Lungs underwater

[Nathan Doty]

Wrong logic. Inverse logic.

Why would the air in the lungs compress? The air in my scuba tank does not compress during a dive. I can clearly see how the SPG shows a constant pressure even when I dive from 1 ATM to 2 ATM. The pressure drops with time but depth does not affect cylinder pressure. This has something to do with the STEEL cylinders.

The air in my lungs would then retain its pressure. PV/T=constant and as my body temp is contant, so PV=contant. Volume goes down, pressure up, or vice versa. At depth there will be an external overpressure (caused by the weight of the water) that compresses the body - and hence the air inside. When the volume has shrunk and pressure increased accordingly (to match the external pressure) an equilibrium is reached.

And oh, it's not my rib cage but my belly that is giving way. The soft tissues.
(I have tried to compress my rib cage during some tight crawling and no, it's not that compressible.)

Scuba diving is different because then we breathe compressed air at the ambient pressure.

The air in your tank is at a greater pressure than depth can create to start with, it can't be compressed by a lesser force. The air would actually compress (very briefly) the water around it if it wasn't contained in a solid structure.

 

[EireDiver606]

It's why scuba divers are advised to never hold their breath

Above - This is another industry standard lie.


The Tuth:
Divers should not hold their breath when ascending due to the risk of the lungs going pop which would be a bad day at the office.

Divers can and should hold their breath for small periods of time only when not ascending


The amount of misconceptions and blanket statements in diving....

 

[Angelo Farina]

What you are missing here is a phenomenon known as "blood shift". Our lungs contain both blood and air. Air compresses following the Boyle's law, so the air volume is effectively 1/5 of the original when the freediver is at 40m.
As the chest cannot compress so much for compensating for the missing air volume, what happens is that a certain amount of blood comes to the lungs. This is called "blood shift", as a volume of blood moves away from peripheral parts of your body and fills the volume in your lungs which is left from air compression.
This explains how it is possible for a free diver to go down to 200m, well beyond the theoretical minimum residual volume of your respiratory system, without his chest crushing.
The Blood Shift phenomenon was discovered by the medical team following Jacques Majol, after Enzo Majorca did proof that the traditional volume compression theory was wrong, by diving well below the limit given by the ratio between total volume and residual volume. All the history here: Blood Shift and the Spleen Effect in Freediving

 

[Joneill]

“And fortunately, our abdominal contents slide under the rib cage to help out with anything beyond that.”

Diving Doc

Makes for a lovely visual of what is going on at depth... lol!

 

[Jcp2]

Makes for a lovely visual of what is going on at depth... lol!

You can make this happen on the surface too! Just blow out as much air as you can.

 

[lowwall]

Divers can and should hold their breath for small periods of time only when not ascending

I'll grant the "can", but please explain the "should".

I can't think of situations where holding your breath is more desirable than slowly exhaling. Even more important, you want to train the way you want to act in an emergency. If you don't breath hold during normal diving, you are much less likely to do so when stressed or task loaded.

 

[Joneill]

You can make this happen on the surface too! Just blow out as much air as you can.

I know what actually happens - it was the mental imagery of Diving Doc’s description “abdominal contents slide under the rib cage” that I found “disturbing”... lol

 

[Angelo Farina]

I'll grant the "can", but please explain the "should".

This has been recently discussed in another thread regarding air consumption and CO2 retention.
I resume here my other post. Our lungs have a variable surface of contact between blood and gas trapped inside them, Only when the lungs are completely filled of air their surface is maximum (around 2 m2), hence for minimizing CO2 retention and for maximizing the effectiveness of breathing (that is, getting the most useful effect with the minimum work load and minimum air consumption) the recommended way of breathing is with a very low number of cycles per minutes (3, maximum 4), always employing all the vital capacity (from maximum inhalation to maximum expiration) and staying much more time with lungs almost extended than with lungs depleted.
To this effect, a 3-5 s long inspiratory pause can be quite effective, as during this time you get optimum CO2 discharge without any muscular effort. Of course this can be dangerous, if the diver goes up without exhaling, hence it is a practice which is usually adopted only by skilled technical or military divers. I prefer to teach to normal leisure divers simply to breath with a very slow pace, with a full volume being vented, and a strongly asymmetrical duty cycle (very slow in the upper part and very fast in the lower part, when the lungs are almost empty). But never stop venting.
This is much safer, while retaining the same favourable effects: optimal discharge of CO2, minimized air consumption and reduced amount of Nitrogen being captured. And this style of breathing also prevents the diver in falling into uncontrolled short and fast breathing, which is very dangerous, as the CO2 cannot be eliminated anymore, and the respiratory effort explodes.
This is what I always did teach to my students. But when I and my wife dive without students, we instead adopt the old approach, making the same asymmetrical breathing cycle, but adding on top the deprecated 5s pause with lungs full of air. This is how we were trained in the seventies using AROs (pure oxygen pendular rebreathers), and after doing this since almost 40 years I really do not see any danger for us (we survived to the danger when young, and we are now on the safe side of the thing):
As said, this is not what should be taught to students. But it is what skilled military or petrol-platform divers do, getting further benefit in terms of reduction of air consumption and better capability of discharging CO2, which prevents narcosis and other problems caused by hypercapnia.

 

[rsingler]

Though other parts of your recommendation may stir controversy (search for "skip breathing"), it is possible to ventilate effectively as you describe, though 3-4 breaths per minute presumes minimal exertion.
However, I disagree with this statement as physiologically unlikely:

reduced amount of Nitrogen being captured

You cannot decrease nitrogen absorption at depth by any special breathing technique. It just doesn't work that way.

 

[Angelo Farina]

You cannot decrease nitrogen absorption at depth by any special breathing technique. It just doesn't work that way.

This is what is commonly accepted.
In reality, studies performed with ultrasonic metering for analyzing microbubbles did show clearly that the harder the work load, the more Nitrogen is absorbed, as the hearth pumps more blood, and more air is vented for discharging the additional CO2 produced, etc..
And some air-integrated diving computers which monitor air consumption take this into account, evaluating more Nitrogen in the tissues when the diver is making anomalous air consumption, which indicates a heavy workload, and hence requires longer deco. This is called the Bühlmann ZHL-16 ADT DD auto-adapting algorithm, which also takes in consideration workload, and was employed by some top-range Uwatec computers.
A style of ventilation as I described reduces the venting workload, which is a significant fraction of the total workload when deep diving and not making other mechanical actions, in what I call a "contemplative" diving.