A bottle model to explain lung ventilation and pneumothorax

bottle model of lung ventilation and pneumothorax

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!

The versatile pharynx

Pharynx depicted with legs, eyes, and nose.

Here’s a body part you’ve probably never seen in isolation — and almost certainly not with legs, and a face of its own!  But these embellishments serve to emphasize that the pharynx is a middle-man, straddling two different organ systems and negotiating the competing demands of each.

Ideally, air travels in through the nasal cavity, all three parts of the pharynx (nasopharynx, oropharynx, laryngopharynx), and then the larynx, on its way to the lungs.  While eating, breathing is paused while food travels through the oral cavity (mouth), the lower parts of the pharynx (oropharynx, laryngopharynx), and the esophagus on its way to the stomach.  But as you can see, these two pathways overlap in the pharynx.  As a result, the pharynx, along with associated structures, plays a vital role in regulating the passage of air and food.

There are four ways that food and air movement can differ from the ideal situation depicted above, and these “mistakes” vary all the way from innocuous (and perhaps entertaining) to life-threatening.

First, food could move through the upper pharynx (nasopharynx).  Sometimes food is expelled this way if you sneeze or laugh while food is in your mouth.  But most of the time, the soft palate, a muscular structure, closes against the back of the pharynx to prevent this.  If you want to “get in touch” with your soft palate, try snorting – you just closed your soft palate and then opened it while inhaling.  A word to the wise: Restrict your snorting to air and mucus!  But unfortunately, if you can think of it, somebody will try it — the FDA had to issue a warning not to snort chocolate powder.

Second, air can move between the middle pharynx (oropharynx) and the mouth.  Wait, you may say, isn’t this exactly how we breathe?  If that’s your impression, then you probably have a congested nasal cavity, and it’s true we can breathe through the mouth, although it’s less than ideal.  The reason is that the nasal cavity has complex folds that help to trap debris and pathogens and do an optimal job of “conditioning” the air before it reaches the lungs.  So if, as I often do, you find yourself walking behind a group of sneezing and coughing students, it may help to close your mouth and breathe through your nose.

Like the rest of your respiratory and digestive tracts, the pharynx is a moist habitat suitable for invading microbes, and being near the entrance to both tracts, it is a common place for infections to begin.  What’s known as a “sore throat” is generally a viral infection of the pharynx.  That’s likely why we have several immune structures — the tonsils — in the walls of the pharynx.

Third, air can move between the lower pharynx (laryngopharynx) and the esophagus – the food tube leading to the stomach.  According to healthline, in most of us about two quarts of air move into the stomach this way, each day, from the small amounts of air trapped in our food or drink.  And of course, when air leaves the body through this route it is called burping — technically known as eructation.

The fourth, and by far the most serious problem, is food traveling from the laryngopharynx into the larynx.  The larynx, or voicebox, sits in front of the esophagus and is the gateway for the lower respiratory tract.  “Inhaling” your food is dangerous — in the US, over 5,000 people die from choking each year. Our main protection from choking is the epiglottis — a laryngeal cartilage between the oropharynx and nasopharynx, which folds down to close off the entrance to the larynx, every time you swallow.

Your pharynx handles a lot of traffic through the body, and deserves some respect – definitely nothing to snort at!