The reason we sigh

Contrary to the song As Time Goes By, a sigh is not just a sigh: it’s a vital reflex that keeps us breathing properly. Bill Condie reports.

Sighing is an emotional response to things like exhaustion and despair. It also has a physiological purpose. – aydinynr / Getty Images

Although associated with emotions ranging from wistfulness or relief to despair, sighs – long, deep breaths – also occur spontaneously every few minutes to reinflate alveoli, the tiny air sacs, in the lungs, in a way an ordinary breath cannot.

The sighs are triggered by a control system in the brain when it senses inadequate oxygen in the system.

That system also controls the frequency, which ranges from several times per hour in humans to dozens of times per hour in rodents during normal breathing, researchers write in a paper published in Nature.

Mark Krasnow of Stanford University and Jack Feldman from the University of California, Los Angeles, identified approximately 230 neurons, which project to a small subset of receptors called the the pre-Bötzinger complex.

“It’s made up of small numbers of different kinds of neurons,” Krasnow told the Guardian.

“Each functions like a button that turns on a different type of breath. One button programs regular breaths, another sighs, and the others could be for yawns, sniffs, coughs and maybe even laughs and cries.”

A micrograph of a cross-section of a lung, showing the alveoli – the tiny air sacs where gases are exchanged. – EYE OF SCIENCE / Getty Images

The researchers tested the function of the system in rats by injecting the pre-Bötzinger complex with neuropeptides – the protein-like molecules that neurons use to communicate.

The rats’ normal sighing rate jumped from about 40 to 400 times per hour, while not changing the rats’ normal breathing pattern.

The only mystery left, it seems, relates to that song.

“While we have a good handle on the physiological reasons we sigh, it’s still a mystery what function or functions it plays during emotional states,” Feldman said.

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