Respiratory System 7, Pleural membranes, teaching demonstration

There are two pleural membranes surrounding the lungs. Firstly, the visceral is the inner membrane with one visceral membrane surrounding the surface of each lung. Secondly, the parietal is the outer membrane lining the inside of the thoracic cavity; this pleural membrane also lines the superior surface of the diaphragm. Between these two membranes there is a film of fluid and a negative pressure of about -4mm of mercury. This negative pressure has the effect of sucking the two membranes together which means the area between the two pleural membranes is only a potential space. A potential space is when two membranes are immediately adjacent to each other but not directly connected by tissue, the result of which is that a space could be created should something be introduced between the two layers.
When the ribs move up and out, while the diaphragm moves down, the parietal pleural membrane will move with these structures as it is adherent to them and invaginates them. Because there is a negative pressure between the parietal and visceral pleural membrane, the visceral pleural membrane will be drawn up and out with the chest wall, and down with the diaphragm. As the visceral pleural membrane is adherent to the surface of the lungs, the lungs expand with their visceral pleural membrane. This is why the lungs expand with the chest wall and diaphragm.
The visceral pleural membrane expands because of the suction of the visceral pleural membrane onto the parietal pleural membrane. Therefore if any air gets into the potential pleural space (that is between the visceral and parietal pleural membranes) for example, through a stab wound, the negative pressure will be lost. Movement of the parietal pleural membrane would then no longer result in movement of the visceral pleural membrane and the lung would collapse. This condition is known as a pneumothorax and requires immediate emergency medical treatment. Fortunately there is usually time to transfer such patients to hospital as the two lungs are each surrounded by a separate visceral and parietal pleural membrane. This means one penetrating injury only results in the collapse of one lung and it is possible to survive on the other single lung for a time.

[RickyC]

Thanks! this answered a lot of questions I had

[Mr.ZEZExSAURUS]

hello I have a question.
can the pleural cavity increase its volume? I always read that when we inspirate the intrapleural pressure becomes more negative, that increase the transpulmonary pressure and that allows the lungs to expand.
I always thought that when the diaphragm contracts and our torax expands and we increase our thoracic volume the volume of the intrapleural cavity will increase and because of the boyle's law the intrapleural pressure will decrease.
But I never was entirely sure and now I see that you are showing that the two membranes (parietal and visceral) never separate from each other (in normal conditions). And I also found that the pleural cavity is a virtual space in reality so now I have my doubts.
Thanks