There is a sense in which the Eucharist is NOT "Christ's physical body and blood." Christ's physical body and blood are physically present in heaven--wherever that may be, physically. It is inaccurate to say that Christ's body and blood are "physically" present in the Eucharist. It IS accurate to say that Christ's real body and real blood, which are physical things, are present in the Eucharist.
"Transubstantiation" means exactly what the words of Jesus (This is my body/blood...) mean: that THIS, that I am holding in my hand, IS (not "represents," not "suggests," not "reminds you of...") my body/blood. The Catholic Church teaches "transubstantiation" because only the concept of "transubstantiation" preserves the clear meaning of the words Jesus spoke when he instituted the sacrament. All contrary doctrines on the Eucharist explain away the words of Jesus.
"Transubstantiation" means that the whole substance of the bread and wine are changed into the whole substance of the Body and Blood of Christ. This is not a physical process or a natural event. It is entirely miraculous. Not one physical atom, proton, neutron, or electron is disturbed by the event of transubstantiation. Thus, absolutely nothing regarding the physical characteristics or appearances of the bread and wine are changed. But the Body and Blood of the Risen Christ are now present, really and substantially, and the bread and wine no longer exist, but only their appearances--their "accidents."
For example: Suppose Jesus, physically present in heaven, weighs 180 lbs. If one ton of bread is consecrated, Jesus does not suddenly weigh one ton. Rather: In the whole one ton of bread, the 180-lb. Jesus is really and substantially present. If a piece of the "bread" weighing 3 grams is broken off, the 180-lb. Body of Jesus is present, really and substantially, in that piece of "bread." And Jesus is present with all HIS accidents: His height, weight, brown hair, etc. HIS accidents are not visible to us, of course, just as his very body is not visible to us.
Similarly, if 1,000 gallons of wine are consecrated, Jesus does not suddenly have 1,000 gallons of blood. Rather, the normal quantity of blood in an adult male (since Jesus is Risen, i.e., alive) is now present, really and substantially, in the whole 1,000 gallons of "wine" and in any portion of it, no matter how large or small.
Wikipedia has excellent articles on this. The first link is to the most thorough article on "transubstantiation." The others treat it in other contexts, such as the Mass and Eucharistic devotion.
https://en.wikipedia.org/wiki/Transubstantiation
https://en.wikipedia.org/wiki/Eucharist_in_the_Catholic_Church#Transubstantiation
https://en.wikipedia.org/wiki/Real_presence_of_Christ_in_the_Eucharist#Catholic:_Transubstantiation
Thomas Aquinas's treatment of the Eucharist in the Summa Theologica:
http://www.newadvent.org/summa/4.htm
You can scroll down on the page to these links, or use them from here:
http://www.newadvent.org/summa/4073.htm
http://www.newadvent.org/summa/4074.htm
http://www.newadvent.org/summa/4075.htm
http://www.newadvent.org/summa/4076.htm
http://www.newadvent.org/summa/4077.htm
http://www.newadvent.org/summa/4078.htm
http://www.newadvent.org/summa/4079.htm
http://www.newadvent.org/summa/4080.htm
http://www.newadvent.org/summa/4081.htm
http://www.newadvent.org/summa/4082.htm
http://www.newadvent.org/summa/4083.htm
I am afraid we are not going to persuade each other on this matter, but for the record I will at least give some basic explanations as to why I disagree. Consider a significant passage regarding communion and Christ’s body:
1 Corinthians 11:23-29
For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, “Take, eat; this is My body which is broken for you; do this in remembrance of Me.” In the same manner He also took the cup after supper, saying, “This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me.” For as often as you eat this bread and drink this cup, you proclaim the Lord’s death till He comes. Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord. But let a man examine himself, and so let him eat of the bread and drink of the cup. For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lord’s body.
This ritual has a view to the past, present, and future. The past is indicated by the word “remember”. We do not remember what is present. Therefore, the bread and cup are symbols of what has already transpired, namely Christ’s death. Because Christ is no longer present in His mortal body to walk among us, we recall His life, His ministry, His teaching, His suffering, and His resurrection. His resurrection is pictured in these symbols because there is no body in the tomb. Christians are not like Muslims who endeavor to make a trip to Mecca. We do not need to go visit the burial site of Christ (although there is nothing wrong with doing so) because His body is no longer there. But we do recall the time of His mortal life through the ritual of breaking of bread as He instructed.
The present and future are also in mind in this tradition. It proclaims or “shows forth” Christ’s death (presently) until He comes again (in the future).
John 16:7
Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you.
Luke 17:22
Then He said to the disciples, “The days will come when you will desire to see one of the days of the Son of Man, and you will not see it.”
Matthew 18:20
For where two or three are gathered together in My name, I am there in the midst of them.
Christ’s “real presence” is not conditional upon breaking of bread. It has to do with gathering in His name. This is the local church. The church is Christ’s body, so any food or drink consumed by believers is essentially, at least in part, transformed into Christ’s body.
The bread and cup (it is never called wine in scripture) are ordinary. But God uses the ordinary for special purposes when they are consecrated for His purposes. Rather than building a shrine or image to Christ, He ordained that we would remember Him through perishable symbols. It was never intended for consecrated symbols or images to become a stumbling block by leading God’s people into idolatry.
In the first Corinthians passage above, it speaks of failing to discern Christ’s body. Discerning Christ’s body requires an understanding about what has transpired as a result of His death, resurrection, and union with believers by the coming of the Holy Spirit.
Discerning Christ’s body requires knowing that there is a natural body and spiritual body:
1 Corinthians 15:42-50
So also is the resurrection of the dead. The body is sown in corruption, it is raised in incorruption. It is sown in dishonor, it is raised in glory. It is sown in weakness, it is raised in power. It is sown a natural body, it is raised a spiritual body. There is a natural body, and there is a spiritual body. And so it is written, “The first man Adam became a living being.” The last Adam became a life-giving spirit. However, the spiritual is not first, but the natural, and afterward the spiritual. The first man was of the earth, made of dust; the second Man is the Lord from heaven. As was the man of dust, so also are those who are made of dust; and as is the heavenly Man, so also are those who are heavenly. And as we have borne the image of the man of dust, we shall also bear the image of the heavenly Man. Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption.
Discerning Christ’s body requires an understanding that there can be no divisions among believers because there can be no division of Christ’s body.
1 Corinthians 10:16-17
The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? For we, though many, are one bread and one body; for we all partake of that one bread.
Discerning the body of Christ requires understanding that the Church (not the hierarchal man-made system of clergy, cathedrals, shrines, relics, and books but all the men, women, and children who have trusted in Christ for salvation) is Christ’s body:
1 Corinthians 12:12-14
For as the body is one and has many members, but all the members of that one body, being many, are one body, so also is Christ. For by one Spirit we were all baptized into one body—whether Jews or Greeks, whether slaves or free—and have all been made to drink into one Spirit. For in fact the body is not one member but many.
We can also see from the language used at the last supper that the bread and cup are symbols:
Luke 22:20
Likewise He also took the cup after supper, saying, “This cup is the new covenant in My blood, which is shed for you.”
One way of expressing His words as recorded by scripture is that the cup IS the new covenant. A covenant is not a physical thing, though it may be represented in a physical object like a document. He also said in a passage I cited above that the bread is His body which IS BROKEN for you. But His body is no longer broken. This is past. Christ is no longer suffering on the cross. He is no longer being beaten. He no longer agonizes in the pains of death. The breaking of bread pictures Christ’s body which WAS broken for us. This sacrifice occurred ONCE and is FINISHED:
Hebrews 7:27
who does not need daily, as those high priests, to offer up sacrifices, first for His own sins and then for the people’s, for this He did once for all when He offered up Himself.
Hebrews 9:28
so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation.
(See also all of Hebrews 10.)
John 19:30
So when Jesus had received the sour wine, He said, “It is finished!” And bowing His head, He gave up His spirit.
It can be seen plainly from the passages of scripture above that the bread and cup of communion are symbolic. They are transformed into the body of Christ in the same sense that all food believers eat become part of Christ’s body because believers’ bodies are parts of Christ’s body. Christ’s real presence does not require the symbols of the bread and cup, but these are given to remember the new covenant by which we have eternal life.