Tension pneumothorax
The air or blood in the chest will push against and squeeze the heart.  This can cause serious problems.  If the heart is squeezed too much, it cannot fill with blood very well.  This means it does not have enough blood to pump to the rest of the body.  Every part of the body needs blood to survive.  If the heart cannot pump enough blood to the body, other parts of the body can start to die.