The heart of Jesus was a human heart like ours. It stopped beating the day Jesus was crucified. According to the Gospel of John, after Jesus had died a soldier made sure of his death by piercing his side with a lance. Blood and water ran out (19:34). Jesus’ pierced heart became a sign of the completeness of his love for us, a love that compelled him to die for us and that prompted him to remain with us in the bread and wine of the Eucharist. The physical heart of Jesus is the symbol of the total love of Jesus, divine and human.
Jesus said that from within him would flow streams of living water (John 7:38), referring to the Holy Spirit. The water from the side of Christ on the cross was the water of salvation, the Holy Spirit.
In art the Sacred Heart is pierced, surmounted by a cross, and encircled with thorns. Light radiates from it to show that it is a burning furnace of love. The Heart of Christ summarizes the paschal mystery—the death and resurrection of Jesus that were prompted by divine love.
Source: Loyolapress.com