The human body is an amazing thing. All of our parts and organs work together in sync in order for our body to operate correctly. The lungs, for example, must work just right to keep us breathing. It is necessary for the intrapleural pressure to be negative for the lungs to remain inflated. This is the normal state of the lungs, the pressure within the pleural cavity is slightly less than atmospheric pressure.
This is where the term negative pressure comes into play. A condition called Pneumothorax will occur if the intrapleural pressure becomes equal to the atmospheric pressure. Pneumothorax can also be referred to as a collapsed lung.