Blocked breathing.
Smoking, cardiopulmonary diseases and intense exercise are causes of blocked breathing. And to complicate things much further, emotional traumas also block breathing.
Blocked breathing happens when our ribs are too high because of rigidity of the breathing muscles (diaphragm, intercostal muscles, etc.). If we pay attention of our breathing without intervening, we notice the small amount of air that comes in and out. However, we are actually full of air. We cannot get this air out unless we do a forced exhalation. We are in inhalation position even when we do breath out. And this leads to a catch-22 situation.
The more rigid our muscles are, the less our ribs descend during exhalation and therefore the less air will be breathed out. If little air is breathed out then little air is also breathed in and we suffer a lack of oxygen. To compensate, we put more effort into inhalation or we increase breathing frequency, leading to more and more contraction of the muscles. We can stop this catch-22 situation, but some of the elasticity that the muscles have suffered following years of contraction can never be regained.