Paul was the earliest writer of the books in the NT. In Galatians he writes that he learned nothing from the Jerusalem Church (James, Peter, and John) but that everything he got came to him directly from Jesus. (Presumably he had "visions".)
Paul is thus the creator of this "Last Supper" Eucharist, with its mystical "blood" and "flesh". The later Gospel writers, who had no personal experience of Jesus either, copied Paul's story, word-for-word in one case. The exception is the John Gospel, who place this "blood" and "flesh" into a totally different context; nothing to do with any "Last Supper in an upper room".
Doug