yawn image from New Medical Terms



Definition The involuntary opening of the mouth, which is thought to be a response to the slowed breathing typical of boredom, exhaustion or sedentary (in)activity, and characterised by increased CO2 and reduced O2 in the circulation. By opening the mouth wide and inhaling deeply, yawns suck in O2 and expel CO2, bringing the gases back to normal. 

Reference answers.google.com/answers/threadview?id=525353

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