70 Likewise the victim is the same, namely, our divine Redeemer in His human nature with His true body and blood. The manner, however, in which Christ is offered is different. On the cross He completely offered Himself and all His sufferings to God, and the immolation of the victim was brought about by the bloody death, which He underwent of His free will. But on the altar, by reason of the glorified state of His human nature, "death shall have no more dominion over Him," and so the shedding of His blood is impossible; still, according to the plan of divine wisdom, the sacrifice of our Redeemer is shown forth in an admirable manner by external signs which are the symbols of His death. For by the "transubstantiation" of bread into the body of Christ and of wine into His blood, His body and blood are both really present: now the eucharistic species under which He is present symbolize the actual separation of His body and blood. Thus the commemorative representation of His death, which actually took place on Calvary, is repeated in every sacrifice of the altar, seeing that Jesus Christ is symbolically shown by separate symbols to be in a state of victimhood.
Source: Mediator Dei (Vatican.va)