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Kennections 01/06/2014 - Fast Blood?

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Kennections 01/06/2014 - Fast Blood?

Postby TheConfessor » Mon Jan 06, 2014 1:31 am

I just saw this week's Kennections in Parade. Question 2 says "What liquid in your body travels over 12,000 miles every day." The answer is blood.

This seems absurd to me. It says that blood travels through our bodies at a speed of 500 miles per hour. Is this an error, or am I missing some non-obvious interpretation that would somehow make this statement correct?
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Re: Kennections 01/06/2014 - Fast Blood?

Postby PhygLeGuy » Mon Jan 06, 2014 12:41 pm

Here’s a different interpretation.

I found one estimate saying that there are 20-30 trillion red blood cells in the blood stream. If each one (of the lower number) travels 0.00004 inches per day, that would add up to 12,000 miles.

This is also absurd.

It does seem to be a popular bit of absurdity. I found the 12,000 miles per day statement mentioned here, here, and here. Perhaps you are the first person ever to question the logic of it. Or perhaps there is no such thing as friction.
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Re: Kennections 01/06/2014 - Fast Blood?

Postby grodney » Mon Jan 06, 2014 2:49 pm

There's also a Google+ post that shows up in searching, and several questioned it in the comments. ... fugNXcno35
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Re: Kennections 01/06/2014 - Fast Blood?

Postby rkd » Fri Jan 10, 2014 3:18 am

Aside from the general unlikeliness of 500mph, I think it'd make your blood pressure drop very low. A few ideas ... just shooting from the hip:

1. It might arise from adding the total length of vessels and multiplying by how often the blood cycles around. I.e. let's say there's a lake that gives rise to seven 100-mile rivers to the sea. It'd make sense to say the water travels 100 miles to the sea, but one might say that 700 miles are traversed by water. In the same vein (heh), if bloods cycles once per minute on average, and the main track consists of 3.4 miles, you could get a number of 12000 miles. Doesn't necessarily make sense to phrase it that way, and it doesn't match the numbers I find in a quarter-assed search, but it's a possibility.

2. Longshot, but it could be analogous to mean free path for an electron in a wire. Hook a battery up to a wire, and electrons will flow through the wire. Each electron bounces wildly, ping-ponging off nuclei, but slowly making its way along. So each electron actually travels an enormous distance, but it might only move along the wire at 1 cm/min. I don't think blood cells would have that same crazy kind of motion, but maybe there's something in that vein (heh) which leads to a giant number. Add up all the little higgly piggly jiggly motion, and maybe you could come up with 12,000 miles somehow.

Nope, that's all I've got. 500mph just wouldn't work, so there's no way an individual erythrocyte could be going 12000 linear miles. Theoretically you could have a main river of blood traveling through an artery at 500+mph with a narrow boundary layer that allows blood gas transfer, but friction alone should prevent that kind of difference. Must be something like I mentioned in #1 above.
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Re: Kennections 01/06/2014 - Fast Blood?

Postby Ken Jennings » Fri Jan 10, 2014 12:19 pm

Ha! It's true, once I read this (on PBS's Nova site, I think) I didn't do my own back-of-the-envelope math.

Here's one guy running numbers who thinks 10,000 miles a WEEK might be more like it. What do you think of his methodology?
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Re: Kennections 01/06/2014 - Fast Blood?

Postby TheConfessor » Fri Jan 10, 2014 6:39 pm

Ken, that still looks way off. The guy acknowledges, then appears to ignore, the fact that blood slows down to a crawl of .02 inch/second before it starts its journey back to the heart, but his assumed average overall velocity for the complete circuit is essentially just the velocity near the heart.

Here's my back of the envelope estimate. It is very widely stated online that it takes about one minute for your blood to complete a full cycle of circulation, including a short round trip to a lung and then a longer round trip to an appendage or vital organ. Let's say a cell is going to your left big toe, so that's maybe 4 feet each way. Add the round trip to a lung and you've got a total distance in the ballpark of 10 feet. So if it does that once per minute, that's 10 feet/minute, or 600 feet/hour, or 14,400 feet/day, or 2.73 miles/day. Which is why the notion of blood circulating 12,000 miles/day struck me as ridiculous.

It seems like one of those false "facts" that may have originally resulted from a typo or a miscalculation, but once it gets loose on the internet, it's hard to kill.
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