Why is low pressure exposure so much worse for the body than increased pressure?

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Offline kaukcz

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Why does the human body react so dramatically when put in a vacuum, a difference of 1 atm from atmospheric pressure, but people can dive hundreds of feet under water with the pressure increasing at a rate of 1 atm every 33ft or so.
« Last Edit: 19/06/2009 09:07:49 by chris »


Offline chris

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Well, there's a very big difference between increasing the pressure on a body and exposing a body to a vacuum!

In the latter circumstance the low pressure causes dissolved gases to come out of solution in dependent body parts, potentially causing gas emboli in blood vessels, fluid fails to be reabsorbed from the extracellular space causing tissues to swell and the lack of any oxygen means that asphyxiation will ensue.

Exposure to increasing pressure, on the other hand, is not too problematic over modest ranges (e.g. down to 40m submerged or 5 atmospheres total) because liquids are incompressible and hence the body tissues, being mainly water, just push back harder against the increased pressure without any net change.

But where problems creep in is in respiration. At greater depths the air breathed by a scuba diver has to be delivered at progressively higher pressures to compensate for the depth (this is the role of the regulator in the diver's mouth).

This means that the density of the gas being inhaled increases with depth, which makes it harder to breathe (it genuinely "tastes" and feels thicker the deeper you go), and the pressure at the alveolar exchange surface (where the gas meets the blood in the lungs) is much higher. Consequently much more gas dissolves in the blood than would do normally. In fact, beyond about 3 atmospheres pressure it's said that you don't actually need red blood cells because so much oxygen dissolves in the plasma alone!

But this increased gas dissolution can cause a number of problems and foremost among them is nitrogen. This is poorly soluble in water but will dissolve in increased amounts as the pressure rises. As a result, at depth the blood becomes saturated and the gas equilibrates with / partitions into the body tissues too. This includes the brain and if the concentration of nitrogen dissolved in the brain tissue reaches a critical threshold it can cause nerve cells to begin to behave abnormally. This is called nitrogen narcosis ("drunkeness of the deep") and is the same effect by which certain general anaesthetics induce unconsciousness. An instructor who taught me to dive tried an experiment on himself, taking a writing tablet under water and doing calculations at progressively greater depths. He quite quickly noticed his performance deteriorating with increasing depth and time at depth, consistent with this claim.

Secondly - the bends! The increase in dissolved gases, especially nitrogen, can cause a painful condition called the bends. This occurs when a diver (or an underground worker too - look up caisson disease) surfaces too quickly. Under these circumstances the excess of dissolved nitrogen in the blood and tissues does not have sufficient time to leave the blood and be exhaled. Instead the lower pressure means that the gas cannot remain dissolved and comes out of solution forming bubbles. These can merge to form large bubbles that can block blood vessels, obstructing the flow of blood through dependent tissues. These tissues are starved of oxygen and become painfully ischaemic. If this occurs in the brain it can provoke a stroke.

The treatment is to quickly recompress the diver to force the gas bubbles back into solution, and then slowly and steadily decompress them back to atmospheric pressure.

Why is it mainly nitrogen that causes this problem? Because nitrogen is so poorly soluble and also because dissolved oxygen is used up by tissues and turned into carbon dioxide, which is both soluble and can also be transported as bicarbonate.

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