A sigh is a deep breath, but not a voluntary deep breath. It starts out as a normal breath, but before you exhale, you take a second breath on top of it," Feldman explained. "When alveoli collapse, they compromise the ability of the lung to exchange oxygen and carbon dioxide. The only way to pop them open again is to sigh, which brings in twice the volume of a normal breath."
Photo by Quinn Dombrowski/Flickr. Image of lungs sourced from ScienceAlert. Available at: https://www.sciencealert.com/images/articles/processed/Lungs_web_1024.jpg