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A human body where, apart for the musculoskeletal and integumentary systems, there is one, and only one, body system formed by vessels, or tubes, that transfer liquids from one place to another.

It's a form of fluidic computing, and the different types of cortex of the brain are essentially agglomerates of tubes (possibly dispatched around the body, but not necessarily). The first problem is, obviously, how would a human brain work as an agglomeration of tubes doing only fluidic computing?

The other functions this system performs are the remaining absent ones, thanks to special cells inspired by white cells, but not white cells, that process external molecules entering the body, transforming them into different things:

  1. Respiration: air molecules get carried into the system.
  2. Nutrition: food materials get metabolized.
  3. Digestion and excretion: all unidentified and processed materials get expelled from the body.

Again, there's only one system for all of this, therefore all the organs are just agglomerates of tubes. That is the other, major problem.

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This is a complex idea, and making it realistic is difficult, but I will try my best to help you.


Speed and size

We can identify two apparent issues : speed and size.

For speed, contrary to what the other response said, nerve signals are more than 10 times slower than the speed of sound for water, with a max speed of 120 m/s for nerve signals vs 1500 m/s for the speed of sound of water. Signals are thus able to propagate much faster in a fluidic system based on water. Speed is thus a non-issue.

About size, capillaries are extremely small, down to a few micrometers, but axons aren't much bigger, being a bit more than one micrometer in diameter. The size of capillaries is actually limited by the diameter of red blood cells, meaning it could be smaller in your case. We could thus get a similar density to human brain, maybe a little less dense


Signal representation

The ducts need to carry information. There are several ways to represent information with this kind of system : either the information is represented by the flow, the pressure, or the volume of liquid. This is similar to current (flow), voltage (pressure) or charge (volume) in electrical systems. I will personally use the pressure, because it is easier to apply a weight to (reducing a duct size will increase it resistance to flow), which is critical for neuron-like behavior.


Pseudo-neuron

Now, we need to have a neuron-like element that can perform computations the same way real neurons do. To make a long story short, there are many inputs that comes from previous neurons, and the strength of the connection varies depending on many factors. The input accumulates in the neuron as the membrane potential and, once a threshold is reached, there is a discharge that is propagated through the axon to the next neurons. The membrane potential decreases with time through leaking, and this is an important aspect of biological neurons. The image below illustrate the process :

Phases of a biological neuron Image source : self

To simulate this behavior, we could imagine a special structure, be it a cell or multiple cells, that acts as a kind of reservoir and accumulate the fluid inside it. Once the volume of the element reaches a certain point, a contraction is triggered, the input ducts closes during the contraction, and the output one opens. The signal is propagated to the next unit, and information is computed like that. All these pseudo-neurons could float in the fluid, allowing them to leak or pump from it. Obviously, this liquid must have the same pressure everywhere. Below a functional diagram of the imaginary pseudo-neuron :

Pseudo-neuron diagram Image source : self

The tubes must be padded by some kind of fat or soft material. This is because the signal, effectively a sound impulsion, would propagate through rigid material and interfere with everything else.


Energy and nutrient source

This is taken care by the fluid itself, which could carry the oxygen and nutrients needed for the cells to function.


Tradeoffs and issues

This brain would be a bit less dense that mammal brains, and sensitive to chocs and sounds. But it could be significantly faster, allowing for better reflexes. If you use a very low viscosity fluid instead of water, like hexane for example, it would be possible to have a higher density, but the nutrient wouldn't dissolve as well, and being filled with a flammable substance isn't always great.

The main issue isn't actually speed or size, it is the sensitivity to choc, sound, acceleration and anything that involve pressure changes. It would impact the brain, and this is a massive issue. Even simply standing up could cause problems.


Conclusion

It is possible, but very unlikely to evolve. But if you accept twisting a bit the reality, you could make it work.

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  • $\begingroup$ Did you literally made those schemes by yourself? I think I've fallen in love. $\endgroup$
    – lollo259
    Sep 13, 2023 at 16:11
  • $\begingroup$ @lollo259 Yes, using draw.io $\endgroup$
    – Elzaidir
    Sep 13, 2023 at 19:44
  • $\begingroup$ @Elzadir may I use those two images in my project, quoting your StackExchange profile as a source? $\endgroup$
    – lollo259
    Sep 14, 2023 at 19:06
  • $\begingroup$ @lollo259 Yes you can, no problem $\endgroup$
    – Elzaidir
    Sep 14, 2023 at 19:29
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In order to have fluids do the computing, you would need ducts and fluid, and the ducts take up space while the fluids have inertia and friction. The brain would be large and slow. And it would be very slow indeed because the ducts cannot be too small, so it makes the brain bigger; the bigger the brain is, the longer the ducts need to be, which means lot of friction and relatively long delays in signal propagation. In addition to this, you need some kind of pumps to make the fluid move, and you need them all over the brain, and largish ducts don't work if the pumps are too small.

Brain that uses neurons is way faster and smaller because the neurons operate electrically, the gaps between the synapses of neurons are tiny, and the dendrites and axons only need to transfer the electrical and/or acoustic wave impulses, so nothing moves physically over long distances. And, the pumps are incredibly small - they pump protons.

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  • $\begingroup$ Well, nerve signals are relatively slow. In fact, they are slower than the speed of sound for water, which would be the maximum velocity of the signal in a water-based fluidic system. $\endgroup$
    – Elzaidir
    Sep 12, 2023 at 16:21
  • $\begingroup$ Speed is relatively slow, true, but in order to effectively send pulses in water based system, you have a serious problem with reverberation when the pulse is reflected from the other end, and then again, and agsin until it dies. I might be wrong, but I think you basically need to make the fluid move so much so that the signal does not get lost in the noise. $\endgroup$ Sep 12, 2023 at 16:39
  • $\begingroup$ In electrical systems, the signal is also reflected when the system isn't perfectly tuned. If you ever have had to use coaxial antenna cable, they are often marked as 50 ohms (or 75 ohm), this is because they have to be specifically tuned to avoid reflections. In incompressible fluids, the phenomenon is the same. There are many ways to fix reflection in an electrical system, and the same methods would work for fluidic. $\endgroup$
    – Elzaidir
    Sep 12, 2023 at 16:46
  • $\begingroup$ @Elzaidir True, however action potential does not show that the reflection is an issue, so the neurons can deal with it. Maybe something similar can be done with fluids, but size requirements are still an issue. Axon-wide duct is probably too narrow for fluids. $\endgroup$ Sep 12, 2023 at 17:55
  • $\begingroup$ Capillaries are in fact close in size compared to axons, and they carry blood which is more viscous than water, see my answer for more details. $\endgroup$
    – Elzaidir
    Sep 12, 2023 at 18:06
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how would a human brain work as an agglomeration of tubes doing only fluidic computing

If we had an answer to this, we could simulate the human brain on a computer, since fluidic computing is not more powerful than our electric computing.
Best we can do today are neuronal networks which are still far away from the way an actual brain works and they need tons of computing power. Due to physical limitations for liquids the fluidic computing will have much less computing power than our electric computing has using the same volume, while also being much slower in signal propagation.

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[Frame Challenge]

how would a human brain work as an agglomeration of tubes doing only fluidic computing?

It wouldn't, Fluid computing is slower than what we can do now with Electric Technology. If we can't do it now, it won't be possible to make a "human-sized" fluid computer that can think and move or even compare to a person, for the same reason we can't do it with Electricity, the cost would be tremendous (even only in theory it's a bad idea).

Also, I didn't quite get how it is possible for waste disposal since it's all a closed loop, else the liquid would just fall out. But the biggest problem I see is with oxygen/food, if the liquid is permanently inside tubes, there would be very difficult to plausibly make a way for the nutrients to go in it, since there's no abortion, also why would a complex system of tubes even need oxygen or nutrients, shouldn't it all go to the actual musculoskeletal since no energy is needed ? (I'm assuming it uses the muscles contracting as both Pumps and Ducts) I know the idea seems fun and I want it to be plausible, but there are just a lot of holes right now, maybe if you work some more on these edges and hand wave the size, and it should be okay.

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First of all, what are the tubes made of? Capillaries are 5 micrometers in diameter, but water can fit (not necessarily flow, as I will later explain) inside carbon nanotubes, which are (obviously) nanoscale, and therefore smaller.

Data transfer

Water has been found to freeze inside carbon nanotubes, and this finding has lead researches to believe that carbon nanotubes filled with ice might be good conductors. This means that data transfer can use electricity. Plus, life as we know it uses carbon anyway. If ice tubes are used though, you would need separate tubes for the blood cells (assuming the logic processing method actually needs these blood cells)

Logic

There are multiple ways the electricity can be used for logic. Tubes do not easily do logic (making a computer with tubes would be like making a computer out of ONLY wires), so you would either have to come up with an innovative way to do the logic, or use alternative structures designed to do logic (Like the psuedo-neurons the guy at the top suggested).

Maybe these all-purpose blood cells could help (I’m not saying how, they could be like neurons with a charge threshold, logic gates, or something completely different, whatever floats your boat). Or maybe they can control smooth muscles that compress around a bigger tube (so the water isn’t frozen like in the nanotubes, because hydraulics doesn’t work well with ice), and at the end of the tube is another smooth muscle that generates electricity (I don’t know if muscles do that naturally, it seems likely, but I can find no sources supporting that, so if they don’t, you might need to invent a new tissue type for that), the voltage depending on the pressure of the water.

Additional cells

The idea I’m going with is the second idea I had, because it seems pretty good. You would need to have some cells to repair the carbon nanotubes just in case something damages them (like a disease, it’s unlikely they would break unintentionally because they are incredibly strong), but all it would have to do is release some carbon atoms, and in fact, it might even work to just have carbon atoms floating around. The logical tubes could be made of a few layers of proteins, but if so you would need some cells adding proteins (or they could float around like the carbon atoms). You could make a water carrying cell that gets attracted to detached proteins (when the proteins get attached they bond with one of the carbon atoms or something so if they break of they can be distinguished) so they can refill the logic cells if they leak. And finally you would need blood that goes to the muscle cells.

This idea makes the being’s brain more autonomous, resistant to failure, durable, and compatible with muscles (you don’t want to invent a whole new muscle that works with water pressure, do you? It’s much easier to use electricity).

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Elzaidir presents an interesting scheme, but it ignores a great deal of reality to make it work.

The biggest issue is going to be that you are trying to double, triple, even quadruple-use a single vascular system. Just trying to combine our blood flow with the lymphatic system would have the efficiency loss equivalent to creatures that eliminate through the same orifice that they consume through.

The human circulatory system acts as a chemical delivery system regulated by vasodilation. This means more blood goes to the parts of the body that we're using. This isn't just convenient and efficient. It's necessary to avoid problems like bleeding into your stomach. If you're overriding that with intellectual needs, you're going to run into conundrums like "do I analyze this, or do I get out of the way?" While a normal human would be restricted from this because they don't have enough cognitive capacity, having your blood flow also be used for thinking would take that to a whole new level.

You also lose the ability to separate chemicals through membranes. I can't even being to describe how crucial that is. Fluids on either side of membranes have vastly different chemical properties, and this is required for many steps of our metabolism. A blood-brain barrier would be a non-starter.

Also, your mental faculties would be easy to disrupt through jolts and pressure waves. Imagine forgetting what you were doing because someone punched you in the stomach. This would make simple behaviors like running into a near-impossible feat.

Overall, it's a fun idea, but it falls apart under even mild analysis.

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