In John 19:34, what does the sign of water and blood mean?
"But one of the soldiers pierced His side with a spear, and immediately blood and water came out" (John 19:34).
This is not a sign in the sense that the water and blood represented something greater, it is a statement of what happened. However, the fact that water and blood came out from the side wound indicated that Jesus had died prior to the spear thrust.
Some medical doctors published an article in the Journal of the American Medical Association several years ago examining in detail what happened to Jesus during his death. A copy of the article appears at http://the-crucifixion.org/. One of their conclusions is that the appearance of water indicates that Jesus had died of heart failure brought about by the inability to breathe while hanging on the cross. This type of heart failure causes the sac in which the heart resides to fill with clear fluid.
Because the solider saw that Jesus had died, he thrust his spear into his side and found water and blood coming from the wound, he knew that Jesus was dead and therefore didn't bother breaking his leg bones as would have been customarily done to speed up the death of a crucifixion victim.
"Then the soldiers came and broke the legs of the first and of the other who was crucified with Him. But 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 immediately blood and water came out. And he who has seen has testified, and his testimony is true; and he knows that he is telling the truth, so that you may believe. For these things were done that the Scripture should be fulfilled, "Not one of His bones shall be broken." And again another Scripture says, "They shall look on Him whom they pierced."" (John 19:32-37).