Curious Kids: why do we sigh?

Curious Kids is a series for children. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

Why do we sigh? – Sophie, aged 4, East St Kilda, Melbourne.

Dear Sophie,

Thanks for your lovely and excellent question about sighing.

Since you asked a question about sighing, it is probably safe to say that you already know something about breathing. But, for the benefit of everyone else (who might not be an expert like you or me) I am going to talk a little bit about breathing first.

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The tiny sacks of life

You have two lungs that you use to breathe. Each lung is a stretchy sack, kind of like a balloon, but not empty like a balloon. Inside your lung are millions and millions and millions of tiny sacks called alveoli. These tiny sacks called alveoli are very important.

To keep living, we need a gas called oxygen to be moved out of the air and into our blood. And that’s not all! We need another gas called carbon dioxide to be moved in the opposite direction: from out of your blood and into the air.

For these two things to happen, the air and your blood need to be brought very, very close together. This is exactly what what happens in the tiny sacks called alveoli that fill your lungs. This is just as well! If you don’t get oxygen in and carbon dioxide out, you will die, which is bad.

When you breathe in, the tiny sacks called alveoli (seriously, your lungs are full of ‘em) get filled with air. At the same time, blood (pumped by your heart) flows around the sides of the tiny sacks.

This brings the air and the blood very close together and so allows the gases to move in the right direction (oxygen in, carbon dioxide out).

Moving these gases is essential for survival, which means that breathing is essential for survival, which means the tiny sacks called alveoli are very, very essential for survival indeed.

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Back to sighing

Here’s the thing about the tiny sacks called alveoli: you have so many of them that, actually, you don’t need to use them all at once. When you are sitting quietly, you can get move enough gas by using just some of them.

This is all fine and well, but if you are sitting quietly for a long while, these tiny unused sacks stay unused. And when a sack stays unused for a long time, it tends to collapse in on itself. When the tiny sacks called alveoli that fill your lungs collapse, they can’t be used to move gas in or out of your blood any more, which is bad.

Fortunately, there is a solution: the sigh. A sigh is breath that is deeper than usual, so sighs fill your lungs with more air than a normal breath would.

This means that any tiny sacks (called alveoli) that are not being used get filled up with lovely air when you sigh. This stops them from collapsing and averts the danger! Hooray!

Most of the time, your brain takes care of breathing for you and, fortunately, it takes care of sighing for you too. This means that you don’t need to worry about remembering to sigh.

If you need to worry about remembering anything, Sophie, it’s that you should never stop asking this sort of lovely question. Congratulations: you are now an expert on breathing and sighing.

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David Farmer receives is employed on a grant awarded by the National Health and Medical Research Council.