Blood filtration currently is essentially the immune system, affecting disease immunity by 100%. Lose one kidney and it's like your colonist has AIDS. Better hope he doesn't get sick. Lose both, instant death. This is not at all how immunity, kidneys, and blood filtration work.
So I suggest this instead:
* Kidneys are mostly redundant. Blood filtration equals 90%*best kidney + 10%*other kidney.
* Blood filtration below 50% causes a permanent "kidney failure" disease, affecting other bodily functions. Below 15%, it progresses to death.
* A new "Immune system" function for humans. This replaces blood filtration as the primary factor in gaining immunity to infectious diseases. Some diseases and conditions can lower this.
* When organs are transplanted, the organ shows up as a "Transplanted kidney" etc. If a colonist has at least one of these it will weaken the immune system by some amount due to anti-rejection drugs. Ships could sell "universal kidney" etc which doesn't have this penalty.
So I suggest this instead:
* Kidneys are mostly redundant. Blood filtration equals 90%*best kidney + 10%*other kidney.
* Blood filtration below 50% causes a permanent "kidney failure" disease, affecting other bodily functions. Below 15%, it progresses to death.
* A new "Immune system" function for humans. This replaces blood filtration as the primary factor in gaining immunity to infectious diseases. Some diseases and conditions can lower this.
* When organs are transplanted, the organ shows up as a "Transplanted kidney" etc. If a colonist has at least one of these it will weaken the immune system by some amount due to anti-rejection drugs. Ships could sell "universal kidney" etc which doesn't have this penalty.