glaucusb t1_j12ly0v wrote
Reply to comment by Scott_Abrams in Could being submersed in a sealed tank of fluid help humans survive heavy G acceleration in outer space? by cheeze_whiz_shampoo
There are liquids that could keep high concentration of oxygen such as perfluorocarbon (here is the liquid breathing link from Wikipedia). But their density is 1.5-2g/ml.
Why do we need a density similar to water and how similar they should be?
Wyattr55123 t1_j12pzdg wrote
I do find it amusing that liquid breathing does absolutely work and has been tested on conscious adult humans, only to discover that shock the feeling of liquid filling your lungs is really goddamn unpleasant.
Ausoge t1_j12ogtc wrote
If you were to fill your lungs with something with greater density than a human body (which is about the same as water on average) and then pull an extremely high-G manoeuvre, the heavier liquid would be more affected than your body. It'd essentially be trying to "sink" through you in the direction of the G force and probably crush something important.
Scott_Abrams t1_j12scj2 wrote
The reason why you need a similar density to water is because of differential density. With a similar density, forces will be exerted roughly equally but when there are non-uniform densities, those effects can be felt. Why does oil float on top of water? Because of the difference in density and the application of buoyant force. Oil is less dense than water and so it floats to the top. But what happens when a person has a different density than the fluid both inside (the cavities) and out? Imagine for a moment if your fluid density is more dense (you "sink") and if your fluid density is less dense (you "float") relative to the fluid immersing you. Higher/lower density doesn't really matter as the only thing that really changes is the direction you go in but regardless, you'll feel the sum total of all the different forces on your body, all at the same time, as your body tries desperately to resist the laws of physics and fails.
Humans are largely composed of fluid. Yes, the fluid is tucked away by membranes (cells) and we think of ourselves as solid, but we are still almost 60% water. Do you know how we use centrifugal force to separate red blood cells from the rest of the plasma? Same concept, differential density, only now the fluids and precipitates that comprise your being are being separated by ascending/descending density.
That's why you need a fluid with a similar density as water, because we're made of water and we need to keep that water inside our cells.
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