Partially from ... your lungs.
I don't think that was the question here. As I understand it, in some OW courses the instructor mentions this (apparently near-mythic) "extra breath" that might, under some circumstances, be delivered in response to the diver inhaling during ascent. I vaguely recall hearing it myself.
The air in a diver's lungs expands on ascent, for sure. But I don't think that addresses the "extra breath" question.