People can't breathe underwater. People can't breathe in most liquids. But it's not the liquid nature that prevents one from breathing from it. Air is just a medium from which we get our oxygen and theoretically, it does not need to be gas. It's called liquid breathing.
However, the solubility of oxygen in water at atmospheric conditions is 40 mg/L [1]. That means if you inhale 6 L/min, which is a typical rate of volume of air inhaled and exhaled at rest (parameter called respiratory minute volume)[2], at most you can 240 mg of oxygen. It is important to realize that this value is the maximum amount since not all of the oxygen can be extracted. 240 mg is not sufficient anyhow to sustain a person at rest.
Let's do a quick calculation:
VO2 (oxygen consumption) at rest is 3.5 ml/min/kg. For a 70 kg person (typical standard weight for medical stuff... Most "nominal" values are based on a 70 kg adult), this is 245 ml of oxygen. 245 ml to mg conversion can be done as follows using the ideal gas law:
PV = nRT
n = PV/RT
n * [MW] = PV/RT * [MW]
m = PV/RT * [MW]
P: pressure, V: volume, n = number of moles of it, R: ideal gas constant, T: temperature in kelvin.
P = 1 atm, V: 0.245 L, MW = 32 g/mol, T = 23 + 273 = 293 K, 0.08206 L atm mol¯1 K¯1.
m = 1 * 0.245 / (0.08206 * 293) *32 = 0.326 g = 326 mg
So, we need at least 326 mg of oxygen every minute, which cannot be supplied by water.
Are there other liquids with high oxygen solubility? What about perfluorocarbons? APF-140HP [4] is a mixture of isomers of certain C10 (meaning it has 10 carbons) has oxygen solubility of 49 ml O2/100ml. That's a maximum of around 2940 ml of oxygen in 1 min (assuming 6L/min inhalation).
In 1966, Clark and Gollan showed that a mouse can live while breathing in the oxygen-saturated liquid perfluorocarbon. Perfluorocarbons are hydrocarbons whose hydrogens have been replaced with fluorines. Oxygen interactions with fluorines is very strong for reasons I will not get depth here. This was shown in a movie called The Abyss (clip) as an incompressible oxygen supply source (when deep underwater the gas compresses due to high pressure and as the diver moves up it decompresses and bursts... maybe more on that later).
There is another important potential application of perfluorocarbon: synthetic blood. Blood transplants have many potential problems - storage, diseases spread, compatibility, availability, etc. Synthetic blood is one way to overcome such problems. But just being able to supply oxygen is not sufficient reason for using them. In addition to efficacy, toxicity, storage, ease of use, reliable manufacturing method are criteria that must be fulfilled. At the time of a review article [5] (2010), there were no perfluorocarbon based synthetic blood substitute that satisfied all of the criteria adequately.
The other sought after route for design of synthetic blood is hemoglobin based. Apparently, there were no hemoglobin based synthetic blood in 2014 [6].
Tomorrow, I will post about haemoglobin and why they're amazing. Maybe will shed light as to why our blood is so amazing and synthetic blood is hard to make.
ref:
[1] http://www.lenntech.com/periodic/water/oxygen/oxygen-and-water.htm
[2] http://biomed.brown.edu/arise/resources/docs/Biopac%20Lesson%2012%20Respiration%20Apnea.pdf
[3] http://www.ncbi.nlm.nih.gov/pubmed/15223593
[4] http://fluoromed.com/products/perfluorodecalin.html
[5] http://www.ncbi.nlm.nih.gov/pubmed/20698841
[6] http://www.fda.gov/biologicsbloodvaccines/scienceresearch/biologicsresearchareas/ucm127061.htm
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3191624/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC137239/
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