before you.

 

[roakey]

Originally posted by NetDoc
However, if I were to hazard a SWAG (Scientific Wild Arse Guess), I would say that the pressure differential you are referring to in the lung tissue, would be from the actual gas pressure within the lungs to the pressure of the gas dissolved within the blood

Whoa, way to technical. I hope my previous post clears this up, we're simply talking about the mechanical force exerted on the body by the water, nothing about blood gasses, etc.

And about the dry suit. If you get vertical (“get vertical” I sound like a snowboarder ) your legs get squeezed while you have a balloon at your shoulders. The reason your legs are squeezed is that the gas pressure within the suit at your leg level is insufficient to resist the external water pressure so the resistance must be supplied by the undergarment and under that undergarment, your legs. You feel the squeeze because your legs are supplying the resistive force against the external water pressure, not the air in your suit (at that depth, the air at shoulder level is another matter entirely).

As an aside, this is why a good undergarment for a dry suit is essential and why DUI’s G-series of Thinsulate is so superior. Unlike most normal “jacket” type Thinsulate which has a lot of loft but can be compressed, DUI’s G series is “boot” Thinsulate that is used in hunting boots and is made to NOT compress. So unlike jacket Thinsulate that will lose its loft and insulation properties when squeezed within a dry suit, DUI’s G series maintains its loft and warmth, even when squeezed.

Roak

 

[The Chairman]

that ultimately I have a reduced lung volume and that impedes off-gassing at fifteen feet (a safety stop)??? I disagree with your assumption about the alveoli being under the same pressure as the outside of your chest. I will however, be open to a medical doctor that tells me there is indeed a significant pressure differential within the chest cavity (overcoming the ribs and muscle structure) that would preclude alveoli to open. If you can expand your chest, then your alveoli are not undergoing the squeeze you are asserting. As my son so aptly pointed out to me, our breathing occurs when we create a relative vacuum within our chest cavity. The lungs do not (can not) press outward, they are sucked outward as we use our muscles and skeleton to expand the cavity. The pressure gradient that is important for off gassing is between the blood gasses and the gasses within the lungs. That differential is increased when you assume the same position as that smug DM... who is enjoying himself and off-gassing rather nicely!

BTW, the method you are referring to, measures the pressure differential of inches of water, not a foot of air. A magnehelic gauge will do the same thing! My contention deals with the gas inside the lungs not the water pressure on the other side of the chest cavity, which has been reinforced with ribs and covered with several layers of muscle! The atmosphere is several miles high... yet it only takes 32 ft of water to accomplish yet another atmosphere of weight.

I again must add the caveat that I am not trained as a physician, but I don’t think anyone involved in the thread is (yet). I have heard this erroneous argument before, and once again it centers around many, many assumptions that (pardon pun) just don’t hold water.

 

[roakey]

Originally posted by NetDoc
that ultimately I have a reduced lung volume and that impedes off-gassing at fifteen feet (a safety stop)??? I disagree with your assumption about the alveoli being under the same pressure as the outside of your chest. I will however, be open to a medical doctor that tells me there is indeed a significant pressure differential within the chest cavity (overcoming the ribs and muscle structure) that would preclude alveoli to open. If you can expand your chest, then your alveoli are not undergoing the squeeze you are asserting.

The reason you can expand your chest, even if the lower alveoli are not being expanded is that we’re not a solid and stuff (medical term) can squish (another medical term) around.

Originally posted by NetDoc
As my son so aptly pointed out to me, our breathing occurs when we create a relative vacuum within our chest cavity. The lungs do not (can not) press outward, they are sucked outward as we use our muscles and skeleton to expand the cavity.

It’s amazing on how much we agree on, but we’re coming to different conclusions. Your son is correct. The diaphragm moves down and the chest rises, creating a vacuum. Nature abhors a vacuum so air rushes in your mouth to equalize the pressure. In this is the key to what I’m saying. Your muscles and/or lungs can generate and/or tolerate only so much of a vacuum. If you need to generate too much of a vacuum (such as trying to breathe through a two-foot snorkel) you simply can’t do it. You can’t raise your chest against the water pressure on the outside of your chest. But it’s not an all-or-nothing deal. Going between a 6-inch snorkel and a two-foot snorkel your breathing gets more and more labored. It is not the case that either all your alveoli are exchanging gas or none of them are, they are progressively unable to inflate the alveoli from the deepest depth moving upwards as you go deeper until you can’t expand enough to get a life-supporting exchange.

Originally posted by NetDoc
The pressure gradient that is important for off gassing is between the blood gasses and the gasses within the lungs. That differential is increased when you assume the same position as that smug DM... who is enjoying himself and off-gassing rather nicely!

Bear with me ND, I’m not going to address this yet since I don’t want to confuse two completely different issues. What I’m saying is that you’ve reduced or eliminated any gas exchange (exchange as in alveoli-atmosphere, not alveoli-blood) in or out of your lowest alveoli when you’re in a vertical position. I’m not talking gas exchange between the blood and the alveoli, I’m talking about the mechanical movement of gas in and out of the alveloi to carry CO2 and N2 off in your exhalations. Once we get this point squared away we can come back to blood gas exchange if you like.

Originally posted by NetDoc
BTW, the method you are referring to, measures the pressure differential of inches of water, not a foot of air. A magnehelic gauge will do the same thing! My contention deals with the gas inside the lungs not the water pressure on the other side of the chest cavity, which has been reinforced with ribs and covered with several layers of muscle! The atmosphere is several miles high... yet it only takes 32 ft of water to accomplish yet another atmosphere of weight.

I’ll try this one more time. We are in violent agreement, we are NOT talking about pressure differential in the gas in your lungs or any airspace. I didn’t know you owned a magnehelic or I would have replied to your statement “I don't own an instrument that could measure it!” with “Your magnehelic can!” rather than “Your regulator can!” Let’s drop this part of the discussion since I think we’re on the same page now. I’m talking about the pressure differential on your lungs from the water pressure OUTSIDE your body, not any pressure differential in the gas INSIDE your lungs.

BTW, the fact that it takes miles of atmosphere to accomplish what 33 feet of water (I’m a freshwater guy ) does is exactly why our body position doesn’t matter in air, but becomes an issue in water. It’s the density and therefore the pressure gradient.

Originally posted by NetDoc
I have heard this erroneous argument before, and once again it centers around many, many assumptions that (pardon pun) just don’t hold water.

“Erroneous argument?” Careful ND, you’re becoming as bad as me!

Roak