Do I need to know anatomy?
No, you do not need to know anatomy.
This is surprising to some people. But remember, Reiki energy goes where the bodymind tells it to go. As a practitioner, you set an intention, but that is as far as it goes. When the client pulls in the Reiki energy, it will go to where it is meant to go. It does not matter where your hands are located or above what organ.