Perhaps a source of confusion is the difference between PSIG (pounds per square inch gauge) and PSIA (pounds per square inch absolute). The pressure inside an open soda bottle at sea level is 0 PSIG and 14.7 PSIA. PSIG is differential, PSIA is absolute.
The fact that pressure is linearly related to depth is pertinent. For example, take a soda bottle to 99fsw and fill it with air, cap it, and then bring it up to 66fsw. The pressure inside the bottle is 14.7 PSIG, i.e., 1 atmosphere more than the pressure outside the bottle. Now fill the bottle at 33fsw and bring it to the surface; the pressure inside the bottle is the same: 14.7 PSIG. The pressure differential is what causes pulmonary embolism; filling your lungs to the point of inelasticity at 99 fsw and ascending to 66fsw will hurt you just as badly as doing so at 33fsw and ascending to the surface.
What is different is the size of the bubble that is the air in your lungs. Ascending from a shallower depth while holding your breath causes the bubble to increase in size more quickly as you ascend and therefore hit that point of inelasticity sooner if your lungs were not quite full when you started up - sooner than if you started deeper and ascended the same number of feet. The gas laws are only relevant until the lungs expand to the point of inelasticity; after that the internal absolute pressure is constant and it's all about the differential between internal and external pressure.
EDIT: I just wanted to add one more point; As your lungs expand when you inhale, the tissues inside them do not stretch, they unfold. Once they are completely inflated (what I somewhat inaccurately called the "point of inelasticity" above) they cannot stand more than a very small differential between inner and outer pressures. A doctor once told me that they have about the resiliency of a wet paper towel. That's why embolism is so dangerous and so easy to cause. Be careful out there.
The fact that pressure is linearly related to depth is pertinent. For example, take a soda bottle to 99fsw and fill it with air, cap it, and then bring it up to 66fsw. The pressure inside the bottle is 14.7 PSIG, i.e., 1 atmosphere more than the pressure outside the bottle. Now fill the bottle at 33fsw and bring it to the surface; the pressure inside the bottle is the same: 14.7 PSIG. The pressure differential is what causes pulmonary embolism; filling your lungs to the point of inelasticity at 99 fsw and ascending to 66fsw will hurt you just as badly as doing so at 33fsw and ascending to the surface.
What is different is the size of the bubble that is the air in your lungs. Ascending from a shallower depth while holding your breath causes the bubble to increase in size more quickly as you ascend and therefore hit that point of inelasticity sooner if your lungs were not quite full when you started up - sooner than if you started deeper and ascended the same number of feet. The gas laws are only relevant until the lungs expand to the point of inelasticity; after that the internal absolute pressure is constant and it's all about the differential between internal and external pressure.
EDIT: I just wanted to add one more point; As your lungs expand when you inhale, the tissues inside them do not stretch, they unfold. Once they are completely inflated (what I somewhat inaccurately called the "point of inelasticity" above) they cannot stand more than a very small differential between inner and outer pressures. A doctor once told me that they have about the resiliency of a wet paper towel. That's why embolism is so dangerous and so easy to cause. Be careful out there.
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