Lung Volume Reduced Underwater

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tceylan

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san diego, ca, usa
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From PADI's encyclopedia of recreational diving, a diver's lung volume decreases
by 15-20 percent (more when vertical) due to water pressure on thorax.

I do understand why lung volume would be reduced when a diver is vertical since regulator second stage will provide x ATM (ambient pressure) whereas the lungs would be at a higher ambient pressure say: x+y ATM.

But when horizontal, why should the lung volume be less than the lung volume on surface? Second stage provides gas at the ambient pressure and our lungs would be at
the same depth as the second stage when horizontal... :confused:
 
Did it by any chance say tidal lung volume?
 
I don't know if it says 'tidal', but dead-air space is being discussed at the same section. I would assume that it would be talking about tidal volume.

It says 'we need to breath slow and deep since there's more dead-air space, more gas resistance due to increased gas density and there's less reduced lung volume due to water pressure on thorax.'

I understand all of this except for less lung volume even when a diver is horizontal. It doesn't
make sense.... As long as we breathe ambient pressure gas how can the lungs get squeezed
due to water pressure on thorax assuming the diver is at the same depth horizontal?
 
I'm not a doctor, but I would guess it is because the lungs aren't flat like pancakes. So the ambient pressure is greater at the bottom. This is the same idea as when the lungs are vertical, but the effect won't be as great.

Of course I could be wrong, especially about that pancake bit. :)
 
So can we conclude that if the lungs are above the second stage (eg. upside down) then there would be no lung volume decrease?
 
Hmmm.... looks like an opportunity to conduct a test... I'll check and report.
Rick
 
So can we conclude that if the lungs are above the second stage (eg. upside down) then there would be no lung volume decrease?
Again, my unqualified opinion is that there would still be a decrease since the top of the lungs, closest to the head, would be at a greater pressure than the bottom, closest to the feet. In otherwords, the pressure difference would still exist, but it would just be upside down.
 
Just ran a test.

Within the rather rough accuracy and precision provided by a BC and a can of lead shot I detect no difference in my lung volume sitting on the edge of the pool and horizontal on the bottom at 6 ft.
 
How did you do the test, Thal? Did you do a total volume exhale into some kind of container?

I suspect the statement is true, and I have a feeling it's due to the difference between the mechanics of breathing on the surface and breathing off a regulator. On the surface, there is a constant communication between the lungs and the ambient atmosphere, unless you deliberately close your glottis. Underwater, the only time your lungs are in communication with ambient pressure is when you are inhaling from your regulator. During exhalation, your lungs are NOT in equilibrium with ambient pressure. And even during inhalation, you have the viscosity and resistance of the surrounding medium to reduce chest wall expansion. During exhalation, you have the pressure difference between ambient and internal, with no column of ambient air to resist the collapse of the chest structures.

My guess is the textbook answer is correct.
 

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