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On average, a human adult  sighs every five minutes. But that’s not because we’re  perpetually exasperated.

Sure there are emotional sighs, but  there are also physiological ones. And we actually have to take  those extra deep breaths — they’re crucial to healthy lung function. But we didn’t figure that out until we  invented machines to help people breathe.

The first tank ventilator – basically a giant box you can put someone in to breathe for  them – was invented back in the 1830s, but the most familiar iteration  came about in the 1920s. This was the infamous Iron Lung — a  phrase coined by an unknown journalist. And it’s mostly associated with the  polio epidemics that swept across America and Europe in the early to mid-20th century.

They work by applying external negative pressure. Dropping the pressure inside the  tank causes a corresponding drop in the patient’s lungs. That causes  outside air to flow into the lungs.

And by increasing the pressure,  the air flows back out. It replaces the job of the diaphragm,  the muscle that controls airflow. In the case of many polio patients,  this muscle would become paralyzed.

Sighing was first described,  medically speaking, in 1919. By the time they were using the  Iron Lung on polio patients, treatment reports stressed the  importance of administering deep breaths interspersed with the regular ones. In fact, one of the device’s early test subjects actually requested deep breaths for comfort.

As you breathe, tiny  fluid-lined sacs in your lungs called alveoli will occasionally collapse. It’s totally normal, but they  can’t re-inflate under a typical, shallow breath because the fluid causes  the sides of the sacs to stick together. And when they’re collapsed, they  can’t exchange CO2 for oxygen like they’re supposed to.

At least not very well. Sighing resets the lungs by  re-expanding the alveoli. So in extreme situations, sighing could  be the difference between life and death.

Extreme situations like needing  an Iron Lung to breathe for you. By the mid-20th century, devices like  the Iron Lung were being phased out in favor of the method we  see in modern ventilators. Instead of negative pressure, they directly  pump air down the patient’s throat.

And research suggests sighs can be  helpful in certain — though not all — cases of positive pressure ventilation as well. We’re also still learning how we sigh.  But it seems to be remarkably simple. We now know that the human body  senses when we need to sigh by using two kinds of sensory receptors.

One detects the amount your  lungs’ volume has collapsed, and the other detects the  concentration of oxygen in your blood. Sensing the need to sigh then gets  converted into the instructions to actually take a deeper breath. Research has also shown that, at least in rodents, sighing is controlled by tiny  clusters of neurons in the brainstem.

This is the smallest number of neurons  ever linked to such a fundamental behavior, suggesting that sighing really is pretty basic. These neurons instruct the production  of two signaling molecules, which tell the lungs to turn  a regular breath into a sigh. Scientists are still working to figure  out what causes emotional sighing, but in the meantime, understanding  the intricacies of physiological sighs will allow us to help regulate healthy  breathing in patients who need it.

It could also lead to medication that  helps people manage breathing problems brought on by anxiety or stress,  as well as neurological disorders. It’s a surprisingly fundamental behavior  that we’re all still trying to understand, but clearly, sighing is about more  than just signaling how bored you are. Thanks for watching this episode  of SciShow.

And don’t forget: you probably sighed at least  once during this video! If you’d like to help us unravel more  bizarre facts about the universe, and about ourselves, why not  consider supporting us on Patreon? We’ve got neat perks to say thanks, like  fancy facts and an exclusive podcast.

If you’re interested, check  out patreon.com/scishow. [♪ OUTRO].