I am not so sure about that. The logic of it was, its a thing that is in your possession. Blood and organs are part of you not really items in your possession. The fact it gathered all the poison despite it being split up and spread through the body makes me think it targets entire things. So I do think is can only target things as a whole, not just take pieces of them.
Also while it hasn't been talked about in this series. Steal can often be resisted if there is a large power gap. Most often explanation is that mana or aura covers equipped items, so you have to get your mana to penetrate that cover. Things like you being weakened, or off guard often make it easier. So again thats general rules for steal in many series, but with him being so weakened that could be why he was able to target internally, it would fit the general rules rather well. But will have to see if it follows the general ones or not.