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Why do we yawn?

Why do we yawn?
Shangita Haq from Greater London (Age 35-44)

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One Response

  1. Increases in lung volume lead to an increased production of a substance called surfactant. Surfactant is a substance which decreases surface tension in the lungs and thus makes the lungs easier to inflate. During quiet breathing the size of our alveoli, the small air sacs in the lungs, changes minimally. Thus no new surfactant molecules are produced and as the ‘old’ surfactant molecules are broken down over time the elastic recoil of the lungs increases as surface tension rises. This slowly reduces functional residual capacity (the amount of air left in the lungs at the end of a normal expiration) and this is detected by stretch receptors which send a signal to the brain which in turn sends a signal to the lungs to tell you to yawn (or sigh). This will expand the lungs, increase production of ‘new’ surfactant and thus make the lungs easier to expand, reducing the amount of work the lungs have to do during quiet breathing.

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