I gotta learn to use sarcasm smileys.
Actually, I always kind of wondered how come fish, in their low-oxygen environment, had such small oxygen-absorbing suface areas (gills.) I learned something today. Eighty percent efficient, huh? Thanks!
I figure I have to learn one thing a day or the day's a waste. So far, I've learned 25,933 things!
In proportion to the body mass of the fish, the gill provides a far larger surface area available for osmosis.
Mammalian lungs provide a smaller surface area per unit of mass.
It is really relative to the size of the animal, not the specific size
per se.
Another factor is geometry.
In the 1960s a Danish scientist names Dr. Kylstra performed a series of experiments first wiht lab rats, then with dogs. What he did was take a tank of saline solution, pressurize it, and oxygenate it to the point where it could hold enough oxygen to sustain mammalian life. The rats used in the experiment, then later, the dogs, seemed to demonstrate one thing;
Although the animals could breath the liquid in the above noted condition, the spherical shaped alveoli of the mammal's lung is the incorrect shape for this. Whether the respiratory medium is air or water, the act of breathing injects the new medium into basically the center area of the respiratory "sphere" -- the alveoli. Thus, the oxygen starts out from there, and through pressure differential ("osmosis") moves toward the wall, where the blood vessels are. Concurrently, the expired carbon dioxide must enter from the wall portion and move into the center.
This model works well when air is the medium. Gas molecules can travel quickly and thus the process chugs along.
But in water, the gas molecules travel far slower. Carbon dioxide thus builds up along the outer portions of the alveoliae, while the oxygen tends to concentrate in the center, doing the animal little good.
The gills of the fish are entirely different. Each gill is composed of what might be described as a series of stacked "plates" -- sort of like people stacking their kitchen plates in a cupboard. A blood vessel moves blood through the center of this
"string of "plates" (called "laminellae" IIRC). Each plate in turn can be flush with blood.
Also, these laminelli are far closer to each other than the diameter of the mammalian alveoli. This helps negate the slow motion of the oxygen/ carbon dioxide molecules in the liquid.
A number of experiments have been conducted over the decades in which artificial gills have been tried. Some have even been demonstrated to work, atleast insofar as to show that they were extracting oxygen from the water, and it was this oxygen that was being breathed by the particular person involved. However, the fact remains the individual was still breathing a gas mixture, as the artificial "gill" was actually separate and apart from the "mammal."
This might sound like the beginnings of a clever underwater breathing aparatus, except it still leaves the human dealing with problems resulting from breathing a gas; the threat of nitrogen narcossis, oxygen poisoning, the bends. Plus, there would be no guarantee that there would be enough oxygen in the water (especially in confined spaces) for any such artificial devices to work. A scuba tank atleast contains a known amount of oxygen and while it can malfunction, the diver remains inside known parameters of danger.
An implantable device that allows direct respiration of water?
Maybe. But it won't be until Buck Rogers or Jim Kirk are buzzing around the galaxy until that happens .....
[/end professorial mode]
