…when they came to Jesus and saw that he was already dead, they did not break his legs. But one of the soldiers pierced his side with a spear, and at once there came out blood and water. (John 19:33–34).

The heart is the core and most expressive element of a person. Although strictly speaking, the evangelist does not use the word heart when Jesus is pierced by the lance, the concept is the same. John wishes to to emphasize, not only through specific words, pleuran and koilia, but with his Scripture quotations, that the very core of the Person of Jesus Christ was opened in order to reveal the depths of his love. The opening of the body or side (even heart) of Jesus releases the life giving waters of the Holy Spirit.
— “The Secret of the Heart: A Theological Study of Catherine of Siena’s Teaching on the Heart of Jesus. Sr. Mary Jeremiah, 1995 Christendom Press, page 28”