This isn't really a very good analogy, because an organ donation is a one time event with no continuous, parasitic relationship between donor and receiver. A m

This isn't really a very good analogy, because an organ donation is a one time event with no continuous, parasitic relationship between donor and receiver. A much better analogy might be where you have a special blood type, and there's a person somewhere who needs regular transfusions of your blood in particular to continue living. No other blood will do, only yours. In this analogy, we can first agree that the healthy person has no obligation to begin donating to the sick person, and this is strongly supported by legal precedent in the US at least. If there exists a frozen embryo, it is not legally justifiable to force some healthy woman to have it implanted. We could also probably agree that if the healthy person is kidnapped and their blood taken against their will, they have no obligation to continue. This would give us an abortion exception for the cases rape and incest. But the open question might be, if the healthy person consents to start donating, do they have the right to rescind their consent at anytime, say after 3 months of consistently donating blood? Do they have the right if they learn that donating blood so regularly is rapidly destroying their own health and is putting their own life at risk? Do they have the right revoke consent if the doctors are quite certain that the sick person will die in 3 months? Do they have a right to revoke consent just because they don't want to do it anymore, even though the sick person will die?