Many of the problems encountered in diving are caused by the compressibility of gases. A simple solution would be to use breathing mixtures that are not compressible: liquids. The obvious danger is drowning. However, newborn animals can survive in water for surprisingly long periods of time. Signs of life have been observed in puppies submerged in water for as long as 54 mins (Edwards 1824). Newborn rats submerged in water at 37 degrees continue to make respiratory movements, can survive underwater for at least 40 minutes, recover when taken out of water, and develop normally into adult rats.
This ability of newborn mammals to survive in an aquatic environment can partly be explained by their tolerance to anoxia. Conceivably, administration of the necessary enzymes to adult mammals could restore their initial tolerance to hypoxia, but this is no more than science fiction at present.
Drowning mammals often inhale water, which can cause tissue damage and alterations in the volume and composition of body fluids which will cause death. However, if the lung is filled with a solution similar to blood, or one which does not mix with water, no such damage should occur. According to Henry's law, the amount of gas dissolved in a liquid is directly proportional to the partial gas pressure at equilibrium on the gas-liquid interface. Thus it should be possible to prevent hypoxic death by having a diver breathe a pressure-oxygenated liquid.
Medical technology has produced 'artificial kidneys' and 'artificial lungs' in which the blood of a patient, flowing outside his body, can exchange gases with the external environment. Thus it would seem reasonable to expect that a gas exchanger could be build modelled after the gills of a fish, enabling a diver with liquid filled lungs to obtain the necessary oxygen from the sea water.
It is clear that the development of liquid breathing and the construction of artificial gills, if feasible, could be of great importance. The possibilities of man exchanging respiratory gases directly with an aquatic environment had not been explored seriously until 1961, and it is, as yet (1978) difficult to predict the outcome of the research.
Some of the more important points to arise out of existing research into the subject are:
1) Clark and Gollan (1966) first used a fluorinated hydrocarbon as a breathing liquid. The solubility of oxygen in this liquid is some 20 times greater than in normal blood plasma type solutions. Frankly this doesn't say much, as the blood plasma itself is a poor carrier of oxygen, which is why the body needs red blood cells and haemoglobin to carry the oxygen. However, research into fluorocarbons has developed since and compounds found that can carry considerable amounts of oxygen compared to their volume.
2) The carriage of oxygen is not the only consideration. The body must also be able to eliminate carbon dioxide. Without this ability the blood's acidity will increase and damage occur. Currently this appears to be one of the major problems. While fluorocarbon mixtures exist which can hold extremely large amounts of dissolved oxygen, they appear to be particularly poor in their ability of absorb 'expired' carbon dioxide. The alternatives, such as water based fluids, or silicone rubber gas- exchange membranes, absorb carbon dioxide from 5 to 25 times faster than the human body wants to eliminate it, causing a mis-balance in the opposite (but equally harmful) direction.
3) The breathing liquid must be particularly pure. The experiments with dogs showed acute inflammation, easing after 3 days, with immune reactions still present after 10 days, being normal after 18 months!
4) Diving problems will still occur.Even when breathing liquids, mammals are still sensitive to pressure, compression, and rates of compression. Experiments with mice have shown that pressure-induced effects, usually trembling of the limbs, are dependent upon body temperature, and start at pressures in excess of 50 bar, although normal body temperatures will increase the offset of the trembling to around 100 bar. Commercial divers are aware of this problem, referred to as 'high pressure syndrome'. The rate of compression seemed to have no effect, although rates in excess of 6 bar / minute appeared less favourable!
5) Liquids are considerably heavier than gases, and exert their own inertial and resistive forces. Experiments on man have demonstrated that maximal breathing rates appear to be about 4 litres per minute. Breathing out 0.5 litres of fluid can take over 10 seconds!
6) In general man tolerates the filling of one lung at a time with liquid remarkably well. After supplemental oxygen by mask for a few hours, they are usually up and around again the next day!
7) Air-filled gills, which the diver breathes out into, and which exchanges gases with the sea water are fundamentally flawed. While oxygen will indeed be absorbed into the gill space, inert gases will pass rapidly into the sea water, and the diver must carry a cylinder of inert gas in order to prevent the rapid collapse of the gill under pressure. Even if a membrane was found that was permeable to oxygen and impermeable to inert gases, the diver would continue to lose gas through his skin, and again the gill would eventually collapse.
8) Blood-filled gills could be made out of silicone rubber 1/1000 inch thick, as has been used in kidney machines and artificial lungs. However, to avoid problems with oxygen partial pressures, the entire cardiac output would have to be routed through the gill. Even if a surgeon could establish such an external link which didn't 'leak' or 'break', and which did not restrict a diver in his underwater activities, the operation might condemn him to live the rest of his life underwater. Open heart surgery in 'Alpha 2' before and after the dive doesn't really sound worth it.
Surely the research will continue, and feature more and more in science fiction and fact. Anyone who hasn't already seen the video 'Abyss' is recommended to hire it from their local video store. No doubt we will see artificial gills work in reality sometime in the future, but I honestly cannot see them becoming popular in sports diving in my lifetime!