Kurzgesagt – In a Nutshell
Kurzgesagt – In a Nutshell
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Sources – 100K Blood Vessels
Sources – 100K Blood Vessels
– One of our favorite fun facts, used in our book and app and a video and… wait…
It was a beloved piece of info that we used on different platforms.
How Large Can a Bacteria get? Life & Size 3
https://youtu.be/E1KkQrFEl2I?t=466
From the video:
"Your body has around 100,000 km of capillaries alone, the tiniest of your blood vessels, with a surface area of around 1000 square meters."
Form the Scale of the Universe app:
"Length of Human Blood Vessels 16,598 km
Human blood vessels are really tiny, but they are everywhere in your body – arteries, veins and capillaries form a giant road system in our body. If you lined up all blood vessels of just one human, it would be around 16,598 km long. That is a little less than half of the Earth's circumference."
– 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.
Well, we didn't search for this for a full year, obviously. We were busy doing our videos and posters and taking care of all the other work in the treehouse. This was more of a sidequest - among many others - that we revisited from time to time. So we wanted to share one of our stories with you.
– You find it in books, blogs, webpages of educational institutions or lecture notes, reviews, scientific papers and articles. We ourselves used it multiple times.
In the following we collected a few examples where we came across the number. It is not meant as pointing out any source obviously. It is just to give an idea about how widespread the number is on the internet.
#British Heart Foundation. Watch: How are blood vessels made? 2019
https://www.bhf.org.uk/informationsupport/heart-matters-magazine/research/how-are-blood-vessels-made
Quote: “A four-week-old human embryo already has miles of blood vessels. By adulthood, we each have 60,000 miles of blood vessels inside our bodies – that’s more than twice the distance around the world.
Those vessels keep blood flowing, supplying your tissues with oxygen and nutrients, and keeping your organs, including the heart, healthy.”
#Joseph Castro. 11 Surprising Facts About the Circulatory System. 2022.
https://www.livescience.com/39925-circulatory-system-facts-surprising.html
Quote: ”If you were to lay out all of the arteries, capillaries and veins in one adult, end-to-end, they would stretch about 60,000 miles (100,000 kilometers). What's more, the capillaries, which are the smallest of the blood vessels, would make up about 80 percent of this length.”
#Thomas Crane. The Total Length of Your Blood Vessels Could Encircle the Earth. 2023.
Quote: “While the total length can vary from person to person, it is estimated to be around 60,000 miles (approximately 96,560 kilometers). This means that if these vessels were stretched out, they could travel more than two and a half times around the Earth’s equator, a distance of roughly 24,901 miles (approximately 40,075 kilometers).”
#Unraveling the Mysteries of the Human Body: The Department of Anatomy Explores the Vascular Network. 2023.
https://www.med.keio.ac.jp/en/features/2023/6/35-139132/index.html
Quote: ““According to one theory, the total length of human blood vessels is 100,000 km, a distance equivalent to two and a half times around the Earth. Don't you think it's peculiar that these blood vessels can run throughout our bodies without getting all tangled up?”
#Mary Elizabeth Dallas. 10 Amazing Facts About Your Blood Vessels. 2015.
https://www.everydayhealth.com/news/10-amazing-facts-about-your-blood-vessels/
Quote: “1. Your blood vessels could circle the globe. Though blood vessels are relatively small, the network is amazingly long. In fact, if they were laid out in a line, they would measure more than 60,000 miles in length, the U.S. National Institute on Aging (NIA) calculates.”
#Quora. Are our blood vessels really 100,000 km long?
https://www.quora.com/Are-our-blood-vessels-really-100-000-km-long
#Ricardo R Bartelme. Anthroposophic Medicine: A Short Monograph and Narrative Review— Foundations, Essential Characteristics, Scientific Basis, Safety, Effectiveness and Misconceptions. 2020.
https://journals.sagepub.com/doi/pdf/10.1177/2164956120973634
Quote: “The total length of blood vessels has been estimated to be 100,000 km or 60,000 miles. Blood is about 5 times more viscous than water and the red blood cells (40% of the composition of blood) are larger than the diameter of the capillaries and must “squeeze” through the narrow capillaries, offering incredible resistance to blood flow”
#Robert Young. Sick and Tired?: Reclaim Your Inner Terrain. 2001
https://archive.org/details/sicktiredreclaim0000youn/page/58/mode/2up?q=%22a+chronically%22
Quote: “A chronically over-acidic body pH corrodes body tissue, slowly eating into the 60,000 miles of our veins and arteries like acid eating into marble. If left unchecked, it will interrupt all cellular activities and functions, from the beating of your heart to the neural firing of your brain.”
#MadSci Network: Anatomy. Re: if you were to stretch all the body's blood vessels end to end, how many. 1999.
https://www.madsci.org/posts/archives/jan99/916069852.An.r.html
Quote: “The exact answer to your question is dependent upon which anatomy textbook one refers to. I've seen the estimate range from 50,000 to 100,000 miles. Go to: http://www.sln.fi.edu/biosci/heart.html and click on vessels. You'll see it stated that there are over 60,000 miles (97,000 kilometers) of blood vessels in a child's body and close to 100,000 miles (161,000 kilometers) in an adult's.”
#Nicholas J. Severs. The cardiac muscle cell. 2000
Quote: “By coordinating its beating activity with that of its 3 billion neighbours in the main pump of the human heart, over 7,000 litres of blood are pumped per day, without conscious effort, along 100,000 miles of blood vessels.”
#Rajendran et al. The vascular endothelium and human diseases. Int J Biol Sci. 2013 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3831119/
Quote: “When NO action is inhibited, endothelial signaling can become impaired, resulting in widespread disease, because the endothelium actively maintains approximately 60,000 miles of blood vessels in the human body.”
We also came across it when we searched in older pages or magazines and local newspapers from decades ago, from computer science magazines to newspaper ads.
#Your Sinclair, June 1986, (page 18 of the pdf)
https://ia800904.us.archive.org/1/items/YourSinclair37Jan89/YourSinclair/YourSinclair06-Jun86.pdf
Quote: “Next bit. Strong arms? It just so happens that I could've been in Commando but I had a cold and Arnie got the part. As for my hands, they have around 13,000 million nerve cells, 60,000 miles of veins, arteries and capillaries connected to them but no hose pipes.”
#Andover Townsman, April 1988 (page 61 of the pdf)
https://mhl.org/sites/default/files/newspapers/ATM-1988-04-28.pdf
Quote: “The human body is built around a frame containing 206 bones. The whole works is linked together with 60,000 miles of blood vessels.”
#CheckUp, July 1990
https://scholarlyworks.lvhn.org/cgi/viewcontent.cgi?article=1222&context=checkup
Quote: “According to HealthCounts, a total of 295 walkers posted a total of 21,556 miles, and many received awards and prizes at ceremonies held in TAH site auditorium The program challenges participants to compete in an effort to symbolically walk their way through the four chambers of the human heart and 60,000 miles of blood vessels in the human body.”
#CheckUp, August 1990
https://scholarlyworks.lvhn.org/cgi/viewcontent.cgi?article=1224&context=checkup
Quote: “The program challenges individuals and teams to compete in an effort to walk their way through the four chambers of the human heart and 60,000 miles of blood vessels in the human body.”
– 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.
If you try today, you may get hits or a different number of papers might turn up but when we did it back in early 2021 we could not get many results on pubmed and pmc. And we can not use the ones behind paywalls because we are not scientists and we would have to share it with you at some point.
– 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 would also be curious to know where it comes from.
We found two textbooks having the number as well, though without a reference. After contacting the authors, we got more motivated because they told us that the number has been circulating around since the 50s and they would be curious to know as well. So it was not only us who couldn't find it but the experts also didn't know. So we thought it might be worth going a bit further.
The section on the Human Circulatory System has this value:
https://www.biology-pages.info/C/Circulation2.html
Quote: “An adult human has been estimated to have some 60,000 miles (96,560 km) of capillaries with a total surface area of some 800–1000 m2 (an area greater than three tennis courts).”
It is stated that most of the book is based on the 6th edition of JW Kimball's general biology textbook which was published in 1994. (The first edition of Kimball's general biology text was published in 1965.)
https://www.biology-pages.info/
There were no references in the online book so we wrote to the contact that we found in the webpage and got a very nice response - but also learnt that our search through this channel won't be fruitful.
We also found an online version of the 1994 edition of the book and the number was there on page 332:
https://archive.org/details/biology0000kimb_c3r7/page/332/mode/2up
Quote: “Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area avail- able for the flow of blood is increased (see figure 17.2). About 60,000 mi of capillaries are estimated to be in the adult human.”
The other book was “Physiology: Textbook of Medical Physiology (Guyton)” that we found in the references section of some university lecture notes where we came across the number a hundredth time. This book is a standard textbook for this subject and has a long legacy – and many editions. So it looked promising and we started searching from the most recent edition.
11th edition of Physiology: Textbook of Medical Physiology (Guyton) does not have it. Though it vaguely mentions:
https://archive.org/details/TextbookOfMedicalPhysiology_201802
Quote: “Blood pumped by the heart flows from the high pressure part of the systemic circulation (i.e., aorta) to the low pressure side (i.e., vena cava) through many miles of blood vessels arranged in series and in parallel. The arteries, arterioles, capillaries, venules, and veins are collectively arranged in series.”
So could it be that the earlier editions had the value but it was corrected in the recent editions? Before diving into the previous ten editions of a thousand-pages textbook, we thought we could also ask the author(s) of the book. Unfortunately Guyton passed away in 2003, but we contacted one of the co-authors from 9th Edition onwards. We again got a very kind response saying that number has been in many books but no knowledge of where it is coming from.
– 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 1929 version cited in the Scientific American magazine was a later edition of the 1922 book. This is why there is a mismatch between the dates. Following version of the magazine has a reference list towards the end.
Scientific American, 1959 January, Vol 200
https://archive.org/details/sim_scientific-american_1959-01_200_1/page/60/mode/2up?view=theater
– 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.
August Krogh (1874–1949) was awarded the Nobel Prize for Physiology or Medicine in 1920 for his 3 papers (see below). They cover the experiments for capillary numbers in skeletal muscle of various animals. The main finding however was the novel role for capillaries: Through his work, the view of capillaries as passive exchange tubes has changed to a one of independent contractile units that are closed at rest and opened actively during muscle contractions. This phenomenon he called ‘capillary recruitment’ also explained the increase in blood-muscle O2 flux from rest to exercise.
Krogh’s three papers:
#Krogh A (1919a). The number and distribution of capillaries in muscles with calculations of the oxygen pressure head necessary for supplying the tissue.
https://pubmed.ncbi.nlm.nih.gov/16993405/
#Krogh A (1919b). The rate of diffusion of gases through animal tissues, with some remarks on the coefficient of invasion. J Physiol 52, 391–408.
https://pubmed.ncbi.nlm.nih.gov/16993404/
#Krogh A (1919c). The supply of oxygen to the tissues and the regulation of the capillary circulation.
https://pubmed.ncbi.nlm.nih.gov/16993410/
The book is a series of lectures that he wrote after the papers. It includes some of the material from the papers but also a body of new experiments and hypotheses.
#Krogh A (1929). The Anatomy and Physiology of Capillaries. Yale University Press.
https://archive.org/details/anatomyphysiolog00kroguoft/page/10/mode/2up?q=length
As reviewed succinctly by E. H. Starling who was himself was a renowned expert on the same topic, the book has contributed greatly to the rather barren field back then:
#Starling, E. H. (1923). The Anatomy and Physiology of Capillaries. Nature, 112(2808), 270–272. doi:10.1038/112270a0
Krogh was not the first one to write about it. Or let’s say the number was speculated before. But Krogh was the first to estimate it based on experiments.
Here is the earliest “mention” of the total length we came across:
# Ephraim Cutter, CARDIATION. Presented to the Section on Physiology and Dietetics, at the Fortyninth Annual Meeting of the American Medical Association, held at Denver, Colo., June 7-10,1898. Harv. 1856, Univ. Pa., 1857. New York City.
https://jamanetwork.com/journals/jama/article-abstract/474134
Quote: “In other words rest the heart in and between its beats, by position, by keeping still, by "laying low," figuratively and literally, by freeing the body from all compression and tight environments, by lightening work, by removing all causes of psychical, mental, intellectual, sensual (in its best and worst meaning) and of physical excitement, by lessening the friction of the more than 100,000 miles of capillaries, in an adult male, a, making the blood or normal dilution with water, b, removing the adhesiveness of the red- and white-blood corpuscles, the massiveness and abundance of the fibrin filaments, the amorphous and crystalline blood gravels or crystallized bodies, such as oxalate of lime, cystine, uric acid and its salts, triple phosphates, cholesterin, etc.; removing thrombi and emboli of fibrin, gravels, fat, leucocytes, cryptogams, in fact, making the blood stream normal by proper feeding and medicines.”
– 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.
In the section where Krogh estimated the total length of capillaries, there is the following table with rather encrypted titles and – without the values for human muscle tissue:
– 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.
So 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.
As a happy affirmative coincidence, in a kind of devastating way, we found out that there is a new paper after we have gone through all our adventures. But it encouraged us to make the story into a video because scientists went into all the trouble of correcting it and publishing a paper. So our efforts to pursue the truth were not in vain, well, but just a bit over the top.
#David C. Poole, Yutaka Kano, Shunsaku Koga, Timothy I. Musch. August Krogh: Muscle capillary function and oxygen delivery. 2021.
https://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC7867635&blobtype=pdf
Quote: “Shack August Steenberg Krogh (1874–1949) was a superb experimentalist who could conceive and fabricate state-of-the art purpose-built research instruments. These capabilities empowered his great intellect and unstoppable curiosity enabling him to address some of the most pressing scientific questions of the day, in physiology and beyond. One of his personal beliefs was that “Questions worthy of attack, show their worth by fighting back” a saying from the Danish polymath, Piet Hein, (27) and his measurements, as depicted in Fig. 1 (left side), emphasized the foundational dilemma regarding capillary function that he solved with his “capillary recruitment” theory. For his estimates of the total length of capillaries in an adult human, 100,000 km (!) (17), there simply was not sufficient blood volume to allow continuous perfusion of this vast network of vessels with a combined blood volume of nearly 5 L! The individual Krogh considered had 50 kg of muscle and for muscle to constitute ~35% of this body mass, this was an extraordinarily large person (143 kg, 315 lb). Their total blood volume at 8% body mass would have been ~12 L and skeletal muscle would have required 34% of that volume - which he quite rightly considered untenable.”
Following image from the paper nicely summarizes the calculation and points out in red where the new data differ from Krogh’s data. Krogh was indeed aware of the impossibility of the size of human his calculations were suggesting but he went on to explain them in a different way: he thought it would be impossible all this length to be filled with blood all the time. So he postulated that some of the capillaries would get recruited only when oxygen needs of the muscle increases, for example during exercise, which is called “capillary recruitment” theory. But today we know that shut capillaries are not opened to be filled with blood when we engage in intense activity. But still his work reshaped how people think about the oxygen exchange mechanism in the capillary bed and opened the way to new research.
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