Do you feel better after breathing a deep sigh? It’s relaxing. And it turns out there’s a good reason why we feel relief after big breaths, It comes down to a special fluid inside our lungs. The liquid — pulmonary surfactant — helps our lungs work. Without it, we’d be in serious trouble. But researchers at Swiss university ETH Zurich found that deep breaths have an interesting effect on how this fluid interacts with the lungs. 

The new research, published in the journal Science Advances, could have meaningful implications for future medicine and therapies. 

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‘Magic’ lung fluid to the rescue 

For a long time, scientists have been interested in how this fluid helps us breathe. In the 1980s, research into this area resulted in a life-saving treatment for babies born prematurely with underdeveloped lungs. 

By extracting fluid from animal lungs and injecting it into those of premature infants, doctors were able to reduce respiratory distress syndrome from developing right after birth. The fluid reduces surface stress in the lungs.

«This surface stress influences how compliant the lungs are,» said ETH Zurich professor Jan Vermant. «The more compliant the lungs are, the less resistance there is to expansion and contraction — and the easier it is to breathe.»

Breathing deep to ease tension

The research team wanted to see how lung fluid behaved when it was stretched and recompressed, so they simulated the movements of normal and deep breaths in the lab. 

They measured the fluid’s surface stress in each case and compared the data. The researchers found that surface stress decreases significantly after a deep breath. 

The utmost layer of the thin film that the fluid leaves on a lung settles and hardens over time. 

«Directly at the boundary with the air, there is a slightly stiffer surface layer,» said Maria Novaes-Silva, a doctoral student in Vermant’s research group and the study’s first author. She said that the pronounced stretching and compression of the pulmonary fluid that comes with a deep sigh actually rearranges the surface layer’s composition.

So, after you let out a deep breath, it becomes easier for you to breathe. That’s why we feel a sense of relief from a deep sigh. 

An ETH Zurich spokesperson said that clinical practice has also found that breathing becomes increasingly complex with constant shallow breaths. So, the lab measurements seem to follow real-world observations. 

The researchers theorize that this study may contribute to a better understanding of adult lung failure.