叹气sigh

Breathe a sigh of relief...It keeps your lung from collapsing. 

Extra deep breaths, or sighs, often express emotions such as sadness or relief, They also play another role which is less obvious, but vital to keeping you alive: sighing. This prevents tiny air sacs in your lungs from collapsing. 

【知乎】：人紧张恐惧时，会僵住，是进化结果，因为捕猎者眼睛首要捕捉的是突然变化(运动)。人倍受压力时，会不自觉触摸不同部位，通过末端神经反馈，进行心理抚慰。叹气是一种慰藉行为(Pacifying Behaviors)，可以恢复氧平衡。

sighing is so vital to your survival, you do it without thinking about it. On average people sigh about 12 times an hour. 叹气对于生存非常重要，我们会不自觉低叹气，人平均一个小时会叹气12次。