One day I thought to myself:

Since I can't do RPG healing spells for a multitude of reasons (waste heat, trash disposal, protein breakdown at high temperatures), I should at least let's try something to save gravely injured characters.

This is how I ended up with the concept of the genetically engineered "Sustenance Creature". Basically, a sustenance creature connects to two catheters, put into a major vein and a major artery, and circulates blood through itself, enriching it with oxygen, nutrients, and medicine, removing toxins and waste, and occasionally infusing lipids into it to draw out lipophilic toxins.

There are other functions to it, like being able to "write test reports" with pheromones, but those are largely irrelevant for now.

Now, an "average" sustenance creature was supposed to be able to perform just as well in maintaining circulation and oxygen levels as a healthy human heart and lungs.

Since the sustenance creature was supposed to be a part of paramedical equipment, I wanted to reduce its size and weight to the smallest functioning form. So, here are the factors that could affect that:

  • The heart is fairly small, but the lungs aren't as much, not to mention the liver, which the creature was also supposed to substitute.

  • Kidneys are probably a-okay.

  • Nutrients are easier, as we can just create a diverse and energy-rich solution and let the creature feed on it constantly like a horse.

  • Removing toxins is also a contentious area, but we can get away with smaller sizes as long as there are no unhandled errors like muscle proteins clogging the filters.

  • The creature was supposed to be capable of moving around, but fairly slowly.

So, what's the smallest size where the creature would probably be able to function?

  • $\begingroup$ What you describe in the body of the question is not the dialysis you mention in the title $\endgroup$
    – L.Dutch
    Jun 2, 2021 at 19:52
  • $\begingroup$ @L.Dutch-ReinstateMonica That's fair, it does more than dialysis, it's just that living life support sounds... weird. $\endgroup$ Jun 2, 2021 at 19:55
  • $\begingroup$ One thing to keep in mind, even though the human liver is fairly large we don't actually need as much of it moment-by-moment as we have. And I suspect this creature would keep the patient un-moving so oxygen requirements should be greatly lowered. $\endgroup$ Jun 2, 2021 at 20:09
  • $\begingroup$ why would a paramedic use anything like this, nutrient supply and filtration are long term care. A paramedic could use an organic ventilator or maybe an heart but most of the stuff is pointless. $\endgroup$
    – John
    Jun 2, 2021 at 21:39
  • $\begingroup$ "maintaining circulation" means that "major artery" must be ascending aorta (which is a little hard to reach). $\endgroup$
    – Alexander
    Jun 2, 2021 at 22:32

4 Answers 4


the same as a human torso

I'm assuming you want the living life-support machine to support an average human. This organic machine would take over 100% of the functions. So I'm disregarding things like too small arteries for full blood circulation.

Biology is generally created in it's smallest size that remains practical. A full heart, full lungs and all the other things are needed to put someone on full life support. So you'll likely end up with the same amount of vital organs as normal, assuming a 100% connection. That would mean you have about a torso worth of organs, minus some attachments and muscles used for the extremities.

There's still ways to reduce further. The person in question is likely not going to be doing any exercise. You can reduce lungs and heart to match. You only need to be sure they don't get a anxiety attack or similar that raises the need for blood and oxigen (debatable), or else lack of blood and oxigen can damage the person.

The constant feeding of the living machine is a problem though. Extracting energy from food is paradoxically an energy intensive task. It would need a better heart and lungs to facilitate both digestion and a person. Moreover, you would need to store much more in liver and fat, as well as have the kidneys work harder. Each if these would increase the size of the organ or the full shaoe. It would be more logical if you have it take it's fill and then have it dormant, saving energy until needed. When feeding during it's life support function it doesn't need extreme digestion either, but regular ones.

This doesn't address the better function of some of the organs, or at least how it is suggested to me. How can it be better at functioning than a liver in metabolising poisons into something harmless if it's the same size as a normal persons liver? In addition, where do you keep all the lymph nodes and other immune system supports? They need to be packed in there as well. Finally it still needs to support itself, which also means supporting some power-hungry organs.

Most likely you'll end up with about the size of a human torso, if not a bit bigger. Only if the life-support isn't required to be 100% you can probably start reducing the size.

  • $\begingroup$ "Extracting energy from food is paradoxically an energy intensive task." Much of that is in the chemical reactor which has to handle everything edible and a few things which aren't. Ruminants have multiple stomachs to handle cellulose, humans only have one. This thing can use something as simple as bloodsucking parasites do - its nutrients are provided in accessible, consistent, purified form. $\endgroup$
    – Anon
    Jun 3, 2021 at 5:55

They are teddy bears.

teddy bear


Because these things were invented for sick kids, and the inventors wanted the kids to like their machines. They can talk. They talk as one would to a sick kid. Some can tell stories. Some can sing songs if you ask, or if they think you need one. When they think you need one, they are always right.

Back to the OP: of course the size would change. Wastes would accumulate from the liver and kidney functions. You would need to drain the bear periodically.

  • $\begingroup$ About like this. OP has explicitly eliminated digestion as an energy source, this greatly simplifies the structure needed. Specified as life support, not full-exertion capability. So about 15% lung and 25% heart capacity is ample. The liver and kidney functions can be done using "sacrificial" means, which allows for a much more compact system.(thus requiring a replacement bear, not merely draining it. Like disposable batteries) $\endgroup$
    – PcMan
    Jun 3, 2021 at 6:30

Give it legs.

Why carry your med-kit around when it can carry itself and do odd-jobs like guarding and fetching supper.

It should be as big as three of these:

Border collie, aaah.

Svenska Mässan under CCL 2021 Via Wikipedia

Border collies (working dogs used to herd sheep) weigh in at a nimble 17 kg (35 lbs) for the males but have pretty fast metabolisms, faster than humans for their size by a factor of about 2.5.

Or one of these:

Big, healthy and alert bullmastiff.

Fausto Moreno under CCL 2021 Via Wikipedia

The Bullmastif weighs in at 55Kg (about 120 lb) for the male and slightly lighter for the female, a fit young dog in peak condition can metabolize 3200 K calories per day, more than an average man's needs, much more than a woman's.

At peak power output these dogs in whatever combination will provide sufficient metabolic oomph to take-over the resting metabolism of a human - with spare - to aid in the healing process. Plus they're good companions, offering emotional support, physical warmth and protection from those with ill intentions to boot. They might even take saddlebags with a little persuasion.

Of course, if you're more of a cat person, a female Bengal Tiger metabolizes over 6000 K calories per day, enough to heal two adults at a time.

  • $\begingroup$ As pretty as they are, they're a bit wasteful for this usage. All that brain matter, legs, tail, the really terrible digestive system... not to mention however many pounds of unnecessary muscle, and the drool fountain. Surely we can do better? Maybe not as aesthetically, but a bit more compact. $\endgroup$
    – Corey
    Jun 4, 2021 at 11:00

As it often happens, the answer is, "it depends".

What you need is power. The creature must produce around 40-60 W of power to handle blood circulation and, but that's a small fraction, air circulation. All the filtering is just a small addition to the blood circulation function.

Then you need oxygen flow. At the same efficiency of human respiration this equals about 8 liters per minute, or fourteen cubic centimeters per second.

Operating with the same efficiency and methods of a human being, you'd get a smallish human torso, but why use large lungs? You can just as well use a continuous-flow mechanism or several smaller lungs breathing faster. As said above, a 14cc piston lung operating once a second would be enough. Using a haemoglobin analogue with twice the oxygen affinity would allow to decrease to 7cc per second.

For the same reasons, the heart could be a perystaltic pump system, just a little larger than a large artery. That, by the way, is a problem: to properly operate, your system must be able to interface in four points, aorta, vena cava and two major pulmonary arteries -- that allows bypassing both heart and lungs, otherwise they might be laboring at odds.

And some form of nervous system control seems in order - this could be done chemically though.

Your creature then has two large bloodvessel trunks that enter the body, one from below the neck and the other from below the breast, each carrying two ducts. Then, several opercles to suck in air, and a "mouth" to get the sugary nutrient syrup it needs both to live and to "power" the patient. It will also need to have an excretory pouch - to avoid messes, it could dehydrate organic matter and wrap each pellet in some sort of impermeable "skin" (inside, all kinds of toxins might be present).

The signaling could be done with skin color changes, like octopi, using different patches of skin. You could tattoo a label near to each patch :-) .

Willk's proposal of a teddy bear size seems about right.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .