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Liquid breathing to resist bone loss
For a broader outline on "Liquid breathing", see: 
From Wikipedia: "Liquid immersion provides a way to reduce the physical stress of G forces. ... Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application."
However, consider if a suitable compound was found. Ignore the issue of acceleration. What is a potential big problem on long duration space flights or indefinite habitation in microgravity is bone loss. The body adapts to lack of stress by eliminating bone and muscle that in no longer used, but this makes returning to a strong gravity field problematical. Savage has proposed future medicine or genetic engineering to overcome this, and one can also in theory create big rotating O'Neill-style structures, and there is also (boring) microgravity exercise, but what if there was another way?
Fish and mammals like whales and dolphins spend their entire lives in a sort-of microgravity suspended in water. They can have strong bones and muscles. Aquatic therapy in a pool is often recommended for humans to improve strength. So presumably, like the mythical Seapeople, if humans could breathe a liquid while living in outer space in microgravity (like during a long trip to Mars), then by just living and moving around in a liquid environment in a space craft, they would maintain their muscle tone and bone mass. The liquid might also provide cosmic ray shielding, and might even be designed to use cosmic rays to clean or re-oxegenate itself.
An important difference between an undersea civilization and a liquid-breathing space-faring one is that there is no water pressure in space in Zero-G (beyond surface tension or compression). Thus, liquid structures could extend in space for miles in three dimensions of endless tubes, all at essentially the same pressure. So there would be no risk of the "bends" when moving around this construction. Another possibility is that a big drop of liquid a mile across might be all one needed for a large space habitat floating in zero-G if the surface tension held the liquid in. This might make it trivial to construct habitats, and micrometeorites might pose less of a problem as the surface would heal itself by surface tension. Comets and asteroids could be mined, but the major result need only be a stream of this breathable liquid, which could be shaped into habitats of desired size by how much liquid was added.
This is all speculative at this point.
Liquid breathing obviously should not be experimented with outside a well-monitored research laboratory situation due to the risk of drowning or lung damage. Various research has already been performed, see Wikipedia for links.
Anyway, all speculative. But kind of cool (to me). I was inspired a little by sci-fi I've read of zero-G pools in space habitats. Might need some simulation work to be more convincing as to surface tension etc. Can't remember if I read this somewhere or not. I did have Savages zero-g bubble design in mind, but those just have a six foot exterior water shield and air in the rest. And as I reflect on it, I do remember reading about zero-G ponds in the book "The Integral Trees" by Larry Niven:
I can now see that maybe refitting people with gills :-) might be easier than spinning a big O'Neill habitat (which could get unbalanced as loads move in it). And this sounds klunky:
But, since most people might not go for that, :-) and these sound klunky:
it does suggest that at the very least dolphins and whales are better adapted for life in space than people are. One could imagine air stations for them in an immense water droplet. Might be a nice payback for all the trouble we've caused them. And easy to build.
So see -- getting intelligence into space maybe quite easy, if we focus on making dolphins happy. :-) See:
It's ironic that scientists are so obsessed with the "brain to body mass ratio" because it lets scientists with small brains (relative to dolphins, whales, and elephants) feel superior. :-)
"You cannot always *have* happiness, but you can always *give* happiness." -- Anonymous
Maybe aquariums should start funding space research? :-)
And it does suggest potential sea/space crossovers like mentioned in the "Maybe we should wait" thread on the OpenVirgle list.
OK, so it's been done before apparently now that I look: :-)
Now if someone could just get a mind uploading thingy working to a Cetacean brain somehow, maybe as a dual personality? (Work your way up to "The Prisoner", episode 13, with Doctor Selzman :-) But that might be bad for the Dolphins -- essentially, humans would be acting like the Goa'uld. :-(
Maybe we should just do one really nice thing for Dolphins and let them inhabit space, and maybe they would remember us someday in their dreams? :-)Theoretical::True