It's not made clear in the book. There are some brief discussions about which organs might be taken, but the reality is that the students will eventually become whole body donors (e.g. with every single organ being available to be harvested) after their third or fourth 'donation'.
The implication seems to be that after several donor cycles (presumably liver / pancreas / lung / kidney, etc), the student is probably too weakened to be used for anything other than donating their heart and other internal organs.
But I’ve gone off a bit. The reason I was talking about all this was
because the idea of things “unzipping” carried over from Tommy’s elbow
to become a running joke among us about the donations. The idea was
that when the time came, you’d be able just to unzip a bit of
yourself, a kidney or something would slide out, and you’d hand it
over. It wasn’t something we found so funny in itself; it was more a
way of putting each other off our food. You unzipped your liver, say,
and dumped it on someone’s plate, that sort of thing. I remember once
Gary B., who had this unbelievable appetite, coming back with a third
helping of pudding, and virtually the whole table “unzipping” bits of
themselves and piling it all over Gary’s bowl, while he went on
determinedly stuffing himself.
Hannah had the theory that it was their duty to make us have sex because otherwise we wouldn’t be good donors later on. According to her, things like your kidneys and pancreas didn’t work properly unless you kept having sex.