The feast of the Most Precious Blood of our Lord was instituted in 1849 by Pius IX, but the devotion is as old as Christianity. The early Fathers say that the Church was born from the pierced side of Christ, and that the sacraments were brought forth through His Blood.
"The Precious Blood which we worship is the Blood which the Savior shed for us on Calvary and reassumed at His glorious Resurrection; it is the Blood which courses through the veins of His risen, glorified, living body at the right hand of God the Father in heaven; it is the Blood made present on our altars by the words of Consecration; it is the Blood which merited sanctifying grace for us and through it washes and beautifies our soul and inaugurates the beginning of eternal life in it."
The Real Presence of Jesus Christ in the Sacrament of the Eucharist: Basic Questions and Answers
O God, who by the Precious Blood of your Only Begotten Son have redeemed the whole world, preserve in us the work of your mercy so that, ever honoring the mystery of our salvation, we may merit to obtain its fruits. Through Christ our Lord. Amen.
But when [the soldiers] came to Jesus and saw that he was already dead, they did not break his legs, but one soldier thrust his lance into his side, and immediately blood and water flowed out. ~John 19:33-34
In this regard, a retired cardio-thoracic surgeon, Dr. Antony de Bono, feels that there is a simple reason for the outpouring of the "blood and water."
He writes:
Jesus had a haemothorax, which in the stillness of the dead body, had separated out as they do into two layers: the heavier red cells below and the light watery plasma above. The haemothorax was the result of the savage flagellation.
The withdrawal of the spear would have been followed first by the red cells (blood), then by the lighter plasma (water).
The body of Jesus had been hanging on the cross, dead, for some time. Obviously the fluid must have accumulated during life by a bleeding into the chest cavity, almost certainly due to the savage flagellation.
It is well known that blood in these circumstances in a still dead body starts to separate out, to sediment, the heavier red cells sinking to the bottom leaving a much lighter, straw colored fluid, the plasma above.
When a hole is made by the spear, the red cells, which John describes as blood, gushes out first, followed by the plasma, which John saw as water.