But do not cough
If you told someone mice could breathe under water, chances are you’d get a strange and pitiful look back. Mice like us breathe air to stay alive and any obstruction of the air-flow to their lungs would represent a life-threatening emergency that would be met with vigorous coughing to free the air passage from the obstruction.
Everybody knows that asthma attacks can kill and so does water when it fills the lungs and then leads to drowning. And yet, it is a fact that mice in specially oxygen-enriched perfluoro-chemical solutions under distinct experimental conditions can survive for hours, even days, submersed in the liquid without a need to swim to the surface to take a breath of air. But surely, the critical mind would object, mice don’t have gills like fish; they would have to use their lungs to breathe and they have to fill them with air. That is certainly true – under normal conditions, but as long as oxygen can be extracted in sufficient quantities from the substance that fills the lungs, be this air or an oxygen-enriched solution as mentioned above and the pressure gradients are optimal, the organism would survive. The big and as yet unsolved problem is to change the experimental underwater-mice back to air-breathing mice: that, to the best of my knowledge, has never yet succeeded.
However, living without lungs (and gills) is actually the rule in the large Lake Titicaca frog, for where it stays (namely at the bottom of the lake at a depth of perhaps 100 or more metres), it would be a herculean effort having to swim ever so often to the surface to take a gulp of air and then swim down again. It simply uses the rather loose and naked skin of its body, which due to its many folds and wrinkles even has an enlarged surface area that functions as a respiratory surface. Sea snakes that have been reported to be able to stay under water for up to 2 hours, possess highly vascularized gums where some gas exchange can occur.
The most successful animals, however, that need no lungs at all and live on land where they inhabit a wide range of terrestrial habitats, are members of the North American family of plethodontid salamanders. There are some 200 or so species of lungless salamanders, all approximately the length of a biro. Of course, they require oxygen and have to get rid of waste material in the form of carbon dioxide, but they use the outer surface of their entire body. Their skin, not bearing scales, feathers or fur and being relatively thin and constantly moist, is in direct contact with the surrounding air. A large proportion of the gas exchange (the equivalent to what happens in our lungs) apparently takes place in the toes of these amphibians, for they contain a particularly intricate meshwork of blood capillaries just under the skin. Consequently, it would be fair to say that these lungless salamanders breathe predominantly with their toes. It may sound a strange way of breathing, but it sure avoids having to cough.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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