I am still on the "where is it in relation to my body" as a point of reference.
If I start a blood transfusion, and they stick needles in my body and start sucking it out, I still have legal right to change my mind and stop giving blood to that other person.
If I start organ donation and they do surgery prep and I'm on the table with my buttocks exposed, I still have legal right to change my mind. It'd be complicated and sorrowful and devastating to others, but I retain that fundamental right to control of my body. I can actively take control and perform actions within my body that may have negative consequences to others, that may not be nice or beneficial to others, but still remain firmly within my control.
It may come to another perspective as well: am I actively murdering the person within me, or am I withdrawing services I am providing to another person within me.