Dear G@Diver:
Liquid breathing
There are some nice references above from Padipro. It is my understanding that the main problem with liquid breathing is the inertia of the fluid. Air breathers take in air, stop the flow and then reverse its direction. This is fine since the air has low mass and inertia.
When you now attempt to do this with liquids, the situation is more difficult. Stopping and reversing direct is a very inefficient process and the work of breathing is considerable. Fluid breathers, such as fish, take in their breathing medium, pass it over gills and exit it, without the need to change direction, through the gill slits.
Dr Deco :doctor: