Pete, I give up. You're imagining that somehow physiology overrides physics. And high school physics at that. Youre overcompexifying, which, not surprisingly, my spell checker says is an incorrect spelling.
We dont need a MD here; we need a physics teacher. But I have no idea how well hed fare, because your argument is at the same level as a 10 pound weight will fall twice as fast as a 5 pound weight. At least THAT claim has a simple experiment that can be done to dispel it if you have a building thats leaning in so you can drop a couple weights from it.
So here you go, all my arguments balled up in one. Step back and think, because youre very, very wrong on this one. Your thought process when you thought this through was flawed, very flawed.
To think that the forces that act on your sinus, protected by bone are somehow not applicable to your lungs, protected by far less is wrong. Were mostly water (ugly bags of mostly water if you were a Next Generation Star Trek fan). That water conducts pressure EVERYWHERE in our body. Our sinuses, our ears, and our lungs.
The ONLY way pressure cannot be conducted within a vessel is if the vessel is COMPLETELY rigid, like a SCUBA cylinder. If a nearly empty cylinder at 50 psia (absolute) is brought down to 99 feet, it will still contain gas at 50 psia, even though the external pressure has gone from 15 psia to 60psia, which now even exceeds the internal pressure!
But were not rigid. Not even close. The fact that you can get sinus squeeze in something surrounded by bone shows us conclusively that the external pressure is conducted throughout our bodies. Theres nothing [natural] than protects any part of our anatomy from external pressure. The fact that we can embolize if we hold our breath and ascend is overwhelming evidence that the lungs SPECIFICALLY are not protected from external pressure.
So Im bobbing vertically at 16 feet with 8 psig (gauge) at my head and at .5 psi per foot, 11 psig at my toes (Im six feet tall). My airway is open to the regulator and Im neither breathing in nor breathing out. The air pressure in my mouth, equalized with the outside water pressure is 8psig, since my mouth is 16 feet deep. The top of my lungs, 1 foot below my mouth is at 17 feet, or subjected to a pressure of 8.5 psig. Whats the pressure of the air within my lungs at that point? 8 psig (remember, this was our very first hang-up where you *thought* we were saying that theres a pressure gradient within the gas and it turned out we were in violent agreement, there is no gradient). The bottom of my lungs, at 18 feet, is exposed to an external pressure of 9 psig. The internal pressure of the lungs at that point? 8 psig. Remember, no gradient in gas.
So what happens to those alveoli that are subjected to 9 psig of external pressure while only containing 8 psig of internal pressure? The collapse, but only partially. Lets step back for a moment.
You are entirely correct to dismiss the glove analogy. Our alveoli are more resistant to collapse than a floppy glove. If a floppy glove is open and its internal pressure was 8 psig and the external pressure was 9 psig, the glove would completely collapse and be wafer-thin (pronounced waffar-thin as in waffarthin mint
). Fortunately, our lungs are composed of a lot of connective tissue and stuff (theres one of those medical terms again) and they resist being compressed to some degree. So a better model for a single alveoli would be a Tupperware tub. If I take a balloon down to 33 feet, it halves in size because its walls do not offer any resistance to collapse. At 33 feet its internal pressure will equal the external pressure. If I take a Tupperware tub down to 33 feet, it will NOT halve in size, despite its best attempt. Itll crumple, but because the walls offer some resistance on their own, they will not give as easily as the ballon walls. If you were to sick your magnahelic on the inside of the tub at depth, you would find that its internal pressure is somewhat lower than the external water pressure. The delta pressure is the amount of resistance that the walls are supplying to avoid being crushed.
Like Tupperware, the alveoli and surrounding tissue resists collapse to some degree, so they dont immediately go flat when exposed to a greater external pressure. But they cant expand quite as well either, resulting in reduced lung capacity.
Now lets model the whole lung. A damp sponge will do nicely. If you were to take a big sponge one foot long and put it in a plastic bag and sink it, youd notice two things. First, thered be a bubble at the top of the plastic bag, which if you put your magnahelic on it would measure the same pressure as the water at that depth. Secondly youd notice that the sponge is crushed into a V shape. Since theres no gas gradient within the bag so all the gas is at the pressure you measured at the top of the bag, all that can resist the mounting external water pressure to the deeper portions of the bag is the sponge within. The less delta in pressure toward the top of the bag, the less the sponge compresses before its resistance counteracts the external water pressure. The greater delta, toward the bottom of the bag, the more the sponge is compressed before its able to successfully able to resist the outside pressure.
Remove the sponge from the water and its sides are parallel, this is equivalent to our lungs in air, which has no pressure gradient to speak of.
Let me anticipate your next point: But our lungs are in a watery system with the same delta (.5 psi from top to bottom, assuming one foot in height) so if my theory was true, wed have the same problem in air. Yes, theres a .5 psi delta and no, we dont have a problem with it. Theres two reasons we dont have a problem with it, first is simply evolution and the second is that were lazy breathers. Weve always had this .5 delta, so to some extent the lungs connective tissue is designed to counteract this self-imposed hydrostatic head generated by our body fluids. But it is a bit harder to fill those lower alveoli, which is why virtually everyone is a chest, rather than a diaphragm breather. It IS harder to suck air down to those lower alveoli, so normally we only breathe with the topmost ones.
The problem again is that you expose the body to yet MORE of a pressure gradient by immersing it in water. We werent designed to do that, and by being horizontal we reduce the external gradient as much as possible to make our lungs work as near-normal as possible.
Let me return to the immersed sponge experiment for a moment. If youre bothered by the fact that there isnt an external container around the sponge, go ahead and put some non-rigid one around it (non rigid because our body is not rigid like a SCUBA cylinder). The results will be the same.
Unlike you, when I first heard about outgassing in a horizontal position, the physics were so intuitively obvious (to me, at least) that this was a head-slapper, as in: Why didnt I think of that! I think your problem is that youve over-thought the problem. If the above doesnt help, I dont think anything can, and you can go on believing that different weight objects fall at different rates.
Roak
