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?