Look at this fun fact: Did you know that YOUR blood vessels taken together add up to 100,000 kilometers, enough to wrap them around the planet twice? One of our favourite fun facts, used in our book and app and a video and… wait… 100,000 kilometers is like. .
. like a lot. We’ve used it so often but where did we get this number from?
And how do we know it's true? Welcome to an epic RESEARCH RIDDLE that took us well over a year to figure out and led us on a strange and baffling journey. It began extremely innocently.
We always collect interesting and fun facts to make our stories more colorful. And this one was just perfect! All of your blood vessels, twice around the world!
Small thing, big number: wow! But one day someone asked where this number was actually from. Did it really matter though, because it had to be true.
If you google variations of “how long are your blood vessels put together” you’ll find the same numbers over and over: 100,000 kilometers or 60,000 miles. You find it in books, blogs, webpages of educational institutions or lecture notes, reviews, scientific papers and articles. We ourselves have used it multiple times.
So it couldn't be that hard to find the source, right? RIGHT? The first perplexing thing we noticed was that not a single one of the many, many websites, books or articles quoted the original source.
Most didn’t give a source at all, some linked to each other. It seemed like the number was just accepted as a truism. Very weird.
Even worse and more suspicious was that the number was used inconsistently: some said it’s the total length of capillaries, some said it’s just veins plus arteries, and others cited it as the length of everything put together. Hm. Something wasn't right.
We couldn't let this go. Getting our facts straight is critical for what we do here and since we’ve used this fact ourselves a few times this now felt personal. We needed to find the original source and solve this research riddle.
There HAD to be an ORIGINAL source! Random googling just turned up thousands of sites that quoted the number, no lead to the original. So we moved on to PUBMED, a search engine for biomedical science papers.
0 results. Ok weird. Maybe if we tried a few different key word combinations.
Nothing. Hm. Hm.
Ok finally, we found a bunch of scientific papers! But still, not a single one referenced the original source. The number also showed up in two different biology textbooks.
We contacted the authors but they told us that the number has been circulating for decades and they'd also be curious to know where it comes from. Maybe we needed to go back to the olden days – so we narrowed our search to the last few decades, starting with the 1990s. And would you look at that….
we found two books! “Vital Circuits” by Steven Vogel. And “Looking at the Body” by David Suzuki.
And it turned out DrSuzuki is an OG science communicator with a PHD in Zoology, who started doing popular programming in the 1970s on Canadian television. He has 29 honorary degrees and has written 52 books. Impressive!
He may either be the original source or at least know what it is! So we ordered “Looking at the Body” to look for it… And here it is: “… If all the body’s blood vessels were laid end to end, they would stretch 96,000 km (60,000 miles). That’s about two and a half times around the world.
… ” But again. No source. It turned out he’s still around doing things in his late 80s – so we thought maybe if we ask Dr Suzuki personally, he might know where the number is from.
He had no public email but ”A personal inquiry is possible by writing a letter to Dr Suzuki. ” Very old school, very respectable. So we wrote a letter!
Dear Dr Suzuki, we are writing to you on a matter of grave importance. What is the original source for the 100,000 km? Our internet video and our sanity depend on it.
Pretty please. Respectfully yours, kurzgesagt fact checkers. Three weeks later we actually got a reply: “Dear weird internet people, unfortunately Dr Suzuki does not recall the source of this data.
And since the book was written over 30 years ago, he doesn't have files that old available to look it up. Good luck with your project, you are going to need it. Kind regards, Public Information Coordinator” Ok they didn't quite use these words and were actually really nice!
But still: bummer. Maybe the 1992 popular-science book by Steven Vogel, a Duke University biomechanics professor would be more helpful. It has this sentence: “Combined length of pipes: 100,000 kilometers (60,000 miles)—more than twice around the earth at the equator.
” So the book talks about this on one of its 300 pages. But where is the source? Unfortunately there's only a list of 93 references and sources with no indication where in the book they're pointing to.
Our original source may be in one of the papers, books and articles in this list. Ouch. Reading all of those would take weeks.
Was this really worth it? Or had we gotten lost in the forest of human knowledge, looking for answers to questions nobody is asking and nobody cares about? But we’d wasted so much time already, so we decided to just do that one by one.
And now for a change we just got stupidly lucky. For no particular reason we decided to check the sources from last to first. And it turned out the very source we checked first, was what we were looking for!
A Scientific American article from 1959: “The Microcirculation of the Blood”. So we got a scan of this 65 year old science magazine and there it was – but this was still not the original source. But it did reference where it got the number from.
“The Anatomy and Physiology of Capillaries” – a 1922 book by August Krogh, winner of a Nobel Prize for Medicine. He probably knew what he was talking about. So we ordered his book and bingo: we got the original source.
The book is a collection of Krogh’s lectures and was highly praised by experts at the time. It summarises his research and adds new experiments, ideas and hypotheses. So here it was.
The original source, used thousands of times for over a century: Supposing a man's muscles to weigh 50 kg and his capillaries to number 2000 per sq. mm. , the total length of all these tubes put together must be something like 100,000 kilometers or 2 ½ times round the globe and their total surface 6300 sq.
meters. While other scientists had speculated on the length of all the capillaries before, Krogh was the first to make a real estimate, based on real experiments. Something very solid.
Ok. And now that we were here – was it correct? Krogh’s book includes this rather cryptic table, without calculations or explanations.
It seems to have been obvious to him but not to us looking at it a century later. We needed to actually read the book. Book reading montage (maybe Zeitraffer) In a very breakthrough sciency kind of way, Krogh just winged it.
In a nutshell, he cut muscle samples from different animals, started counting and made some rule of thumb assumptions. Today we know that his assumptions about the density of capillaries in humans was quite off. On top of that, he used a kind of idealised body builder human, weighing 143 kg with 50 kg of pure muscle mass.
And this finally gave him the very pleasing number of 100,000 km. It would be unfair to blame Krogh – this was just a small ancilliary fact he probably calculated for fun and out of curiosity, not elemental to his body of work. But he was a world expert, so his incorrect number was used in scientific papers, spread and became dogma, eventually entirely detached from the original source.
At this point in our research, over a year had passed. So just to be sure, we did another google search and… While we were caught up in our personal mission to find the source, up to the neck in old books, writing letters to Canada – scientists quietly published a paper, not really getting a lot of attention from anybody. They calculated a new number, a way more accurate estimate.
If we had just waited another year, we could have saved doing all that work. The deepest of sighs So now, here it is: According to the newest science, the length of all the capillaries in a human is somewhere between 9000 and 19,000 km. Very impressive but not enough to go around the world.
A Sort of Conclusion If you look up the question today, you'll likely still not find the real answer but thousands of sources, among them one kurzgesagt video, using the wrong number. And it will probably stay like that for a while. In all this time, why did nobody bother to double check this?
Well because the reality is, it’s extremely hard and time consuming. As we mentioned before it took us a year and a great deal of luck to get to the bottom of this on our own. Investing this time and effort while you’re writing a paper is just not feasible for most scientists or science communicators.
If a number seems safe and is used by credible sources, most people, including us, end up quoting a secondary source or worse. Also: it’s just such a nice, round number that will stick with you once you’ve heard it. Facts that seem beautiful tend to survive much longer.
But that is exactly the problem, isn’t it? The most interesting stories survive on the internet and often with each retelling, they get more exciting and memorable. It takes a lot of energy to get to the bottom of things, and a factoid is easily repeated, so misinformation can persist.
When we started this research we didn’t think we’d end up here. You, dear viewer, got a little peek into the kinds of mazes we have to navigate just to tell you our short sciency stories. They are as true as we can possibly make them.
We'll run into the same traps as everybody else from time to time. But we are doing our best to correct misinformation, give more context, and to bring you the latest science on any topic we cover. See you next time!
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