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The conventional model of the circulatory system assumes the heart pumps blood out of the ventricles through a series of progressively branching arteries of decreasing diameter transitioning to innumerable microscopic capillaries which then progressively converge as veins of increasing diameter connected to the atriums then back to the ventricles. Although the return trip through veins is assisted by skeletal muscle contractions squeezing blood through one-way valves, arteries lack valves and are assumed to be passive conduits.

The total length of blood vessels, if strung end to end, is said to be 60,000 miles for a human. Are there 60,000 miles of blood vessels in the human body?. Obviously the vast majority of this distance is from capillaries. So even if we assume skeletal contractions account for 100% of power for venous blood, that still leaves at least 30,000 miles of arteries and capillaries. We can further assume capillaries are multiply redundant and not actually in use simultaneously (regulated by arterial sphincters). But that still leaves thousands of miles of conduits for blood to traverse before reaching the veins.

How can a pump the size of a fist drive a fluid more viscous than water through thousands of miles of conduits, most of which are invisibly small? The ratio of circumference to cross-sectional area is inversely proportional to radius, so the total wall friction to overcome increases with distance from the heart. If you want to pump water thousands of miles, a fist-sized mechanical pump won't cut it. Maybe 100 of them at regular intervals would do it, but not one.

OK after typing all that I just realized I neglected gravity. However, blood circulates whether the person is standing, lying down, in outer space, or standing on their head. How could all of the power needed to move blood through thousands of miles of conduits be provided by such a small muscle, even at rest?

I think skeletal muscles must be the primary driver of circulation, and the heart more of a flow rate regulator.

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    Welcome to Skeptics.SE! This site is for investigating specific claims made by notable people or that are widely believed, not random speculation. This question would be better suited to Biology.SE.
    – F1Krazy
    Nov 3, 2020 at 10:32
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    And you make the false assumption that blood is pumped over a single pipe many miles long. It isn't: arteries branch and the total distance from end to end is not very long on any specific branch. Also, this should be moved to biology.se as there is not published claim to be skeptical of.
    – matt_black
    Nov 3, 2020 at 10:34
  • It is widely believed that the heart pumps blood through arteries and veins. I think this is perfectly notable. The OP's analysis is flawed, and this should be easy to show empirically.
    – Oddthinking
    Nov 3, 2020 at 10:37
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    We have a Biology site. This question would be a much better fit there. Nov 3, 2020 at 14:50
  • I’m voting to close this question because the answer isn't verifying facts, but explaining the working processes of the human body. It's a science question, not a skeptics question. I'd migrate to Biology SE, but they'd probably prefer the form "Does the heart alone move blood or are there other forces?"
    – fredsbend
    Nov 6, 2020 at 5:05

1 Answer 1

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Your reasoning is faulty. You assume that the capillaries are a single tube of length 60,000 miles. In fact they are many tubes running in parallel, so the flow of blood is divided between parallel capillaries, reducing resistance.

Further, empirically if most of the flow of blood was caused by skeletal muscles then (a) the contractions required for this would be detectable and (b) people would not die of heart attacks.

The nature of the circulatory system is well understood. The heart pumps blood around the body.

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