Hail, Precious Blood, flowing from the wounds of our crucified Lord Jesus Christ, and washing away the sins of the whole world.
From The New Roman Missal by Fr. Lasance:
Blessed Pius IX instituted this feast in thanksgiving for the deliverance of the Apostolic See from the violent revolutionaries who had expelled the pope to Gaeta. In 1849, with the assistance of the French army, they were vanquished and the pope was able to return to Rome.
The meaning of this devotion is closely akin to that of the Sacred Heart. The Precious Blood is the price of universal redemption, which love would not have to be anything less than itself. There is very close connection between the Heart and the Blood not only because, according to St. John, after the death of Jesus, blood and water flowed from His wounded Heart, but because the first chalice in which that divine Blood was consecrated and vivified was precisely the Heart of the incarnate Word.
The Mass is of quite recent composition. In the ancient Roman rite the Mass of Passion Sunday was especially intended to recall to the remembrance of the faithful the infinite value of the Blood of Jesus Christ.
ALMIGHTY, eternal God, Who hast appointed Thine only-begotten Son to be the Redeemer of the world, and hast willed to be appeased by His blood, grant us, we beseech Thee, so incessantly to worship the price of our salvation, and to be so defended by its power from the ills of this life on earth, that we may enjoy its everlasting fruit in heaven. Through the same. Collect. Mass of the Most Precious Blood of Our Lord.