Sam's Weblog

The Lonely Lung

26 June 2017 — permalink

This is yet another tidbit from the catacombs of my physiology studies. I’m supposed to be revising for exams, but I’ve given myself this somewhat-productive procrastination time.

Humans have quite a few body parts that we don’t need. We call them vestigial features (named after the latin word vestigium, which means “footprint”). There are some pretty weird ones. Did you know that our outer ears (that part you can grab) have muscles that never move? Ever wonder why guys have nipples? You’re almost certainly heard of your appendix, and have quite possibly had it removed after it got infected1.

We can come up with pretty plausible explanations for why most of our vestigial features were useful at some time. A bit of educated guessing and looking for similar features in animals helps a lot. This year, however, I’ve stumbled upon one that has us science nerds stumped. Airway muscles. Let me explain a bit…

Your mouth has two tubes coming off the back of it: your oesophagus and your windpipe. The windpipe travels down to your lungs and splits in two: one side for each lung. From there, it makes lots of smaller branches to get air to all the places in your lung. They’re called bronchi, and they’re what hurt when you have bronchitis.

As we look down into the lung, we see that the bronchi are surrounded by a layer of muscle that can contract to squeeze them, making them narrower. People who have asthma have airway muscles that contract a lot, making it much harder for them to breathe2. Here’s the problem though: unless you have a disease that makes these muscles contract, they basically never do anything.

If we use drugs to deactivate all of the muscles in the lungs, nothing happens. If we use genetic engineering to create mice where these muscles don’t work, nothing happens. Nobody has been able to find a single adverse reaction to losing these muscles, and the only time they’re ever important is when they make people sick by suddenly working too well!

This is especially strange because these muscles have a very good connection to the nervous system, so controlling them wouldn’t be a problem. This is a lot like paying for a super fast broadband connection, but then reading books while sitting next to your computer all day.

Some Wrong Ideas

Everybody’s ideas so far are wrong, but these are some interestingly wrong ones.

  1. They’re useful in coughing. The idea is that they can give the lungs and extra squeeze from the inside to help them get all the crud out, but that’s not what ends up happening. A good cough needs the bronchi to be wide open so that air can be pushed out as quickly as possible. Smaller bronchi means slower airflow, making for less effective coughs that take more effort.
  2. Matching blood and oxygen flow in the lungs. The techy term for this is V’/Q’ matching, and it’s done to make sure that the parts of the lung that have lots of oxygen to give are surrounded by enough blood to give it to. Thing is, the muscles around the bronchi aren’t responsive to oxygen levels so they’d have no idea where to send the air. Better yet, we already have a very clear picture of how the muscles surrounding your blood vessels redirect blood to follow the available oxygen.
  3. Protecting the lung from harmful chemicals. Superficially, this sounds like a good idea. If air coming into the lung contains something bad for the lung, shut it off or slow it down. Alas, remember that if one part of the lung is detecting something harmful, every other part will be too. Closing down all of the bronchi in the lung would probably kill you a lot faster than whatever exotic pollen you were breathing in.

Conclusion

There really isn’t one! I just thought this was a fantastic example of one of the little puzzles in our biology just waiting to be solved. I hope you enjoyed it too. Don’t forget to check out the footnotes too, there’s some really neat info in there!

I got most of this work from my lecturer, who in turn based it heavily on Mitzner, W. (2004). Airway smooth muscle: The appendix of the lung. American Journal of Respiratory and Critical Care Medicine, 169(7), 787-90.

  1. Why continue to have an appendix at all, you ask? The problem is that an organ can’t disappear in one generation, so it instead becomes progressively smaller over thousands of generations (think “use it or lose it”). Getting smaller is easy, but disappearing is hard. When an organ is very small, its chances of getting infected get much larger. People throughout history who had genes giving them an appendix just small enough to become infected generally didn’t survive long enough to have many (if any) kids. When they didn’t have kids, people with genes giving them an appendix just big enough were left to make all the babies. These two forces – appendix shrinkage due to disuse, and growth to avoid infection – are playing a species-wide game of tug-of-war and have settled at the size we have now. There is still some variation (in the same way that not everybody is the same height) but the people on the more infection-prone end of that scale don’t have such a problem anymore because it’s now quite cheap and safe to remove an infected appendix. ↩

  2. People with asthma have a few other problems too. Even when the muscles in their lungs are fully relaxed, their airways are a lot smaller than they should be. A person with asthma has chronically inflamed airways, which is like having a nasty rash inside your lungs at all times. Just like how your skin gets a bit puffy when you have a rash, so do your bronchi. The other problem is mucus. It usually helps your lungs by trapping and removing things like dust and pollen, but people with asthma produce too much of it. When they can’t get it out of the way quickly enough, it takes up space inside the bronchi. They end up coughing a lot to get rid of it, but sometimes it can get very serious if left untreated. ↩