Well, the pressure inside this space of our lungs is known as the intrapulmonary pressure. And the intrapulmonary pressure is the same as the atmospheric pressure because the lungs are actually exposed to our environment. And what that means is, inside our lungs, if the pressure outside the body is, let's say, one atmospheric pressure, then the pressure inside the lungs is also one ATM. Now, as our diaphragm flattens down and as the ribcage increases, that increases the volume inside the intraplural space, and that decreases the pressure inside that space. And eventually, the pressure inside the space will basically decrease to a point where it will be lower than the pressure inside our lungs than atmospheric pressure. And at this point, there exists a pressure gradient, a difference in pressure, such that inside the intraplural space, we have a low pressure and in the outside of our body and inside the lungs, we have a higher pressure.