|MadSci Network: Medicine|
This is a good question, I don't completely buy the carbon dioxide/oxygen explanation either, although there might be something to it. I have heard an interesting theory, though. I was told this by a Physiology professor here at the Med. School. He tells a story of a hospital, around the end of the Second World War, that was having a terrible track record with patients dying during surgery, with about every surgeon at the hospital losing at least one patient a week. All the patients were dying of fluid in the lungs. However, there was one surgeon that wasn't losing any patients, and none of the people he operated on had fluid in their lungs. In order to curb the trend in the hospital, the other surgeons watched this one surgeon during surgery, to see what he was doing that was different from them. After watching several of his surgeries, they concluded that this surgeon preformed just like the other surgeons, with one exception. The successful surgeon liked to monitor the respiration of his patients during surgery by watching the inflation of the respiration bag. When this bag filled with air, he couldn't see the breathing anymore, so he emptied it by squeezing the air out of it, inflating the patient's lungs. This is the only difference between his technique and the other surgeons, but invariably his patients survived with no complications.
Now, the theory goes like this: The inflation of the patient's lungs acted as a simulated yawn, which had the effect of redistributing the alveolar surfactant. Therefore, the possible purpose of yawning is help in surfactant redistribution. It may be that our bodies spontaneously inflate our lungs to refresh or speed up the surfactant redistribution. Anecdotal evidence supports this role, for every animal that uses a surfactant like substance in their respiratory systems to decrease surface tension and prevent lung edema, yawns, including fish. Now, why people tend to yawn when they are tired, or how/why yawning seems to be contagious remains a mystery, but may be related to a stimulus modifiable reflex. Hope this answers it for you!
Benjamin Walker, Ph.D.
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