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In a world of cyborgs, it seems pretty likely that many people will have some number of nanobots in their blood, completing tasks such as clearing away blockages, monitoring health, and boosting the immune system. However, what I'm wondering is how many little robots you can put into your bloodstream before there are adverse health effects.

Assume the lil' bots are smart enough to stay in the blood stream and not get stuck anywhere. Each one is about the size of a red blood cell (which may not actually classify as 'nano', but it's close enough for me), and for the purposes of this question let's assume they do not have any capability to carry oxygen or do any of the other things red blood cells can do. I'd like an answer either in quantity or volume for an average human, or proportion of red blood cells to nanobots. I would also like to know the main health concerns that would come from having too many.

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Blood is complicated. People with red blood cell counts (RBC) that are too high, suffer higher incidences of stroke/blood clots. People with RBC that are too low pass out or die. Each nanobot displaces some essential function of the blood (oxygen/food transport, immune system, clotting factors, etc.). What really happens as you increase nanobots is you start hampering these other blood functions. In other words, there will be a range of answers that are unique to each individual. The range starts when nanobots noticeably hamper performance and ends with death. – Jim2B Jan 4 at 16:24
By extension: In the limit, is it possible to replace blood cells by oxygen carrying nanobots? – xeon Jan 4 at 17:01
@jean I meant replacing all blood buy various nanobots. Some nanobots will carry oxygen, others will perform white cell functions and so on. – xeon Jan 4 at 19:09
Read the paper on Respirocyte nanobots, which goes into considerable detail. – JDługosz Jan 4 at 23:12
An episode of The Outer Limits gives a different perspective : – WoJ Jan 8 at 20:28
up vote 63 down vote accepted

For simplicity, let's pretend these are well designed nanobots that don't outright kill their host by blood clots or accidentally ripping holes in capillaries. So let's pretend these great scientists already killed a bunch of small woodland laboratory creatures and got those little issues figured out relatively early on. We'll assume these nanobots are the size of red blood cells (as you mentioned), deform the same way, can squeeze through capillaries and such one at a time just like normal red blood cells (or they avoid situations like that on their own, etc).

So, let's start with...

Replace (Some) of The Red Blood Cells!

Since we don't want to cause edema or high blood pressure, we will screen out some red blood cells and just replace those with our nanobots. It turns out that in this method there is an existing known condition caused by losing too many red blood cells: anemia. The normal count of red blood cells is noted as 4.7 to 6.1 million cells per microliter for men and a similar 4.2 to 5.4 for women.

That's a range of 1.4 million cells per micro-liter for men, so if we get someone on the "high but normal" RBC count level and replace their blood cells so they are down to the "low but normal" RBC count, that gives us 1.4 million cells per micro-liter; with 5 liters of blood in the human body (5 million micro-liters), that gives us a cool 7 trillion nanobots for starters.

Given these numbers there would normally be 31.5 trillion red blood cells in a human male body who that on the "high" side of the count, so that sounds nice to me. So our nanobots would exist in about a 4.5:1 ratio with red blood cells, or just under 20%.

Current total: 7,000,000,000,000 nanobots (some may experience some mild side-effects, ask your doctor of nanobots are right for you)

Replace (Some) of The Blood

People give a liter of blood all the time, which is 20% of their total blood volume (give or take). So why can't we just take that blood and replace it all with nanobots?

Well, blood isn't all cells - a lot of it is water. So just filling someone 1/5th full of what would probably be like a really fine powder...that'd probably screw up the blood pretty bad. Blood is already pretty tightly packed with cells, so I don't think we can dissolve that much powder into the remaining 4 liters without effecting key properties of the liquid.

But fear not, intrepid scientists - we'll pump people with as many bots as we can!

It turns out that when you separate blood, only about 40% of it is a combination of white blood cells, platelets, red blood cells, etc - with 60% being plasma. So why not take out a liter of blood, filter out all the plasma for mixing, add back in a liquid mixture of about the right viscosity as the blood cells, and pump that back into our frightened willing test subjects?

Well, since we already have the figure for red blood cells and it seems that white blood cells are puny by comparison, let's go with that previous number of up about 6 million red blood cells per micro-liter of blood.

If we filter out a liter of blood, that would give us room for 6 trillion bots! ...wait, that's almost the same figure we got earlier. What gives? Oh, right - giving blood is expected to cause mild (short-term) anemia! And we were figuring taking subjects who already had a high amount of red blood cells, so suddenly this makes sense.

So we've still got the same figure: 6-7 trillion nanobots (if we tell you we are giving your extra blood cells to sick children, will that make you stop screaming feel better?) We're still at 20% or less of the blood in nanobots...we have to go higher!


Come on, what kind of scientists are we, anyhow? We know from our calculations that we easily can shove around 37 trillion nanobots into people by removing all their red blood cells, so why don't we just do that?

"Oh, but won't the subjects die?" What are you, a communist? Let's face facts here - we have high technology, nanobots that are as small as red blood cells. Why can't our nanobots do the job of red blood cells, but do the job even better? Think outside the box, here! 100% nanobots!

And why do all nanobots need to be the same? We'll make 4-5 trillion nanobots to do no job related to being a red blood cells, so they can do whatever. The other 33-34 trillion nanobots will do everything a red blood cell does, plus it'll probably do other cool stuff. We aren't banging rocks together here! As we figure out how to make nanobots do stuff even better than red blood cells (maybe using moon dust), we'll be able to shift the proportion of nanobots in favor of non-blood cell lasers, or something.

Everybody gets 37,000,000,000,000 nanobots! whether they like it or not

Early on, after we've worked our number up this high without publicly acknowledged any deaths, we start jacking up the counts to replace all blood cells with nanobots, and we can probably start allowing for thicker blood because our ThriveBots™ deal with any side-effects from high blood pressure, so I'd say we can start getting our count up over 40 trillion in the blood alone, no problems (none we allow to be reported, anyway). Maybe even up to 50 trillion, but it's going to get pretty hard to go beyond that.

Say, skin cells are awfully many layers of skin do you need, really, when you could have a few layers of EpiThrive™ - the superior human skin alternative! Guaranteed to not give you any cancer that is presently known to man! We'll have a few more trillion ThriveBots™ packed into you in no time!

Final Nanobot Count

  • 3-5,000,000,000,000 nanobots for early, squeamish subjects - minimal side-effects, nanobots can't do red blood cell work

  • 6-7,000,000,000,000 nanobots with still minimal side-effects, but will cause anemia in all but high red blood count subjects

  • 37,000,000,000,000 nanobots, for those who've transcended their childish need for human red blood cells (may cause a change in skin tone, if we don't up-sell them to colored plasma additives)

  • 40 trillion and beyond, for real champions who understand that Maximum OverThrive Technology™ is the true future of whatever it is we are calling human-technology hybrids now adays

Minor side-effects may include: grumpiness and irritability, feeling tired, fatigue especially after exercise, headaches, problems with thinking and concentration, desire to eat ice cream, desire to eat non-food items (organic and inorganic), blue colors to the whites of the eyes, light-headedness, pale skin, shortness of breath, heart murmur, rapid heart beat, low blood pressure, vitamin deficiencies, double vision, sudden and severe unexplained pain, a desire to be part of something greater, a strengthening dedication to The Collective, sudden instant death, restless legs (and arms), memory loss, vivid hallucinations, heart attack, stroke, loss of impulse control, decreased emotion or flat affect, and an increased awareness of the vileness of the common unimproved animal.

Take nanobots only as directed, your mileage may vary. Please report all unexpected symptoms or side-effects to your personal ThriveBot™ representative immediately, before inconveniencing others or your doctor with things they are not capable of understanding.

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I couldn't find more info on how the body knows how many blood cells to create, other than "the amount is precisely balanced" - but maybe another person would know more than I. As a scientist of TM, I would just recommend getting into the beta for our ThriveBone replacement as soon as possible. – DoubleDouble Jan 4 at 20:28
@DoubleDouble In that case, we'd need to induce aplastic anemia, which seems to be known to be caused by various drugs and viruses, so well within the technology level needed for nanobots. Or nanobots could hijack the process and effectively use the pathways to make more nanobots, which would mean filtering out the bots would be temporary at best - or produce severe anemia that would be hard to treat, so people would be dependent on the bots. Maybe better if bots are produced externally, for sales :) – BrianDHall Jan 4 at 20:37
Cave Johnson, is that you? – Darthfett Jan 4 at 21:55
@Darthfett: I was going to ask if it was Randall Monroe. This has a very "What if..." feel to it. – Darrel Hoffman Jan 5 at 18:26
I wish all answers on this site could be this hilarious and thorough. – Ethan The Brave Jan 6 at 14:18

I'm wishing I'd kept up my fluid dynamics now. A significant quantity could start affecting the viscosity of your blood which would place extra strain on your heart, making the long term benefits questionable.

Read the bit about blood viscoelasticity. If your nano-bots don't deform in a similar manner to red blood cells you could have some very interesting effects. When I say interesting, I'm calling gangrene interesting, it may get as interesting as braindeath.

Blood cells, most importantly this article includes a couple of measurements. In this case 7μm and 3μm are your critical values, the first being the size of a red blood cell and the second being the smallest capillary it has to get through. (Ignoring the fact whoever wrote that site can't insert μ as needed). I would suggest that your nanobots should be able to mimic this behaviour, having the same plasticity as red blood cells. it'll make the later calculations much simpler.

What we need to know next is how much material the circulatory system can handle and what the penalties are for exceeding that.

RBC count

  • male – 4.7 to 6.1 million cells per microlitre (cells/mcL)
  • female – 4.2 to 5.4 million cells/mcL

Those are some big numbers and largely meaningless in real terms. What you're looking for is the difference between them, around 20%. Now if that's not a simpler calculation I don't know what is.

The trick now is stopping your body from treating them like dead cells and just filtering them out.

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You could also have nanobots "stored" in an organ, and released when others nanobots sense (nanobots specialized into closing wound released by others nanobots feeling pressure).

Of course at a given time you can not have more than the given % active, but you can have a lot more inactive.

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Great question.

The main concern I'm thinking of is the following : too many nanobots will prevent the necessary amount of red blood cells from flowing through the veins normally. So, blood circulation problems, oxygen delivery problems, heat distribution problems... You do not want to deal with that, and you don't want to reduce the amount of red blood cells in the blood either. So every nanobot (or whatever it is) you add in the body will be added to the already flowing red blood cells.

Size will apparently not be a problem since

Each one is about the size of a red blood cell.

So the thing you want to focus on is just the proportion of bots compared to the blood cells, so that the red blood cells can keep working among the nanobots. But since you don't want to replace blood cells with nanobots, keep in mind that each bot you add in the blood will take one more room in the veins.

I am no mad scientist, so I have no accurate numbers to give you, but I doubt you can put more than 10% of the total red blood cells in nanobots inside the veins.

More than 10% would probably cause trafic jam, so to speak.

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I'll make a note of @user16295 's answer about the deformation of blood cells, "If your nano-bots don't deform in a similar manner to red blood cells you could have some very interesting effects." – Draco18s Jan 4 at 15:41
Oh. This answer was not there when I wrote mine. Totally wasn't aware of that. – Nico Jan 4 at 15:54
I didn't know it either, but looked pretty important! :) – Draco18s Jan 4 at 15:55
RBCs are just a tiny part of your blood, really. Your estimate is probably a safe lower bound (unless the nanobots have some other detriminal properties, e.g. the deformation issues noted in another answer), but a reasonable upper bound might easily be much more than the RBC count. – Luaan Jan 4 at 17:00

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