The double inhale in a physiological sigh reinflates collapsed alveoli, increasing surface area to allow for greater offloading of carbon dioxide.

“The second inhale is really important because your lungs are not just two big bags of air. They're two big bags of air with lots of little sacks, millions of sacks. And if you were to lay out those sacks, their volume is as big as a tennis court. And that allows both the intake of more oxygen but also the offload of carbon dioxide. So when you do the double inhale, it reinflates any of these little sacks that have collapsed and in doing so, it allows you to offload more carbon dioxide.”