Hypothetical question: organ transplants.

This is a purely hypothetical question and is not based on any current (Allah la ye2adar) events.  If you could donate an organ like a kidney to your mom, dad, spouse, child, sibling or best friend; would you?  I say kidney because my assumption is that you are not dead, you have two of them and you really only need one and you can make that decision while you are alive.  Would you choose to give them a couple of years/months more for them to live?  If you are already dead then this question does not apply, but do you have the ability to let go of your own kidney just to try to help someone else?  And the bigger question is, could you do that knowing that the organ might be rejected and you would have lost a kidney in vain?  your special person would have to go through the rest of their life on medication to prevent the rejection of that organ, they would always have low immunity and other general unpleasantness?

Which is more merciful?  To help them enjoy what is left as much as possible or to give them more time but one that might not be as enjoyable?