Taking the point of view of a single RBC travelling through blood vessels, approximately how many cells will it affect throughout its lifetime? As blood tends to provide O2 as a whole, I am taking "affect" as being in the general oxygen transfer vicinity of a body cell.

Assuming that:

  • a RBC lives about ~120 days (source)
  • There are 60,000 miles of vessels in the avg adult human body (source)
  • There are ~5 trillion non-RBC type cells in the body that presumably need O2 (source)
  • Avg speed of 3 mph (source)
  • Blood cells are uniformly distributed, so any blood cell has the chance to be at any part of the body

So 120 days at 3 mph = 8640 miles travelled by a single blood cell. This represents 14.4% of the human bodies' vessels or about ~720 million cells affected by a single RBC.

Does this back of the envelope calculation make sense?

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    $\begingroup$ Not sure what you're trying to achieve here. Why wouldn't you take into account the probability of following the same path twice or more - and why are you concerned about a single blood cell when they act in concert? What's the objective of the calculation except to pass time.? $\endgroup$ Commented Jan 30, 2022 at 4:08
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    $\begingroup$ I have no problem with this kind of speculation; it's fun to wonder about the importance of a single tiny red blood cell, so kudos to you! My only objection is that it seems to presume to never follow the same path twice. Did you take into account that it must pass through the lungs every go round? Also, only about 80% of those miles are capillaries, where exchange is greatest. Anyway, I like this question, but it has no precise answer because of chance/randomness. $\endgroup$ Commented Jan 30, 2022 at 11:39
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    $\begingroup$ @ARogueAnt. - What's wrong with passing time wondering about a single red blood cell? It seems a decent use of time to me (given, an elderly biologist). $\endgroup$ Commented Jan 30, 2022 at 11:55

2 Answers 2


TL;DR: No. It's a fun little thought exercise, but unfortunately off the mark.* Most of my statements will be given, i.e. repeated numerous times in the medical literature as factual, though I can't say with certainty how they came to be so.

Assumption(?) #1 Assumption that the 60,000 miles of vasculature in the body all comprise of capillaries where exchange might take place.

Counterargument: In reality, only 80% (50,000 miles) is comprised of capillaries, where presumed "visits" take place. The rest is comprised of the heart itself, all arteries, arterioles, venules, and veins. There are (depending on how they are counted, not including the azygous system) about 30 major veins collecting blood from capillary beds; the larger they are, the more often they will be traversed by the blood cell, until they enter the superior or inferior vena cava on every return. The cell will see the same insides of the heart every go round. I don't want to do the math (I hate math) but (if) there are about 5 liters of blood in the circulatory system, and (if) the heart beats 100,000 times per day (average adult male, of course, just because), and (if) you know exactly how much of that volume is taken up by red blood cells, you can calculate how many times that one cell will visit the same heart every day (I think.)

Assumption(?) #2: Assumption that the red cell will never travel through the same vessel twice.

Counterargument: (See above for where it will travel for sure every day.) Your assumption(?) is based on equal probabilities encountered at every bifurcation in the arterial system, in other words, that every bifurcation in the vascular system is created such that the probability of going in either direction is equal. (Was that a tautology?) That isn't quite so. The probability of a red cell entering vessel A or vessle B (in the literature, described as daughter 1/d1 or daughter 2/d2) at a particular bifurcation varies with the diameters of a and b (with the larger diameter receiving more blood) and the angles of bifurcation (less important but still matters.) (This is just the mouth of one of those rabbit holes.) Randomness is not perfect at bifurcations, especially at the arteriolar level (active muscles will get more red cell visits than inactive muscles, etc.) This doesn't even take into consideration the effect of endothelial damage (which causes drag in blood flow, thereby affecting the probabilities of entering d1 or d2) which commonly occurs with aging but can start in utero, and which is more common in youth than one normally supposes.**

All things are never really equal, so, as I said in the beginning, No. The longer answer is, "I like this question, but the calculations are off due to the human circulatory system and the lack of randomness at particular bifurcations within it.

*But I like your curiosity and tenaciousness. That you bothered to do some research and put it together here is refreshing, and though my answer is without doubt incomplete/flawed, you deserve one. So here it is (with my apologies for its quality.) I wish an anatomist was interested in answering; the quality would be so much better!

**As an intern, I once had the very sorrowful experience of admitting an healthy appearing, exuberant 4 year old child to the pediatric surgical service. The only presenting symptom was that the child started squatting during exertion (not good), and on exam, had a murmur which was caused by aortic stenosis. This was long before imaging studies were as sophisticated as they are now. The pediatric cardiac surgeon took him to the operating room (OR) to replace the valve, but there was so much atherosclerotic aortic damage that there was no healthy tissue which could hold sutures in place. The child died in the OR. I don't know what would have been done today, but had the child stayed home, they might have had a couple more years with the parents, who hoped for an uneventful procedure. So the exercise involved in this answer was fun, but the memory it brought back is still quite sad.


I would focus on capillaries, much like anongoodnurse's answer. I'd go about thinking about the problem a bit differently, though.

From https://www.vhlab.umn.edu/atlas/physiology-tutorial/blood-vessels.shtml there are about 10 billion capillaries in the human body: capillaries being the places where vessels narrow to where only a single red blood cell can transit. These are where the "visits" happen. You can't base it on distance traveled, because most of the distance traveled is through very large vessels, where blood is really just being transported from place to place rather than "doing" anything.

On each "loop" through the circulation, a given blood cell is only going to enter one capillary. It'll be in a very small arteriole on one side, then go through a capillary, then come out in a very small venule. Before the arteriole it would have been in progressively larger vessels; after the venule it'll be in progressively larger vessels. However, the work of "visiting" other cells is mostly during that brief sojourn through the capillary.

To know how many of the 10 billion capillaries a blood cell visits in a lifetime of 120 days (=10,368,000 seconds), all we need to know is about how long a "round trip" is.

It's hard to find any solid reputable estimate for this round trip time. This very old paper:

Tarr, L., Oppenheimer, B. S., & Sager, R. V. (1933). The circulation time in various clinical conditions determined by the use of sodium dehydrocholate. American Heart Journal, 8(6), 766-786.

estimated a transit time of an average 13 seconds specifically from an arm vein, through heart and lungs, to the mouth. Travel time is going to be a lot slower to the legs, for example. I see various estimates thrown out of averages somewhere between 20 seconds and a minute. Overall it doesn't really matter for an order-of-magnitude estimate; let's just pick 30 seconds.

That would mean a blood cell has about 10,368,000 / 30 = 345,600 trips through the circulation. That number is small enough relative to the 10 billion capillaries that we can approximately assume no "repeat visits" and say 350,000/10,000,000,000 = 0.000035 = 0.0035% of capillaries are visited by one blood cell during the lifetime of that cell.

Capillary beds can be a bit complex and we've made a bunch of assumptions along the way, but I think even if you were very generous with some of those estimates you'd still stay under 0.1%.

  • $\begingroup$ If capillary beds make up 80% of our circulatory system (easy to believe), why do you say, most of the distance traveled is through very large vessels? Higher velocity travel is in larger vessels (travel in the aorta is - I believe - more than three orders of magnitude greater than through capillary beds), but (and it's a big but) as I stated, I hate math. If capillaries are 50K miles of vasculature, it doesn't compute if most of the time, the RBC's are actually in capillary beds, miles and miles of them, and it's there that they spend most of their time (they crawl along......slowly.) $\endgroup$ Commented Jan 31, 2022 at 22:19
  • $\begingroup$ (I purposely didn't take physical distance in account.) $\endgroup$ Commented Jan 31, 2022 at 22:20
  • $\begingroup$ @anongoodnurse Well, think about a capillary in your leg. A blood cell may travel more than a meter through the aorta, iliac, and femoral arteries before taking a relatively short trip over a mm or less through a capillary and then heading back through progressively larger veins. A huge proportion of the circulatory system is small vessels only because there are so many of them. I'd think of it the same way as roads...traveling between cities you might spend hours on a highway, and only a minute or two on a local road, though there is way more pavement dedicated to local road than highway. $\endgroup$
    – Bryan Krause
    Commented Jan 31, 2022 at 22:23
  • $\begingroup$ In any event, it doesn't really matter for the calculation I presented. All that matters is that on one trip, a blood cell is going to go through just one capillary (or a few; it's kind of hard to define what exactly "one capillary" is in the capillary beds, just talking orders of magnitude here). Whatever % of time it spends in any kind of vessel, that's one visit per round-trip through the heart and lungs. $\endgroup$
    – Bryan Krause
    Commented Jan 31, 2022 at 22:25
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    $\begingroup$ @anongoodnurse Yes, we're definitely on the same page there; OP's calculation was based on distance. I'm saying you can't think in terms of distance, because much of the distance traveled is through vessels where nothing is being delivered, that is, there are no "visits". $\endgroup$
    – Bryan Krause
    Commented Jan 31, 2022 at 23:02

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