Since the lung and chest are elastic, we can represent them with springs. Under normal conditions, they are coupled together: the “lung” springs are stretched and the “chest” springs are compressed. During a pneumothorax, the lungs and chest are independent and the springs representing them go to their relaxed positions, causing the lung to collapse and the chest wall to enlarge.
This analogy is honestly completely flying over my head. How are the lung and chest wall “coupled”? Heck, earlier in this book it’s mentioned that one of the two forces keeping the lungs from collapsing is “the surface tension between the lungs and the chest wall” (the other being the intrapulmonary pressure).
This and the said analogy make it sound like there is a physical-contact adhesion force between the lungs and the chest wall, which, from what I’ve looked into (and several other analogical figures the book has provided), seems to not be the case.
I believe this analogy of “coupling” is probably related to pressure relations, but I don’t understand how those are exactly set up either, and why during a pneumothorax they just cease as the two become “uncoupled”.
For those interested, the book is Medical Physics by John R. Cameron.