An oft-quoted "fact" on the internet1,2,3 (and at least once in print4) is the claim:
Your body has about 5.6 liters (6 quarts) of blood. This 5.6 liters of blood circulates through the body three times every minute. In one day, the blood travels a total of 19,000 km (12,000 miles)—that's four times the distance across the US from coast to coast.
None of the instances I've found provide any justification for the 12,000 miles figure; nor do they cite any authoritative source.
Although the 12,000 miles figure occurs often, it is fairly obvious that most (if not all) instances are copied from one of the other instances. Sometimes word-for-word, sometimes paraphrased; sometimes with a mention of the other instance, more often with no mention of the source. It is therefore quite possible that someone "plucked a number out of the air" and the meme-dispersal properties of the internet have done the rest.
In trying to justify this claim (see Background and Further Research below), it becomes clear that an awful lot depends on what precisely is meant by "the blood" and – to a lesser extent – what is meant by "the distance traveled". Some plausible definitions could put this figure at anywhere from around 70-90 miles up to around 90 million miles (and, in extremis, even a couple of thousand trillion miles!). It is entirely possible that there is no meaningful answer to the question of how far blood travels in a day.
Question(s)
Is there any extant, authoritative (ideally peer-reviewed) source for the 12,000 miles claim (i.e. that might have been the original source for the above "fact")?
If the 12,000 miles has simply been plucked from the air, is there any extant, authoritative source for a different figure?
If there is no extant authoritative source for any such figure, can someone provide a justifiable and evidence-backed answer to the question "How far does blood travel in a day"? (But see note below.)
Please note: I am slightly hesitant at including the third option above, and may remove it if felt appropriate. I recognize that the underlying question is likely ambiguous, and that any attempt at an answer might well be "opinion based" (as it will probably need to interpret exactly what is meant by "the blood" and "the distance traveled"). At the risk of being over-demanding, any attempt at answering under this option must include fact-backed justification for any such interpretation(s) and ideally would justify why such interpretation(s) are the only valid ones. I an not looking for speculation along the lines of "If we define "the blood" this way and follow these calculations, we end up with this figure.".
Background
In the pub a while back, someone had the book 1,342 QI Facts To Leave You Flabbergasted4. This was opened at random to the "fact" that in one day, "blood travels 12,000 miles". I don't have the book in front of me to give its exact wording, but the accompanying Source Finder website5 points to the PBS quote at the top of this question.
Our first thought was "that's Quite Interesting" (which, after all, is the point of the show/book). Our second thought was "that sounds very high"... if "the blood" travels 12,000 miles in a day, it travels 500 miles in an hour, and clearly (at least at a simplistic level), blood isn't coursing through our body at 500 mph!
Some further pondering, and a little web-searching on mobiles didn't clarify things: our conclusion was (as noted above), that too much hinged on what exactly is meant by "the blood" and how you measure "the distance traveled". Some clearly wrong interpretations:
We found a figure of 3-4 mph for the average speed of blood-flow6 . Treating "the blood" as a homogeneous fluid, this would give a "total distance traveled" of between 72 and 96 miles over a 24-hour period.
Clearly 12,000 miles isn't the distance traveled by an individual blood cell, as that would necessitate it traveling at the aforementioned 500 mph.
It's clearly not based on the total distance traveled by all blood cells. An adult male has around 25 trillion (25,000,000,000,000) red blood cells (RBCs)7. If each is traveling on average at 3-4 mph, the total distance for all cells would be roughly 2,000 trillion miles in a day (or about 350 light-years – a little more than the distance to Canopus, the second brightest star in the sky). Conversely, if the total distance was 12,000 miles, each RBC would travel less than a thousandth of a millimetre (about the wavelength of red light).
We did have some thoughts about trying to account for the multiple paths that "the blood" flows through. In an extremely simplified example, suppose blood from the heart split into two parallel paths, each of which went around the body before merging just before re-entering the heart. It could be argued that "the total distance" was 2 x 4 mph x 24 hours, since "the blood" was going down two channels. However, it could also be argued that what is going down each channel is not "the blood" but "half the blood".
Things get more complicated when more realistic topologies are considered: if the blood flows through a common vessel (e.g. the aorta) before splitting, if you count each path separately (as above), should the shared path count double or only once? You probably want something in between based on the relative lengths of the single path and the multiple paths.
My view is that while this might be a way of deriving a plausible figure, it is not without problems. One is practical: you'd need to map the entire circulatory system in order to correctly assign weights to each branch and arrive at an overall multiplier. Another is numerical: as we scale up to the full system, there is a danger the multiplier grows too quickly (remember: 80% of the system is formed from capillaries, and the smallest of these are no wider than a blood cell... there will be a lot of paths). Finally, what's being measured feels increasingly arbitrary: the contribution from the aorta is from all the blood at once; but from the capillaries it is almost cell-by-cell.
Between the two extremes above – 70-90 miles when treating blood "as a whole", and 2 thousand trillion by counting each cell – you could probably arrive at almost any number for the total distance traveled. Our problem was that we could not think of a natural way of "dividing the blood" and counting "the distance" that would arrive at 12,000 miles. Further research would be required...
Further Research
Over the next couple of days, I did some dedicated web-trawling... with little success.
The 12,000 miles kept cropping up, but with no justification as to where the number came from, and no references to an authoritative source. I also collected the following, potentially relevant "facts" about the heart and circulatory systems (generally, where it matters, for an adult male):
Blood travels an average of 3-4 mph6. Faster in the aorta; slower in the capillaries (where cells need to pass in single-file, and sometimes have to "push" their way through). Another source10 gives a slightly wider range (2.2 to 4.5 mph).
The total length of all arteries, capillaries and veins is about 60,000 miles8,3,9 (with capillaries making up about 80% of this total).
There is about 5.6 litres (6 quarts) of blood in the body1,6,9.
Blood takes about 20 seconds to circulate throughout the entire vascular system1,9.
The only discussion I could find on the subject was a Reddit thread11, which starts:
Does human blood really travel 12,000 miles every day?
Someone posted that on Facebook and I thought it was total bullshit. So I Googled it and it seems it's not bullshit? I guess my real question then would be: isn't there something more to this? That's 500 miles per hour. I can't understand how that wouldn't rip your body apart.
which neatly mirrors our experience: lots of pages quoting 12,000 miles, no references, and a feeling that 500 mph is clearly wrong. There follows some loose speculation, including that the total distance traveled by all blood cells is clearly massively too high; as is a figure of about 90 million miles calculated by multiplying the number of times the blood circulates in a day (1,440) by the total length of the circulatory system (60,000 miles). To me, the most pertinent comment is the closing one from user rm999:
I searched online, there's nothing reputable backing this. The figures I see are closer to <1 mph, which would be on the order of ~20 miles a day.
although rm999's figure for the overall speed is less than the most common figures I saw (although 3 quotes this speed for the aorta).
References
1 Amazing Heart Facts on the Nova section of the PBS website.
2 The Journey of Blood (PDF). Lesson plan on the American Heart Association website.
3 Slideshow on "Circulation" on slideplayer.com by Dwain Gibbs.
4 1,342 QI Facts To Leave You Flabbergasted: a spin-off book from the British TV comedy panel game-show "QI" (for "Quite Interesting") – see entry on Wikipedia. Despite being primarily an entertainment show, it has a pretty good reputation for research, although it does sometimes make mistakes and issue corrections. I suspect the book is less thoroughly researched than items appearing on the TV show.
5 The book lists its sources on https://qi.com/1342/index.php (this can be seen using the Look Inside! feature on the Amazon page above, under "Read This First"). The "12,000" miles fact is the second entry on page 186, leading to the aforementioned http://www.pbs.org/wgbh/nova/heart/heartfacts.html.
6 Our Bodies' Velocities, By the Numbers on the Discover magazine website. Note, though, that after giving the 3-4 mph figure, it goes on to say "But this blood speed is just an average. It starts out by rushing through the aorta at an impressive 15 inches a second, then slows to different rates in various parts of the body." I suspect the "inches" is a mistake: 15 inches/second equates to less than 1 mph; whereas 15 feet/second equates to about 10 mph, which fits better.
7 How many blood cells are in the human body? from the article "How Many Cells Are in the Human Body?" on the Healthline.com website.
8 11 Surprising Facts About the Circulatory System on the Live Science website.
9 Heart Facts on the Health Essentials website of the Cleveland Clinic. The "20 seconds to circulate" fact was originally taken from Amazing Heart Facts on the Arkansas Heart Hospital website, but that link (https://www.arheart.com/heart-health/amazing-heart-facts/) is no longer valid.
10 What is the speed of blood circulation in human body? on Quora.
11 Does human blood really travel 12,000 miles every day? on Reddit.
