A consistent feature in many scifi movies is how people can actually breathe liquids like a baby in the womb. We got that in Matrix, Resident Evil, Star Wars or the X Files, just to mention a few. The amazing truth is that it is actually possible to breathe liquids, as portrayed in Cameron’s film the Abyss. In fact, the rat in the film was indeed put into the colored liquid for real and survived the experience, as seen in the DVD extras on the film special edition. Too bad it seems that animals won”t survive much yet afterwards after it, but there is at least a company, Alliance Pharmaceutical Corporation which manufactures Liquivent, a so called perfluorocarbon. This substance is basically a hydrocarbon with some of the hydrogen replaced by fluorine, and it allows gases -in our case oxygen- to be easily dissolved into in the liquid, up to a 65% against the usual 21% in the air.
Breathing liquids would be quite useful for deep sea diving. Usually, sport diving runs with a two stage regulator. Lungs won’t open for air unless it is at the same pressure than the water around, so, basically, air is first compressed into a portable enough bottle at around 200 atmospheres so that it lasts for a while. A first stage reduces this to 10 atm, approximately. goes to a second stage at the mouth piece that adapts the air flow to the person’s breath thanks to the effect of the surrounding pressure. When the person exhales, used air either goes into the water as bubbles (open circuit) or into a canister full of soda lime to reuse the remaining 80% of oxygen remaining in exhaled gas, so that equipment is more efficient (close circuit). This breathing technique works similarly in diving bells, used to work underwater, where air is inyected into an inverted box at the same pressure that water is around and, hence, keep the inside dry even though the bottom of the bell may be open. An example of this phenomenon can be see, for example, in Lost season 3 “Looking Glass” underwater station.
Nevertheless, time spent at the bottom of the sea depends on two factors. First, the deeper we are, the more pressured the air we need to breathe and, hence, the shortest the time the bottle lasts. This limitation can be softened with different gas mixes, mostly nitrox, but not too long. Second, tiny bubbles of air dissolved into our blood tend to expand when the outside pressure gets lower, i.e. we are going up, and may block the blood flow. This phenomenom is known as decompression and may kill the diver. Hence, it is necessary to go up slowly and making stops to send these bubbles out of our system.
Time limits allowed in a dive, even though they vary for each person and situation, can be roughly calculated in a very simple way. At the surface, we get 1 atmosphere (atm) of pressure and ordinarily breathe around 15 litres of air per minute. Pressure increases 1
atm each 10 meters roughly, so at 20 meters (3 atm) we would consume 3 x 15 = 45 litres of gas per minute. If we fill a 12 litre bottle up to 200 bar and typically want to keep a 25% to go up we have 75% × 200 × 12 = 1800 litres to breathe underwater. At 45 l/min we get a maximum of 1800/45 = 40 min, that may go up to one hour because your typical diver won’t spend all the time so deep, but could also shorten if we indeed go deeper. If we go fully close-circuit now, your average diver consumes about 1 litre of oxygen per minute, so a 3 litre oxygen cylinder filled to 200 bar and leaving 25% in reserve will be able to do a 450 minute = 7.5 hour dive (3 L × 200 bar × 0.75 / 1), that is, however, limited by the life of the soda lime scrubber right now.
Calculations aside, long dives at significant depths require the so called breathing liquids to fill the lungs, and Liquivent is supposed to provide this some time soon.
Note: Breathe today is a song by rock group Flyleaf