I read your post AGuest but part of the problem is there are quite a few here that don't believe the Bible is God's inspired word. The writing of Leviticus was completed in 1512 B.C.E. Apart from the Old Testament and related sources, there are only a few surviving records of any sort from the Mosaic era, mostly in the form of inscribed stone slabs called stelae. There's certainly no record of blood transfusions as a medical practice, but both historical and Biblical records recount that humans worshiped a pantheon of gods made up of N'filim and fallen angels. Most polytheistic religions required human sacrifices and drank the sacrificial blood. Assyrians were noted to drink the blood of conquered nations while sieging their next conquest for intimidation purposes. As the book of Enoch includes the records of semi-divine entities drinking blood straight from a human, so do many other ancient religions. The Egyptian Sekhmet, the Greek Hecate, the Indian Kali and the Babylonian Lilitu are just a few examples.
The Bible book of John was completed in 98 C.E. I already stated that blood transfusions weren't a medical practice in the 1st century (the century the last book of the Bible was completed). The first historical attempt at a blood transfusion was described in the 17th century, although there was no way to safely store blood, screen blood for diseases or match blood types in the 17th century. Jesus obviously wasn't referring to channeling a person's blood to someone else as a means of salvation. There's very little sacrifice involved in such a procedure. There would be more sacrifice in giving up a vital organ to someone on a long waiting list for an organ transplant, but that's not the message Jesus was conveying either in John 15:13.
John 6:51-56 is obviously speaking in illustrative terms. "So the disciples came up and said to him: “Why is it you speak to them by the use of illustrations?” In reply he said: “To you it is granted to understand the sacred secrets of the kingdom of the heavens, but to those people it is not granted." (Matthew 13:10-11)
John 6:35, 40 clearly indicates that the eating and drinking would be done by exercising faith in Jesus Christ. Since the Eucharistic transubstantiation: flesh and blood ritual is a principal rite of the Catholic Church, one might expect the Scriptures to support it. They do not. The Catholic Encyclopedia (1913 edition) explained why: “The chief source of our doctrine . . . is tradition, which from the earliest times declares the impetratory [entreating] value of the Sacrifice of the Mass.”
http://www.usccb.org/catechism/document/protocol.shtml
Only validly ordained priests can preside at the Eucharist and consecrate the bread and the wine so that they become the Body and Blood of the Lord. (1411)
I'd say it's better for Catholics that the miracle can't be performed by an ordained priest because eating anything turned into human blood and flesh once a week would probably make you ill after a while. What was stated in Matthew 9:13, Matthew 12:2-7 and Matthew 23:23 was to men that constantly tried to trap Jesus in his speech. "Then the Pharisees went their way and took counsel together in order to trap him in his speech." (Matthew 22:15) Jesus replaced animal sacrifices as atonement for sin when he replaced the Mosaic law covenant. The Law covenant became in a sense “obsolete” when God announced by means of the prophet Jeremiah that there would be a new covenant. In 33 C.E. the Law covenant was canceled on the basis of Christ’s death (Collisions 2:14), the New Testament replacing it. Observing the sabbath and animal sacrifices were part of the law covenant under Moses, but the astonishment about abstaining from blood was reaffirmed in Acts 15:20, so what does Matthew 9:13, Matthew 12:2-7 or Matthew 23:23 have to with Acts 15:20?