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There are some health myths we've heard so many times that we just take them as fact.
Has someone ever told you not to crack your knuckles because it'll give you arthritis, going out with wet hair can give you a cold, or eating breakfast can help you lose weight?
None of these are actually true, but while they've been regularly debunked, there's an even bigger health myth we continue to fall for.
Over on YouTube, Kurzgesagt – In a Nutshell delved into the idea that the average blood vessels in the human body can wrap around the world twice.
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The idea that grouping our blood vessels together can stretch for 100,000 km (60,000 miles) is quoted numerous times across the internet, in books, and even in studies, but is it true?
Trying to find out where the mythical 100,000 km came from, the video notes that most mentions don't link to an original source or even give a source at all.
Inconsistencies continued when they noted that there's no real confirmation whether it's the total lengths of capillaries, veins and arteries, or the whole lot.
Looking for the original source turned up thousands of sites, although they eventually narrowed it down to two books from the 1990s.
Both Steven Vogel's Vital Circuits and David Suzuki's Looking at the Body mentioned the 10,000 km, with Suzuki's saying: "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..."
Although there were no contact details to email Suzuki, they did it the old-fashioned way and wrote him a letter.
Unfortunately, the reply read reiterated that Suzuki doesn't recall the source, and he doesn't have files going back that long.
When looking at Vital Circuits, there was a mention of a Scientific American article from 1959 called “The Microcirculation of the Blood”. In turn, this mentioned 1922's The Anatomy and Physiology of Capillaries from Novel Prize winner August Krogh.
This appeared to be the original source, with Krogh saying that if we assume a man's muscles weigh 50 kg and his capillaries number 2000 per sq. mm, the total length would be 100,000 km.
While Krogh had used cross sections of animal capillaries and suggested something of an ideal body form, it's thought that his numbers could be off.
According to newer science, the actual number is between 9,000 and 19,000 km - far less than twice around the world.
As the video noted, it took a year and a LOT of luck to get to the bottom of the statistics, which explains why people are happy to use the wrong number. It concludes: "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."
So, the next time a pub quick asks how long the blood vessels in your body are, you can give the correct answer.
