Wait... The Lungs are NOT the size of a Tennis Court?

In Search of Proof

I was researching the aging respiratory system for a geriatric trauma class when I came across a source that said something to the effect of:

‘Geriatrics lose about 1/3rd of their lung function/capacity over so many years.’

The first thing that popped into my head was:

‘Wow, that’s like losing a third of a tennis court as you get older!’ (I had heard this tennis court metaphor repeated many times.)

My analytical brain kicked in and wanted to quantify this and build an illustration to visualize how substantial this decrease was. Of course, I needed to reference this work, since it’s a rather specific claim about the human body. I opened Google Scholar and PubMed and began searching: “human lungs size tennis court” -> {search} (click)

Multiple sources populated substantiating this claim, including a pulmonary medicine book I had on a shelf nearby. Sure enough, I also found it printed on the physical page.

Did you notice my error?

When researching, it’s essential to approach a claim with a level of what I’ll call ‘neutral skepticism’ or perhaps ‘a fresh (unbiased) mindset.’ I was already primed to believe that the surface area of the lungs was, in fact, the size of a tennis court. Searching for a quick reference that agrees with my presupposition is known as seeking confirmation bias, and it’s mentally lazy. The process goes:

1. We believe something.

2. We want to find someone who agrees with us.

3. We specifically search for evidence that substantiates, rather than challenges, the original claim.

4. We find that agreement.

5. We stop looking, satisfied that we were correct all along.

While I did indeed find multiple sources substantiating the tennis court metaphor, they were not consistent in their agreement. Stated surface areas were quite different, yet still claimed a tennis court as their (what now seemed to be) arbitrary unit of comparison. Now I was skeptical… better late than never.

Counterpoints

I was now on a hunt for disagreement with the stated metaphor. Think of evidence as a scale. Stack the agreement on one side, stack the disagreement on the other, and then ignore which stack is bigger, throw away the scale, and look for the explanation that survives testing. Maybe that’s a little harsh. But what I’m trying to say is that it’s not a volume game. One good explanation can outweigh a large volume of bad explanations.

In 1931, a pamphlet titled “One Hundred Authors Against Einstein” was published. Scientists, philosophers, and critics were attempting to dismantle the groundbreaking theory of relativity that had upended people’s understanding of the universe. Albert Einstein: “Why one hundred? If I were wrong, then one would have been enough!” Theories stand or fall on evidence, not on the volume (interpret “volume” however you would like) of voices raised against them.

Getting back on track… I searched for evidence counter to my presupposition and found the most interesting article thus far: “Tennis Courts in the Human Body: A Review of the Misleading Metaphor in Medical Literature.”

The paper recounts numerous citations (some that I had read moments prior) that repeat this apparently bogus claim. It explores Euclidean geometry vs. fractals, and the possible reasons for significant variations in estimated surface areas of the alveoli, along with similar claims about the intestines and vasculature. It turns out the lungs are not the size of a tennis court, and the visual illustration I was working on was off to a rough start. While one may be able to approximate the surface area of the lungs, it’s better to think in the irregular pattern of fractals from the beginning.

Metaphores

It’s probably important to note at this point that I do not believe anyone has ever been on the verge of a clinical intervention and thought:

‘WAIT! Aren’t the lungs the size of a tennis court? Ah, yes, it’s good I’ve remembered this! How unnecessary this intervention would have been. If they were any smaller, I would have continued!’

The point is that we all want good and accurate information, and a simple metaphor doesn’t always get us there.

To make it catchy:

We choose metaphors over mechanisms, stories over statistics, and narratives over nuance.

The problem is, we all enjoy metaphors, stories, and narratives quite a bit. They’re memorable, engage us emotionally, and give us a sense of real-world application, transforming facts and figures into observable reality (even if only in our imagination).

I was recently working on a hypothermia class involving cold injuries such as frostbite. I started creating an illustration of a bottle of Coca-Cola in the freezer (an illustration I had heard and believed). As it freezes, the liquid expands by 9% and shatters the glass bottle. This, my friends, is what happens when your cells freeze - popping like a glass bottle in the freezer!… except that isn’t how it works at all. The real mechanism has to do with extracellular ice crystals, pure water formation, solute concentration, relative hypertonicity surrounding cells, cell shrinkage, shearing forces on cytoskeletons and phospholipid membranes, and intercellular ice finality.

When I was writing a blog about the Oxyhemoglobin Desaturation Curve, I had discovered that 2,3 DPG does not function ‘like a crowbar’ at all - it stabilizes deoxyhemoglobin after the oxygen is offloaded.

I really need to start researching more before I start drawing - I’ve thrown out a lot of graphics.

Reach

This may all seem rather academic. No, the lungs are not the size of a tennis court. No, cells don’t explode like Coke bottles. No, 2,3 DPG does not act like a crowbar. Although it’s fun, my point isn’t just to dispel some interesting dogma. My point is that good care begins with good explanations.

I’ve been reading a book titled The Beginning of Infinity by David Deutsch, which is probably what subconsciously inspired this blog. His exploration of “good” and “bad” explanations seems to apply to our topic. He distinguishes between good and bad explanations by their depth, reach, and resistance to being easily varied. On one hand, a good explanation is hard to change without breaking its internal logic (it fits reality tightly and leaves little room for arbitrary tweaks). On the other hand, bad explanations are vague, too convenient, or rely on authority or coincidence (they can be endlessly adjusted to fit almost anything).

Every metaphor, anecdote, narrative, and illustration becomes a part of how we think and pass on information. How we talk about medicine builds our mental models. Are our quick explanations good, or oversimplified, and building a weak foundation of knowledge?

There’s no such thing as harmless oversimplification.

Stay skeptical, and thanks for reading!

References:

Ananda Rao, A., & Johncy, S. (2022). Tennis Courts in the Human Body: A Review of the Misleading Metaphor in Medical Literature. Cureus, 14(1), e21474. https://doi.org/10.7759/cureus.21474

Deutsch, D. (2011). The beginning of infinity: Explanations that transform the world. Viking Publishing.