The main man vs. nature conflict in the story has to do with how our bodies betray us. The premise of the book is that children can be “unwound” for body parts.
An example of the man vs. nature conflict is present when Connor is sent to be unwound and escapes. He wonders if the police will kill him.
Would they shoot an unarmed kid in the back, he wonders, or would they shoot him in the legs and spare his vital organs. (p. 18)
In this society, organs and body parts are more important than children. Life itself has no value. People who are sick or injured and need new organs take them from children who have been chosen to be unwound.
In this society, when people’s bodies betray them (character vs. nature), they can get body parts from the unwinds.
An accident victim who would have died from internal injuries could get fresh organs. A wrinkled arthritic hand could be replaced with one fifty years younger. (p. 224)
The fact that body parts can be harvested is also a man vs. nature conflict for the unwinds, as well as a man vs. man conflict, because if it was not possible to harvest body parts there would be no need for unwinding. Of course, there is a also a chracter vs. society conflict here, because the society has passed the Bill of Life and approves the unwinding.