To understand how your lungs work, you need to understand how they function within the context of your chest, or thorax. In other words, your lungs don’t work within a vacuum. Wait — correction — your lungs DO work within a vacuum! (Well, almost a vacuum.) That’s because in order for your lungs to expand, there needs to be low pressure between the lungs and the wall of the thorax.
It’s easy to make a simple model to demonstrate how breathing works. Take a large soda bottle and cut off the bottom. Replace the bottom with a flexible rubber sheet from the hardware store, to represent the diaphragm, and attach it securely to the bottle using a rubber band or super glue. To represent a lung, you take a party balloon, insert it into the top of the balloon, and stretch the opening of the balloon over the bottle to attach it securely.
To inhale, pull down on the diaphragm. This increases the volume of the space between the bottle and the balloon, decreasing its pressure as a result. That space represents the pleural cavity, which is a very narrow space in the healthy human body but plays a crucial role in ventilation. When the chest expands, the lungs expand only because the pleural cavity is “vacuum sealed”. Its low pressure counteracts the natural tendency of the lung tissues to recoil. Thus, when the chest expands, the lung expands as well, and air is sucked into the lung.
The “vacuum seal” of the pleural cavity can be broken if the chest wall is perforated (such as by a bullet or knife wound). It can also happen if only the lung itself is damaged (which can sometimes be caused by physical trauma). In either case, when you attempt to breathe by expanding the thorax, air quickly enters the pleural cavity, where you don’t want it to be. This condition is known as pneumothorax (“air within the thorax”). Without the near-vacuum in your pleural cavity, there is nothing to keep your lung expanded and it collapses.
To represent pneumothorax in the “bottle model”, you can poke a hole either in the side of the bottle, or in the balloon itself — you’ll find that the balloon no longer expands. When I present this model to my class, I ask my students to imagine this last step. I’m always very excited (as shown above) just to get the thing working at all, so I’ll be darned if I’m going to poke any holes in it!